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href=/2019/12/release-the-gil>Release the GIL</a></ul><ul class="sidebarItemList_Yudw clean-list"><li class=sidebarItem__DBe><a class=sidebarItemLink_mo7H href=/2019/09/binary-format-shootout>Binary format shootout</a></ul><ul class="sidebarItemList_Yudw clean-list"><li class=sidebarItem__DBe><a class=sidebarItemLink_mo7H href=/2019/06/high-performance-systems>On building high performance systems</a></ul><ul class="sidebarItemList_Yudw clean-list"><li class=sidebarItem__DBe><a class=sidebarItemLink_mo7H href=/2019/05/making-bread>Making bread</a></ul><div role=group><h4>Allocations in Rust</h4><ul><ul class="sidebarItemList_Yudw clean-list"><li class=sidebarItem__DBe><a class=sidebarItemLink_mo7H href=/2019/02/understanding-allocations-in-rust>Foreword</a><li class=sidebarItem__DBe><a class=sidebarItemLink_mo7H href=/2019/02/the-whole-world>Global memory</a><li class=sidebarItem__DBe><a class=sidebarItemLink_mo7H href=/2019/02/stacking-up>Fixed memory</a><li class=sidebarItem__DBe><a 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href=/2018/09/isomorphic-apps>Isomorphic desktop apps with Rust</a></ul><ul class="sidebarItemList_Yudw clean-list"><li class=sidebarItem__DBe><a class=sidebarItemLink_mo7H href=/2018/09/primitives-in-rust-are-weird>Primitives in Rust are weird (and cool)</a></ul><ul class="sidebarItemList_Yudw clean-list"><li class=sidebarItem__DBe><a class=sidebarItemLink_mo7H href=/2018/06/dateutil-parser-to-rust>What I learned porting dateutil to Rust</a></ul><ul class="sidebarItemList_Yudw clean-list"><li class=sidebarItem__DBe><a class=sidebarItemLink_mo7H href=/2018/05/hello>Hello!</a></ul><div role=group><h4>Captain's Cookbook</h4><ul><ul class="sidebarItemList_Yudw clean-list"><li class=sidebarItem__DBe><a class=sidebarItemLink_mo7H href=/2018/01/captains-cookbook-part-1>Project setup</a><li class=sidebarItem__DBe><a class=sidebarItemLink_mo7H href=/2018/01/captains-cookbook-part-2>Practical usage</a></ul></ul></div></div><div role=group><h3>2016</h3><ul class="sidebarItemList_Yudw 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class=sidebarItemLink_mo7H href=/2015/12/testing-cramer>Testing Cramer</a></ul><ul class="sidebarItemList_Yudw clean-list"><li class=sidebarItem__DBe><a class=sidebarItemLink_mo7H href=/2015/11/autocallable>Autocallable Bonds</a></ul><ul class="sidebarItemList_Yudw clean-list"><li class=sidebarItem__DBe><a class=sidebarItemLink_mo7H href=/2015/11/welcome>Welcome, and an algorithm</a></ul></div></nav></aside><main class="col col--7"><article><header><h1 class=title_f1Hy>Release the GIL</h1><div class="container_mt6G margin-vert--md"><time datetime=2019-12-14T12:00:00.000Z>December 14, 2019</time> · <!-- -->9 min read</div><div class="margin-top--md margin-bottom--sm row"><div class="col col--12 authorCol_Hf19"><div class="avatar margin-bottom--sm"><div class="avatar__intro authorDetails_lV9A"><div class=avatar__name><span class=authorName_yefp>Bradlee Speice</span></div><div class=authorSocials_rSDt><a href=https://github.com/bspeice target=_blank rel="noopener noreferrer" class=authorSocialLink_owbf title=GitHub><svg viewBox="0 0 256 250" width=1em height=1em class="authorSocialLink_owbf githubSvg_Uu4N" style=--dark:#000;--light:#fff preserveAspectRatio=xMidYMid><path d="M128.001 0C57.317 0 0 57.307 0 128.001c0 56.554 36.676 104.535 87.535 121.46 6.397 1.185 8.746-2.777 8.746-6.158 0-3.052-.12-13.135-.174-23.83-35.61 7.742-43.124-15.103-43.124-15.103-5.823-14.795-14.213-18.73-14.213-18.73-11.613-7.944.876-7.78.876-7.78 12.853.902 19.621 13.19 19.621 13.19 11.417 19.568 29.945 13.911 37.249 10.64 1.149-8.272 4.466-13.92 8.127-17.116-28.431-3.236-58.318-14.212-58.318-63.258 0-13.975 5-25.394 13.188-34.358-1.329-3.224-5.71-16.242 1.24-33.874 0 0 10.749-3.44 35.21 13.121 10.21-2.836 21.16-4.258 32.038-4.307 10.878.049 21.837 1.47 32.066 4.307 24.431-16.56 35.165-13.12 35.165-13.12 6.967 17.63 2.584 30.65 1.255 33.873 8.207 8.964 13.173 20.383 13.173 34.358 0 49.163-29.944 59.988-58.447 63.157 4.591 3.972 8.682 11.762 8.682 23.704 0 17.126-.148 30.91-.148 35.126 0 3.407 2.304 7.398 8.792 6.14C219.37 232.5 256 184.537 256 128.002 256 57.307 198.691 0 128.001 0Zm-80.06 182.34c-.282.636-1.283.827-2.194.39-.929-.417-1.45-1.284-1.15-1.922.276-.655 1.279-.838 2.205-.399.93.418 1.46 1.293 1.139 1.931Zm6.296 5.618c-.61.566-1.804.303-2.614-.591-.837-.892-.994-2.086-.375-2.66.63-.566 1.787-.301 2.626.591.838.903 1 2.088.363 2.66Zm4.32 7.188c-.785.545-2.067.034-2.86-1.104-.784-1.138-.784-2.503.017-3.05.795-.547 2.058-.055 2.861 1.075.782 1.157.782 2.522-.019 3.08Zm7.304 8.325c-.701.774-2.196.566-3.29-.49-1.119-1.032-1.43-2.496-.726-3.27.71-.776 2.213-.558 3.315.49 1.11 1.03 1.45 2.505.701 3.27Zm9.442 2.81c-.31 1.003-1.75 1.459-3.199 1.033-1.448-.439-2.395-1.613-2.103-2.626.301-1.01 1.747-1.484 3.207-1.028 1.446.436 2.396 1.602 2.095 2.622Zm10.744 1.193c.036 1.055-1.193 1.93-2.715 1.95-1.53.034-2.769-.82-2.786-1.86 0-1.065 1.202-1.932 2.733-1.958 1.522-.03 2.768.818 2.768 1.868Zm10.555-.405c.182 1.03-.875 2.088-2.387 2.37-1.485.271-2.861-.365-3.05-1.386-.184-1.056.893-2.114 2.376-2.387 1.514-.263 2.868.356 3.061 1.403Z"/></svg></a></div></div></div></div></div></header><div id=__blog-post-container class=markdown><p>Complaining about the <a href=https://wiki.python.org/moin/GlobalInterpreterLock target=_blank rel="noopener noreferrer">Global Interpreter Lock</a>
(GIL) seems like a rite of passage for Python developers. It's easy to criticize a design decision
made before multi-core CPU's were widely available, but the fact that it's still around indicates
that it generally works <a href=https://wiki.c2.com/?PrematureOptimization target=_blank rel="noopener noreferrer">Good</a>
<a href=https://wiki.c2.com/?YouArentGonnaNeedIt target=_blank rel="noopener noreferrer">Enough</a>. Besides, there are simple and effective
workarounds; it's not hard to start a
<a href=https://docs.python.org/3/library/multiprocessing.html target=_blank rel="noopener noreferrer">new process</a> and use message passing to
synchronize code running in parallel.</p>
<p>Still, wouldn't it be nice to have more than a single active interpreter thread? In an age of
asynchronicity and <em>M:N</em> threading, Python seems lacking. The ideal scenario is to take advantage of
both Python's productivity and the modern CPU's parallel capabilities.</p>
<p>Presented below are two strategies for releasing the GIL's icy grip without giving up on what makes
Python a nice language to start with. Bear in mind: these are just the tools, no claim is made about
whether it's a good idea to use them. Very often, unlocking the GIL is an
<a href=https://en.wikipedia.org/wiki/XY_problem target=_blank rel="noopener noreferrer">XY problem</a>; you want application performance, and the
GIL seems like an obvious bottleneck. Remember that any gains from running code in parallel come at
the expense of project complexity; messing with the GIL is ultimately messing with Python's memory
model.</p>
<div class="language-python codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-python codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token plain">load_ext Cython</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">from</span><span class="token plain"> numba </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">import</span><span class="token plain"> jit</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">N </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">1_000_000_000</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id=cython>Cython<a href=#cython class=hash-link aria-label="Direct link to Cython" title="Direct link to Cython">​</a></h2>
<p>Put simply, <a href=https://cython.org/ target=_blank rel="noopener noreferrer">Cython</a> is a programming language that looks a lot like Python,
gets <a href=https://en.wikipedia.org/wiki/Source-to-source_compiler target=_blank rel="noopener noreferrer">transpiled</a> to C/C++, and integrates
well with the <a href=https://en.wikipedia.org/wiki/CPython target=_blank rel="noopener noreferrer">CPython</a> API. It's great for building Python
wrappers to C and C++ libraries, writing optimized code for numerical processing, and tons more. And
when it comes to managing the GIL, there are two special features:</p>
<ul>
<li>The <code>nogil</code>
<a href=https://cython.readthedocs.io/en/latest/src/userguide/external_C_code.html#declaring-a-function-as-callable-without-the-gil target=_blank rel="noopener noreferrer">function annotation</a>
asserts that a Cython function is safe to use without the GIL, and compilation will fail if it
interacts with Python in an unsafe manner</li>
<li>The <code>with nogil</code>
<a href=https://cython.readthedocs.io/en/latest/src/userguide/external_C_code.html#releasing-the-gil target=_blank rel="noopener noreferrer">context manager</a>
explicitly unlocks the CPython GIL while active</li>
</ul>
<p>Whenever Cython code runs inside a <code>with nogil</code> block on a separate thread, the Python interpreter
is unblocked and allowed to continue work elsewhere. We'll define a "busy work" function that
demonstrates this principle in action:</p>
<div class="language-python codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-python codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token plain">cython</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token comment" style="color:hsl(230, 4%, 64%)"># Annotating a function with `nogil` indicates only that it is safe</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token comment" style="color:hsl(230, 4%, 64%)"># to call in a `with nogil` block. It *does not* release the GIL.</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">cdef unsigned </span><span class="token builtin" style="color:hsl(119, 34%, 47%)">long</span><span class="token plain"> fibonacci</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">unsigned </span><span class="token builtin" style="color:hsl(119, 34%, 47%)">long</span><span class="token plain"> n</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> nogil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">if</span><span class="token plain"> n </span><span class="token operator" style="color:hsl(221, 87%, 60%)">&lt;=</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">1</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">return</span><span class="token plain"> n</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> cdef unsigned </span><span class="token builtin" style="color:hsl(119, 34%, 47%)">long</span><span class="token plain"> a </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">0</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> b </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">1</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> c </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">0</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> c </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> a </span><span class="token operator" style="color:hsl(221, 87%, 60%)">+</span><span class="token plain"> b</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">for</span><span class="token plain"> _i </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">in</span><span class="token plain"> </span><span class="token builtin" style="color:hsl(119, 34%, 47%)">range</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token number" style="color:hsl(35, 99%, 36%)">2</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> n</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> a </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> b</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> b </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> c</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> c </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> a </span><span class="token operator" style="color:hsl(221, 87%, 60%)">+</span><span class="token plain"> b</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">return</span><span class="token plain"> c</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">def</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">cython_nogil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">unsigned </span><span class="token builtin" style="color:hsl(119, 34%, 47%)">long</span><span class="token plain"> n</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token comment" style="color:hsl(230, 4%, 64%)"># Explicitly release the GIL while running `fibonacci`</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">with</span><span class="token plain"> nogil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> value </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> fibonacci</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">n</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">return</span><span class="token plain"> value</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">def</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">cython_gil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">unsigned </span><span class="token builtin" style="color:hsl(119, 34%, 47%)">long</span><span class="token plain"> n</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token comment" style="color:hsl(230, 4%, 64%)"># Because the GIL is not explicitly released, it implicitly</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token comment" style="color:hsl(230, 4%, 64%)"># remains acquired when running the `fibonacci` function</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">return</span><span class="token plain"> fibonacci</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">n</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<p>First, let's time how long it takes Cython to calculate the billionth Fibonacci number:</p>
<div class="language-python codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-python codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token plain">time</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">_ </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> cython_gil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">N</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<blockquote>
<pre><p>CPU times: user 365 ms, sys: 0 ns, total: 365 ms
Wall time: 372 ms</pre>
</blockquote>
<div class="language-python codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-python codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token plain">time</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">_ </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> cython_nogil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">N</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<blockquote>
<pre><p>CPU times: user 381 ms, sys: 0 ns, total: 381 ms
Wall time: 388 ms</pre>
</blockquote>
<p>Both versions (with and without GIL) take effectively the same amount of time to run. Even when
running this calculation in parallel on separate threads, it is expected that the run time will
double because only one thread can be active at a time:</p>
<div class="language-python codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-python codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token plain">time</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">from</span><span class="token plain"> threading </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">import</span><span class="token plain"> Thread</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token comment" style="color:hsl(230, 4%, 64%)"># Create the two threads to run on</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t1 </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> Thread</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">target</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain">cython_gil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> args</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">[</span><span class="token plain">N</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">]</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t2 </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> Thread</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">target</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain">cython_gil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> args</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">[</span><span class="token plain">N</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">]</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token comment" style="color:hsl(230, 4%, 64%)"># Start the threads</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t1</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">start</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> t2</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">start</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token comment" style="color:hsl(230, 4%, 64%)"># Wait for the threads to finish</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t1</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">join</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> t2</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">join</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<blockquote>
<pre><p>CPU times: user 641 ms, sys: 5.62 ms, total: 647 ms
Wall time: 645 ms</pre>
</blockquote>
<p>However, if the first thread releases the GIL, the second thread is free to acquire it and run in
parallel:</p>
<div class="language-python codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-python codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token plain">time</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t1 </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> Thread</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">target</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain">cython_nogil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> args</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">[</span><span class="token plain">N</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">]</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t2 </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> Thread</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">target</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain">cython_gil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> args</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">[</span><span class="token plain">N</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">]</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t1</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">start</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> t2</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">start</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t1</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">join</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> t2</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">join</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<blockquote>
<pre><p>CPU times: user 717 ms, sys: 372 µs, total: 718 ms
Wall time: 358 ms</pre>
</blockquote>
<p>Because <code>user</code> time represents the sum of processing time on all threads, it doesn't change much.
The <a href=https://en.wikipedia.org/wiki/Elapsed_real_time target=_blank rel="noopener noreferrer">"wall time"</a> has been cut roughly in half
because each function is running simultaneously.</p>
<p>Keep in mind that the <strong>order in which threads are started</strong> makes a difference!</p>
<div class="language-python codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-python codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token plain">time</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token comment" style="color:hsl(230, 4%, 64%)"># Note that the GIL-locked version is started first</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t1 </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> Thread</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">target</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain">cython_gil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> args</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">[</span><span class="token plain">N</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">]</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t2 </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> Thread</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">target</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain">cython_nogil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> args</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">[</span><span class="token plain">N</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">]</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t1</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">start</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> t2</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">start</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t1</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">join</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> t2</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">join</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<blockquote>
<pre><p>CPU times: user 667 ms, sys: 0 ns, total: 667 ms
Wall time: 672 ms</pre>
</blockquote>
<p>Even though the second thread releases the GIL while running, it can't start until the first has
completed. Thus, the overall runtime is effectively the same as running two GIL-locked threads.</p>
<p>Finally, be aware that attempting to unlock the GIL from a thread that doesn't own it will crash the
<strong>interpreter</strong>, not just the thread attempting the unlock:</p>
<div class="language-python codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-python codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token plain">cython</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">cdef </span><span class="token builtin" style="color:hsl(119, 34%, 47%)">int</span><span class="token plain"> cython_recurse</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token builtin" style="color:hsl(119, 34%, 47%)">int</span><span class="token plain"> n</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> nogil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">if</span><span class="token plain"> n </span><span class="token operator" style="color:hsl(221, 87%, 60%)">&lt;=</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">0</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">return</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">0</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">with</span><span class="token plain"> nogil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">return</span><span class="token plain"> cython_recurse</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">n </span><span class="token operator" style="color:hsl(221, 87%, 60%)">-</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">1</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">cython_recurse</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token number" style="color:hsl(35, 99%, 36%)">2</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<blockquote>
<pre><p>Fatal Python error: PyEval_SaveThread: NULL tstate<p>Thread 0x00007f499effd700 (most recent call first):
File "/home/bspeice/.virtualenvs/release-the-gil/lib/python3.7/site-packages/ipykernel/parentpoller.py", line 39 in run
File "/usr/lib/python3.7/threading.py", line 926 in _bootstrap_inner
File "/usr/lib/python3.7/threading.py", line 890 in _bootstrap</pre>
</blockquote>
<p>In practice, avoiding this issue is simple. First, <code>nogil</code> functions probably shouldn't contain
<code>with nogil</code> blocks. Second, Cython can
<a href=https://cython.readthedocs.io/en/latest/src/userguide/external_C_code.html#conditional-acquiring-releasing-the-gil target=_blank rel="noopener noreferrer">conditionally acquire/release</a>
the GIL, so these conditions can be used to synchronize access. Finally, Cython's documentation for
<a href=https://cython.readthedocs.io/en/latest/src/userguide/external_C_code.html#acquiring-and-releasing-the-gil target=_blank rel="noopener noreferrer">external C code</a>
contains more detail on how to safely manage the GIL.</p>
<p>To conclude: use Cython's <code>nogil</code> annotation to assert that functions are safe for calling when the
GIL is unlocked, and <code>with nogil</code> to actually unlock the GIL and run those functions.</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id=numba>Numba<a href=#numba class=hash-link aria-label="Direct link to Numba" title="Direct link to Numba">​</a></h2>
<p>Like Cython, <a href=https://numba.pydata.org/ target=_blank rel="noopener noreferrer">Numba</a> is a "compiled Python." Where Cython works by
compiling a Python-like language to C/C++, Numba compiles Python bytecode <em>directly to machine code</em>
at runtime. Behavior is controlled with a special <code>@jit</code> decorator; calling a decorated function
first compiles it to machine code before running. Calling the function a second time re-uses that
machine code unless the argument types have changed.</p>
<p>Numba works best when a <code>nopython=True</code> argument is added to the <code>@jit</code> decorator; functions
compiled in <a href=http://numba.pydata.org/numba-doc/latest/user/jit.html?#nopython target=_blank rel="noopener noreferrer"><code>nopython</code></a> mode
avoid the CPython API and have performance comparable to C. Further, adding <code>nogil=True</code> to the
<code>@jit</code> decorator unlocks the GIL while that function is running. Note that <code>nogil</code> and <code>nopython</code>
are separate arguments; while it is necessary for code to be compiled in <code>nopython</code> mode in order to
release the lock, the GIL will remain locked if <code>nogil=False</code> (the default).</p>
<p>Let's repeat the same experiment, this time using Numba instead of Cython:</p>
<div class="language-python codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-python codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token comment" style="color:hsl(230, 4%, 64%)"># The `int` type annotation is only for humans and is ignored</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token comment" style="color:hsl(230, 4%, 64%)"># by Numba.</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token decorator annotation punctuation" style="color:hsl(119, 34%, 47%)">@jit</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">nopython</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token boolean" style="color:hsl(35, 99%, 36%)">True</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> nogil</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token boolean" style="color:hsl(35, 99%, 36%)">True</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">def</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">numba_nogil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">n</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token builtin" style="color:hsl(119, 34%, 47%)">int</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">-</span><span class="token operator" style="color:hsl(221, 87%, 60%)">></span><span class="token plain"> </span><span class="token builtin" style="color:hsl(119, 34%, 47%)">int</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">if</span><span class="token plain"> n </span><span class="token operator" style="color:hsl(221, 87%, 60%)">&lt;=</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">1</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">return</span><span class="token plain"> n</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> a </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">0</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> b </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">1</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> c </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> a </span><span class="token operator" style="color:hsl(221, 87%, 60%)">+</span><span class="token plain"> b</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">for</span><span class="token plain"> _i </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">in</span><span class="token plain"> </span><span class="token builtin" style="color:hsl(119, 34%, 47%)">range</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token number" style="color:hsl(35, 99%, 36%)">2</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> n</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> a </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> b</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> b </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> c</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> c </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> a </span><span class="token operator" style="color:hsl(221, 87%, 60%)">+</span><span class="token plain"> b</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">return</span><span class="token plain"> c</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token comment" style="color:hsl(230, 4%, 64%)"># Run using `nopython` mode to receive a performance boost,</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token comment" style="color:hsl(230, 4%, 64%)"># but GIL remains locked due to `nogil=False` by default.</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token decorator annotation punctuation" style="color:hsl(119, 34%, 47%)">@jit</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">nopython</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token boolean" style="color:hsl(35, 99%, 36%)">True</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">def</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">numba_gil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">n</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token builtin" style="color:hsl(119, 34%, 47%)">int</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">-</span><span class="token operator" style="color:hsl(221, 87%, 60%)">></span><span class="token plain"> </span><span class="token builtin" style="color:hsl(119, 34%, 47%)">int</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">if</span><span class="token plain"> n </span><span class="token operator" style="color:hsl(221, 87%, 60%)">&lt;=</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">1</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">return</span><span class="token plain"> n</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> a </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">0</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> b </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">1</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> c </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> a </span><span class="token operator" style="color:hsl(221, 87%, 60%)">+</span><span class="token plain"> b</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">for</span><span class="token plain"> _i </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">in</span><span class="token plain"> </span><span class="token builtin" style="color:hsl(119, 34%, 47%)">range</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token number" style="color:hsl(35, 99%, 36%)">2</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> n</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> a </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> b</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> b </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> c</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> c </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> a </span><span class="token operator" style="color:hsl(221, 87%, 60%)">+</span><span class="token plain"> b</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">return</span><span class="token plain"> c</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token comment" style="color:hsl(230, 4%, 64%)"># Call each function once to force compilation; we don't want</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token comment" style="color:hsl(230, 4%, 64%)"># the timing statistics to include how long it takes to compile.</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">numba_nogil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">N</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">numba_gil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">N</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<p>We'll perform the same tests as above; first, figure out how long it takes the function to run:</p>
<div class="language-python codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-python codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token plain">time</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">_ </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> numba_gil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">N</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<blockquote>
<pre><p>CPU times: user 253 ms, sys: 258 µs, total: 253 ms
Wall time: 251 ms</pre>
</blockquote>
<small><p>Aside: it's not immediately clear why Numba takes ~20% less time to run than Cython for code that should be
effectively identical after compilation.</small>
<p>When running two GIL-locked threads, the result (as expected) takes around twice as long to compute:</p>
<div class="language-python codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-python codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token plain">time</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t1 </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> Thread</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">target</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain">numba_gil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> args</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">[</span><span class="token plain">N</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">]</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t2 </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> Thread</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">target</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain">numba_gil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> args</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">[</span><span class="token plain">N</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">]</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t1</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">start</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> t2</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">start</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t1</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">join</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> t2</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">join</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<blockquote>
<pre><p>CPU times: user 541 ms, sys: 3.96 ms, total: 545 ms
Wall time: 541 ms</pre>
</blockquote>
<p>But if the GIL-unlocking thread starts first, both threads run in parallel:</p>
<div class="language-python codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-python codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token plain">time</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t1 </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> Thread</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">target</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain">numba_nogil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> args</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">[</span><span class="token plain">N</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">]</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t2 </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> Thread</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">target</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain">numba_gil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> args</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">[</span><span class="token plain">N</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">]</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t1</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">start</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> t2</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">start</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t1</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">join</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> t2</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">join</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<blockquote>
<pre><p>CPU times: user 551 ms, sys: 7.77 ms, total: 559 ms
Wall time: 279 ms</pre>
</blockquote>
<p>Just like Cython, starting the GIL-locked thread first leads to poor performance:</p>
<div class="language-python codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-python codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token operator" style="color:hsl(221, 87%, 60%)">%</span><span class="token plain">time</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t1 </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> Thread</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">target</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain">numba_gil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> args</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">[</span><span class="token plain">N</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">]</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t2 </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> Thread</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">target</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain">numba_nogil</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> args</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">[</span><span class="token plain">N</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">]</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t1</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">start</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> t2</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">start</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">t1</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">join</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> t2</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain">join</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<blockquote>
<pre><p>CPU times: user 524 ms, sys: 0 ns, total: 524 ms
Wall time: 522 ms</pre>
</blockquote>
<p>Finally, unlike Cython, Numba will unlock the GIL if and only if it is currently acquired;
recursively calling <code>@jit(nogil=True)</code> functions is perfectly safe:</p>
<div class="language-python codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-python codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token keyword" style="color:hsl(301, 63%, 40%)">from</span><span class="token plain"> numba </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">import</span><span class="token plain"> jit</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token decorator annotation punctuation" style="color:hsl(119, 34%, 47%)">@jit</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">nopython</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token boolean" style="color:hsl(35, 99%, 36%)">True</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> nogil</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token boolean" style="color:hsl(35, 99%, 36%)">True</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">def</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">numba_recurse</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">n</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token builtin" style="color:hsl(119, 34%, 47%)">int</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">-</span><span class="token operator" style="color:hsl(221, 87%, 60%)">></span><span class="token plain"> </span><span class="token builtin" style="color:hsl(119, 34%, 47%)">int</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">if</span><span class="token plain"> n </span><span class="token operator" style="color:hsl(221, 87%, 60%)">&lt;=</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">0</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">return</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">0</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">return</span><span class="token plain"> numba_recurse</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">n </span><span class="token operator" style="color:hsl(221, 87%, 60%)">-</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">1</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">numba_recurse</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token number" style="color:hsl(35, 99%, 36%)">2</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id=conclusion>Conclusion<a href=#conclusion class=hash-link aria-label="Direct link to Conclusion" title="Direct link to Conclusion">​</a></h2>
<p>Before finishing, it's important to address pain points that will show up if these techniques are
used in a more realistic project:</p>
<p>First, code running in a GIL-free context will likely also need non-trivial data structures;
GIL-free functions aren't useful if they're constantly interacting with Python objects whose access
requires the GIL. Cython provides
<a href=http://docs.cython.org/en/latest/src/tutorial/cdef_classes.html target=_blank rel="noopener noreferrer">extension types</a> and Numba
provides a <a href=https://numba.pydata.org/numba-doc/dev/user/jitclass.html target=_blank rel="noopener noreferrer"><code>@jitclass</code></a> decorator to
address this need.</p>
<p>Second, building and distributing applications that make use of Cython/Numba can be complicated.
Cython packages require running the compiler, (potentially) linking/packaging external dependencies,
and distributing a binary wheel. Numba is generally simpler because the code being distributed is
pure Python, but can be tricky since errors aren't detected until runtime.</p>
<p>Finally, while unlocking the GIL is often a solution in search of a problem, both Cython and Numba
provide tools to directly manage the GIL when appropriate. This enables true parallelism (not just
<a href=https://stackoverflow.com/a/1050257 target=_blank rel="noopener noreferrer">concurrency</a>) that is impossible in vanilla Python.</div></article><nav class="pagination-nav docusaurus-mt-lg" aria-label="Blog post page navigation"><a class="pagination-nav__link pagination-nav__link--prev" href=/2019/09/binary-format-shootout><div class=pagination-nav__sublabel>Older post</div><div class=pagination-nav__label>Binary format shootout</div></a><a class="pagination-nav__link pagination-nav__link--next" href=/2011/11/webpack-industrial-complex><div class=pagination-nav__sublabel>Newer post</div><div class=pagination-nav__label>The webpack industrial complex</div></a></nav></main><div class="col col--2"><div class="tableOfContents_bqdL thin-scrollbar"><ul class="table-of-contents table-of-contents__left-border"><li><a href=#cython class="table-of-contents__link toc-highlight">Cython</a><li><a href=#numba class="table-of-contents__link toc-highlight">Numba</a><li><a href=#conclusion class="table-of-contents__link toc-highlight">Conclusion</a></ul></div></div></div></div></div><footer class=footer><div class="container container-fluid"><div class="footer__bottom text--center"><div class=footer__copyright>Copyright © 2024 Bradlee Speice</div></div></div></footer></div>
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