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_drafts/case-study-optimization.md
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@ -6,42 +6,42 @@ category:
tags: []
---
One of my first conversations about programming went like this:
One of my earliest conversations about programming went like this:
> Programmers have it too easy these days. They should learn to develop
> in low memory environments and be efficient.
> in low memory environments and be more efficient.
>
> -- My Father (paraphrased)
Though it's not like the first code I wrote was for a
[graphing calculator](https://education.ti.com/en/products/calculators/graphing-calculators/ti-84-plus-se),
...though it's not like the first code I wrote was for a
[graphing calculator](https://education.ti.com/en/products/calculators/graphing-calculators/ti-84-plus-se)
packing a whole 24KB of RAM. By the way, *what are you doing on my lawn?*
The principle remains though: be efficient with the resources you're given, because
[what Intel giveth, Microsoft taketh away](http://exo-blog.blogspot.com/2007/09/what-intel-giveth-microsoft-taketh-away.html).
My professional work has been focused on this kind of efficiency; low-latency financial markets demand that
you understand at a deep level *exactly* what your code is doing. As I've been experimenting with Rust for
personal projects, it's exciting to bring that mindset with me. There's flexibility for the times where I'd rather
have a garbage collector, and flexibility for the times that I really care about efficiency.
My professional work is focused on this kind of efficiency; low-latency financial markets demand that
you understand at a deep level *exactly* what your code is doing. As I continue experimenting with Rust for
personal projects, it's exciting to bring a utilitarian mindset with me: there's flexibility for the times I pretend
to have a garbage collector, and flexibility for the times that I really care about efficiency.
This post is a (small) case study in how I went from the former to the latter. And it's an attempt to prove how easy
it is for you to do the same.
# The Starting Line
# Curiosity
When I first started building the [dtparse] crate, my intention was to mirror as closely as possible
the equivalent [Python library][dateutil]. Python, as you may know, is garbage collected. Very rarely is memory
usage considered in Python, and I likewise wasn't paying too much attention when `dtparse` was first being built.
That works out well enough, and I'm not planning on making that crate hyper-efficient.
But every so often I've wondered: "what exactly is going on in memory?" With the advent of Rust 1.28 and the
That works out well enough, and I'm not planning on making that `dtparse` hyper-efficient.
But every so often, I've wondered: "what exactly is going on in memory?" With the advent of Rust 1.28 and the
[Global Allocator trait](https://doc.rust-lang.org/std/alloc/trait.GlobalAlloc.html), I had a really great idea:
*build a custom allocator that allows you to track your own allocations.* That way, you can do things like
writing tests for both correct results and correct memory usage. I gave it a [shot][qadapt], but learned
very quickly: **never write your own allocator**. It went from "fun weekend project" into
very quickly: **never write your own allocator**. It went from "fun weekend project" to
"I have literally no idea what my computer is doing" at breakneck speed.
Instead, let's highlight another (easier) way you can make sense of your memory usage: [heaptrack]
Instead, I'll highlight a separate path I took to make sense of my memory usage: [heaptrack].
# Turning on the System Allocator
@ -59,12 +59,12 @@ use std::alloc::System;
static GLOBAL: System = System;
```
Or look [here](https://blog.rust-lang.org/2018/08/02/Rust-1.28.html) for another example.
...and that's it. Everything else comes essentially for free.
# Running heaptrack
Assuming you've installed heaptrack <span style="font-size: .6em;">(Homebrew in Mac, package manager in Linux, ??? in Windows)</span>,
all that's left is to fire it up:
all that's left is to fire up your application:
```
heaptrack my_application
@ -84,14 +84,10 @@ And even these pretty colors:
# Reading Flamegraphs
We're going to focus on the heap ["flamegraph"](http://www.brendangregg.com/flamegraphs.html),
which is the last picture I showed above. Normally these charts are used to show how much time
you spend executing different functions, but the focus for now is to show how much memory
was allocated during those functions.
As a quick introduction to reading flamegraphs, the idea is this:
The width of the bar is how much memory was allocated by that function, and all functions
that it calls.
To make sense of our memory usage, we're going to focus on that last picture - it's called
a ["flamegraph"](http://www.brendangregg.com/flamegraphs.html). These charts are typically
used to show how much time you spend executing different functions, but they're used here
to show how much memory was allocated during those functions.
For example, we can see that all executions happened during the `main` function:
@ -101,7 +97,7 @@ For example, we can see that all executions happened during the `main` function:
![allocations in dtparse](/assets/images/2018-10-heaptrack/heaptrack-dtparse-colorized.png)
...and within *that*, allocations happened in two main places:
...and within *that*, allocations happened in two different places:
![allocations in parseinfo](/assets/images/2018-10-heaptrack/heaptrack-parseinfo-colorized.png)
@ -112,7 +108,7 @@ as an issue: **what the heck is the `Default` thing doing?**
# Optimizing dtparse
See, I knew that there were some allocations that happen during the `dtparse::parse` method,
See, I knew that there were some allocations during calls to `dtparse::parse`,
but I was totally wrong about where the bulk of allocations occurred in my program.
Let me post the code and see if you can spot the mistake:
@ -132,14 +128,13 @@ pub fn parse(timestr: &str) -> ParseResult<(NaiveDateTime, Option<FixedOffset>)>
---
The issue is that I keep on creating a new `Parser` every time you call the `parse()` function!
Now this is a bit excessive, but was necessary at the time because `Parser.parse()` used `&mut self`.
In order to properly parse a string, the parser itself required mutable state.
Because `Parser::parse` requires a mutable reference to itself, I have to create a new parser
every time it receives a string. This seems excessive! We'd rather have an immutable parser
that can be re-used, and avoid needing to allocate memory in the first place.
Armed with that information, I put some time in to
[make the parser immutable](https://github.com/bspeice/dtparse/commit/741afa34517d6bc1155713bbc5d66905fea13fad#diff-b4aea3e418ccdb71239b96952d9cddb6).
Now I can re-use the same parser over and over! And would you believe it? No more allocations of default parsers:
Now that I can re-use the same parser over and over, the allocations disappear:
![allocations cleaned up](/assets/images/2018-10-heaptrack/heaptrack-flamegraph-after.png)
@ -153,12 +148,12 @@ All the way down to 300KB:
# Conclusion
In the end, you don't need to write a custom allocator to test memory performance. Rather, there are some
great tools that already exist you can put to work!
In the end, you don't need to write a custom allocator to be efficient with memory, great tools
already exist to help you understand what your program is doing.
**Use them.**
Now that [Moore's Law](https://en.wikipedia.org/wiki/Moore%27s_law)
Given that [Moore's Law](https://en.wikipedia.org/wiki/Moore%27s_law)
is [dead](https://www.technologyreview.com/s/601441/moores-law-is-dead-now-what/), we've all got to
do our part to take back what Microsoft stole.