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<main>
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<h1 id="基准测试-benchmark"><a class="header" href="#基准测试-benchmark">基准测试 benchmark</a></h1>
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<p>几乎所有开发都知道，如果要测量程序的性能，就需要性能测试。</p>
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<p>性能测试包含了两种：压力测试和基准测试。前者是针对接口 API，模拟大量用户去访问接口然后生成接口级别的性能数据；而后者是针对代码，可以用来测试某一段代码的运行速度，例如一个排序算法。</p>
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<p>而本文将要介绍的就是基准测试 <code>benchmark</code>，在 Rust 中，有两种方式可以实现：</p>
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<ul>
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<li>官方提供的 <code>benchmark</code></li>
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<li>社区实现，例如 <code>criterion.rs</code></li>
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</ul>
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<p>事实上我们更推荐后者，原因在后文会详细介绍，下面先从官方提供的工具开始。</p>
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<h2 id="官方-benchmark"><a class="header" href="#官方-benchmark">官方 benchmark</a></h2>
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<p>官方提供的测试工具，目前最大的问题就是只能在非 <code>stable</code> 下使用，原因是需要在代码中引入 <code>test</code> 特性: <code>#![feature(test)]</code>。</p>
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<h4 id="设置-rust-版本"><a class="header" href="#设置-rust-版本">设置 Rust 版本</a></h4>
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<p>因此在开始之前，我们需要先将当前仓库中的 <a href="https://course.rs/appendix/rust-version.html#不稳定功能"><code>Rust 版本</code></a>从 <code>stable</code> 切换为 <code>nightly</code>:</p>
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<ol>
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<li>安装 <code>nightly</code> 版本：<code>$ rustup install nightly</code></li>
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<li>使用以下命令确认版本已经安装成功</li>
|
|
|
</ol>
|
|
|
<pre><code class="language-shell">$ rustup toolchain list
|
|
|
stable-aarch64-apple-darwin (default)
|
|
|
nightly-aarch64-apple-darwin (override)
|
|
|
</code></pre>
|
|
|
<ol start="3">
|
|
|
<li>进入 <code>adder</code> 项目(之前为了学习测试专门创建的项目)的根目录，然后运行 <code>rustup override set nightly</code>，将该项目使用的 <code>rust</code> 设置为 <code>nightly</code></li>
|
|
|
</ol>
|
|
|
<p>很简单吧，其实只要一个命令就可以切换指定项目的 Rust 版本，例如你还能在基准测试后再使用 <code>rustup override set stable</code> 切换回 <code>stable</code> 版本。</p>
|
|
|
<h4 id="使用-benchmark"><a class="header" href="#使用-benchmark">使用 benchmark</a></h4>
|
|
|
<p>当完成版本切换后，就可以开始正式编写 <code>benchmark</code> 代码了。首先，将 <code>src/lib.rs</code> 中的内容替换成如下代码：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span>#![feature(test)]
|
|
|
|
|
|
<span class="boring">fn main() {
|
|
|
</span>extern crate test;
|
|
|
|
|
|
pub fn add_two(a: i32) -> i32 {
|
|
|
a + 2
|
|
|
}
|
|
|
|
|
|
#[cfg(test)]
|
|
|
mod tests {
|
|
|
use super::*;
|
|
|
use test::Bencher;
|
|
|
|
|
|
#[test]
|
|
|
fn it_works() {
|
|
|
assert_eq!(4, add_two(2));
|
|
|
}
|
|
|
|
|
|
#[bench]
|
|
|
fn bench_add_two(b: &mut Bencher) {
|
|
|
b.iter(|| add_two(2));
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>可以看出，<code>benchmark</code> 跟单元测试区别不大，最大的区别在于它是通过 <code>#[bench]</code> 标注，而单元测试是通过 <code>#[test]</code> 进行标注，这意味着 <code>cargo test</code> 将不会运行 <code>benchmark</code> 代码：</p>
|
|
|
<pre><code class="language-shell">$ cargo test
|
|
|
running 2 tests
|
|
|
test tests::bench_add_two ... ok
|
|
|
test tests::it_works ... ok
|
|
|
|
|
|
test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
|
|
|
</code></pre>
|
|
|
<p><code>cargo test</code> 直接把我们的 <code>benchmark</code> 代码当作单元测试处理了，因此没有任何性能测试的结果产生。</p>
|
|
|
<p>对此，需要使用 <code>cargo bench</code> 命令：</p>
|
|
|
<pre><code class="language-shell">$ cargo bench
|
|
|
running 2 tests
|
|
|
test tests::it_works ... ignored
|
|
|
test tests::bench_add_two ... bench: 0 ns/iter (+/- 0)
|
|
|
|
|
|
test result: ok. 0 passed; 0 failed; 1 ignored; 1 measured; 0 filtered out; finished in 0.29s
|
|
|
</code></pre>
|
|
|
<p>看到没，一个截然不同的结果，除此之外还能看出几点:</p>
|
|
|
<ul>
|
|
|
<li>单元测试 <code>it_works</code> 被忽略，并没有执行: <code>tests::it_works ... ignored</code></li>
|
|
|
<li>benchmark 的结果是 <code>0 ns/iter</code>，表示每次迭代( <code>b.iter</code> )耗时 <code>0 ns</code>，奇怪，怎么是 <code>0</code> 纳秒呢？别急，原因后面会讲</li>
|
|
|
</ul>
|
|
|
<h4 id="一些使用建议"><a class="header" href="#一些使用建议">一些使用建议</a></h4>
|
|
|
<p>关于 <code>benchmark</code>，这里有一些使用建议值得大家关注:</p>
|
|
|
<ul>
|
|
|
<li>将初始化代码移动到 <code>b.iter</code> 循环之外，否则每次循环迭代都会初始化一次，这里只应该存放需要精准测试的代码</li>
|
|
|
<li>让代码每次都做一样的事情，例如不要去做累加或状态更改的操作</li>
|
|
|
<li>最好让 <code>iter</code> 之外的代码也具有幂等性，因为它也可能被 <code>benchmark</code> 运行多次</li>
|
|
|
<li>循环内的代码应该尽量的短小快速，因为这样循环才能被尽可能多的执行，结果也会更加准确</li>
|
|
|
</ul>
|
|
|
<h4 id="谜一般的性能结果"><a class="header" href="#谜一般的性能结果">谜一般的性能结果</a></h4>
|
|
|
<p>在写 <code>benchmark</code> 时，你可能会遇到一些很纳闷的棘手问题，例如以下代码:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span>#![feature(test)]
|
|
|
|
|
|
<span class="boring">fn main() {
|
|
|
</span>extern crate test;
|
|
|
|
|
|
fn fibonacci_u64(number: u64) -> u64 {
|
|
|
let mut last: u64 = 1;
|
|
|
let mut current: u64 = 0;
|
|
|
let mut buffer: u64;
|
|
|
let mut position: u64 = 1;
|
|
|
|
|
|
return loop {
|
|
|
if position == number {
|
|
|
break current;
|
|
|
}
|
|
|
|
|
|
buffer = last;
|
|
|
last = current;
|
|
|
current = buffer + current;
|
|
|
position += 1;
|
|
|
};
|
|
|
}
|
|
|
#[cfg(test)]
|
|
|
mod tests {
|
|
|
use super::*;
|
|
|
use test::Bencher;
|
|
|
|
|
|
#[test]
|
|
|
fn it_works() {
|
|
|
assert_eq!(fibonacci_u64(1), 0);
|
|
|
assert_eq!(fibonacci_u64(2), 1);
|
|
|
assert_eq!(fibonacci_u64(12), 89);
|
|
|
assert_eq!(fibonacci_u64(30), 514229);
|
|
|
}
|
|
|
|
|
|
#[bench]
|
|
|
fn bench_u64(b: &mut Bencher) {
|
|
|
b.iter(|| {
|
|
|
for i in 100..200 {
|
|
|
fibonacci_u64(i);
|
|
|
}
|
|
|
});
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>通过<code>cargo bench</code>运行后，得到一个难以置信的结果：<code>test tests::bench_u64 ... bench: 0 ns/iter (+/- 0)</code>, 难道 Rust 已经到达量子计算机级别了？</p>
|
|
|
<p>其实，原因藏在<code>LLVM</code>中: <code>LLVM</code>认为<code>fibonacci_u64</code>函数调用的结果没有使用，同时也认为该函数没有任何副作用(造成其它的影响，例如修改外部变量、访问网络等), 因此它有理由把这个函数调用优化掉！</p>
|
|
|
<p>解决很简单，使用 Rust 标准库中的 <code>black_box</code> 函数:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span> for i in 100..200 {
|
|
|
test::black_box(fibonacci_u64(test::black_box(i)));
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>通过这个函数，我们告诉编译器，让它尽量少做优化，此时 LLVM 就不会再自作主张了:)</p>
|
|
|
<pre><code class="language-shell">$ cargo bench
|
|
|
running 2 tests
|
|
|
test tests::it_works ... ignored
|
|
|
test tests::bench_u64 ... bench: 5,626 ns/iter (+/- 267)
|
|
|
|
|
|
test result: ok. 0 passed; 0 failed; 1 ignored; 1 measured; 0 filtered out; finished in 0.67s
|
|
|
</code></pre>
|
|
|
<p>嗯，这次结果就明显正常了。</p>
|
|
|
<h2 id="criterionrs"><a class="header" href="#criterionrs">criterion.rs</a></h2>
|
|
|
<p>官方 <code>benchmark</code> 有两个问题，首先就是不支持 <code>stable</code> 版本的 Rust，其次是结果有些简单，缺少更详细的统计分布。</p>
|
|
|
<p>因此社区 <code>benchmark</code> 就应运而生，其中最有名的就是 <a href="https://github.com/bheisler/criterion.rs"><code>criterion.rs</code></a>，它有几个重要特性:</p>
|
|
|
<ul>
|
|
|
<li>统计分析，例如可以跟上一次运行的结果进行差异比对</li>
|
|
|
<li>图表，使用 <a href="http://www.gnuplot.info"><code>gnuplots</code></a> 展示详细的结果图表</li>
|
|
|
</ul>
|
|
|
<p>首先，如果你需要图表，需要先安装 <code>gnuplots</code>，其次，我们需要引入相关的包，在 <code>Cargo.toml</code> 文件中新增 :</p>
|
|
|
<pre><code class="language-toml">[dev-dependencies]
|
|
|
criterion = "0.3"
|
|
|
|
|
|
[[bench]]
|
|
|
name = "my_benchmark"
|
|
|
harness = false
|
|
|
</code></pre>
|
|
|
<p>接着，在项目中创建一个测试文件: <code>$PROJECT/benches/my_benchmark.rs</code>，然后加入以下内容：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use criterion::{black_box, criterion_group, criterion_main, Criterion};
|
|
|
|
|
|
fn fibonacci(n: u64) -> u64 {
|
|
|
match n {
|
|
|
0 => 1,
|
|
|
1 => 1,
|
|
|
n => fibonacci(n-1) + fibonacci(n-2),
|
|
|
}
|
|
|
}
|
|
|
|
|
|
fn criterion_benchmark(c: &mut Criterion) {
|
|
|
c.bench_function("fib 20", |b| b.iter(|| fibonacci(black_box(20))));
|
|
|
}
|
|
|
|
|
|
criterion_group!(benches, criterion_benchmark);
|
|
|
criterion_main!(benches);
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>最后，使用 <code>cargo bench</code> 运行并观察结果：</p>
|
|
|
<pre><code class="language-shell"> Running target/release/deps/example-423eedc43b2b3a93
|
|
|
Benchmarking fib 20
|
|
|
Benchmarking fib 20: Warming up for 3.0000 s
|
|
|
Benchmarking fib 20: Collecting 100 samples in estimated 5.0658 s (188100 iterations)
|
|
|
Benchmarking fib 20: Analyzing
|
|
|
fib 20 time: [26.029 us 26.251 us 26.505 us]
|
|
|
Found 11 outliers among 99 measurements (11.11%)
|
|
|
6 (6.06%) high mild
|
|
|
5 (5.05%) high severe
|
|
|
slope [26.029 us 26.505 us] R^2 [0.8745662 0.8728027]
|
|
|
mean [26.106 us 26.561 us] std. dev. [808.98 ns 1.4722 us]
|
|
|
median [25.733 us 25.988 us] med. abs. dev. [234.09 ns 544.07 ns]
|
|
|
</code></pre>
|
|
|
<p>可以看出，这个结果是明显比官方的更详尽的，如果大家希望更深入的学习它的使用，可以参见<a href="https://bheisler.github.io/criterion.rs/book/getting_started.html">官方文档</a>。</p>
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