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Benchmarking MediaWiki with PHPBench

This post gives a quick introduction to a benchmarking tool, phpbench, ready for you to experiment with in core and skins/extensions.[1]

What is phpbench?

From their documentation:

PHPBench is a benchmark runner for PHP analagous to PHPUnit but for performance rather than correctness.

In other words, while a PHPUnit test will tell you if your code behaves a certain way given a certain set of inputs, a PHPBench benchmark only cares how long that same piece of code takes to execute.

The tooling and boilerplate will be familiar to you if you've used PHPUnit. There's a command-line runner at vendor/bin/phpbench, benchmarks are discoverable by default in tests/Benchmark, a configuration file (benchmark.json) allows for setting defaults across all benchmarks, and the benchmark tests classes and tests look pretty similar to PHPUnit tests.

Here's an example test for the Html::openElement() function:

namespace MediaWiki\Tests\Benchmark;

class HtmlBench {

	/**
	 * @Assert("mode(variant.time.avg) < 85 microseconds +/- 10%")
	 */
	public function benchHtmlOpenElement() {
		\Html::openElement( 'a', [ 'class' => 'foo' ] );
	}
}

So, taking it line by line:

  • class HtmlBench (placed in tests/Benchmark/includes/HtmlBench.php) – the class where you can define the benchmarks for methods in a class. It would make sense to create a single benchmark class for a single class under test, just like with PHPUnit.
  • public function benchHtmlOpenElement() {} – method names that begin with bench will be executed by phpbench; other methods can be used for set-up / teardown work. The contents of the method are benchmarked, so any set-up / teardown work should be done elsewhere.
  • @Assert("mode(variant.time.avg) < 85 microseconds +/- 10%") – we define a phpbench assertion that the average execution time will be less than 85 microseconds, with a tolerance of +/- 10%.

If we run the test with composer phpbench, we will see that the test passes. One thing to be careful with, though, is adding assertions that are too strict – you would not want a patch to fail CI because the assertion for execution was not flexible enough (more on this later on).

Measuring performance while developing

One neat feature in PHPBench is the ability to tag current results and compare with another run. Looking at the HTMLBench benchmark test from above, for example, we can compare the work done in rMW5deb6a2a4546: Html::openElement() micro-optimisations to get before and after comparisons of the performance changes.

Here's a benchmark of e82c5e52d50a9afd67045f984dc3fb84e2daef44, the commit before the performance improvements added to Html::openElement() in rMW5deb6a2a4546: Html::openElement() micro-optimisations

❯ git checkout -b html-before-optimizations e82c5e52d50a9afd67045f984dc3fb84e2daef44 # get the old HTML::openElement code before optimizations
❯ git review -x 727429 # get the core patch which introduces phpbench support
❯ composer phpbench -- tests/Benchmark/includes/HtmlBench.php --tag=original

And the output [2]:

image.png (842×2 px, 176 KB)

Note that we've used --tag=original to store the results. Now we can check out the newer code, and use --ref=original to compare with the baseline:

❯ git checkout -b html-after-optimizations 5deb6a2a4546318d1fa94ad8c3fa54e9eb8fc67c # get the new HTML::openElement code with optimizations
❯ git review -x 727429 # get the core patch which introduces phpbench support
❯ composer phpbench -- tests/Benchmark/includes/HtmlBench.php --ref=original --report=aggregate

And the output [3]:

image.png (798×2 px, 177 KB)

We can see that the execution time roughly halved, from 18 microseconds to 8 microseconds. (For understanding the other columns in the report, it's best to read through the Quick Start guide for phpbench.) PHPBench can also provide an error exit code if the performance decreased. One way that PHPBench might fit into our testing stack would be to have a job similar to Fresnel, where a non-voting comment on a patch alerts developers whether the PHPBench performance decreased in the patch.

Testing with extensions

A slightly more complex example is available in GrowthExperiments (patch). That patch makes use of setUp/tearDown methods to prepopulate the database entries needed for the code being benchmarked:

/**
 * @BeforeMethods ("setUpLinkRecommendation")
 * @AfterMethods ("tearDownLinkRecommendation")
 * @Assert("mode(variant.time.avg) < 20000 microseconds +/- 10%")
 */
public function benchFilter() {
	$this->linkRecommendationFilter->filter( $this->tasks );
}

The setUpLinkRecommendation and tearDownLinkRecommendation methods have access to MediaWikiServices, and generally you can do similar things you'd do in an integration test to setup and teardown the environment. This test is towards the opposite end of the spectrum from the core test discussed above which looks at Html::openElement(); here, the goal is to look at a higher level function that involves database queries and interacting with MediaWiki services.

What's next

You can experiment with the tooling and see if it is useful to you. Some open questions:

  • do we want to use phpbench? or are the scripts in maintenance/benchmarks already sufficient for our benchmarking needs?
  • we already have a benchmarking tools in maintenance/benchmarks that extend a Benchmarker class; would it make sense to convert these to use phpbench?
  • what are sensible defaults for "revs" and "iterations" as well as retry thresholds?
  • do we want to run phpbench assertions in CI?
    • if yes, do we want assertions about using absolute times (e.g. "this function should take less than 20 ms") or relative assertions ("patch code is within 10% +/- of old code)
    • if yes, do we want to aggregate reports over time, so we can see trends for the code we benchmark?
    • should we disable phpbench as part of the standard set of tests run by Quibble, and only have it run as a non-voting job like Fresnel?

Looking forward to your feedback! [4]


[1] thank you, @hashar, for working with me to include this in Quibble and roll out to CI to help with evaluation!

[2]

> phpbench run --config=tests/Benchmark/phpbench.json --report=aggregate 'tests/Benchmark/includes/HtmlBench.php' '--tag=original'
PHPBench (1.1.2) running benchmarks...
with configuration file: /Users/kostajh/src/mediawiki/w/tests/Benchmark/phpbench.json
with PHP version 7.4.24, xdebug ✔, opcache ❌

\MediaWiki\Tests\Benchmark\HtmlBench

    benchHtmlOpenElement....................R1 I1 ✔ Mo18.514μs (±1.94%)

Subjects: 1, Assertions: 1, Failures: 0, Errors: 0
Storing results ... OK
Run: 1346543289c75373e513cc3b11fbf5215d8fb6d0
+-----------+----------------------+-----+------+-----+----------+----------+--------+
| benchmark | subject              | set | revs | its | mem_peak | mode     | rstdev |
+-----------+----------------------+-----+------+-----+----------+----------+--------+
| HtmlBench | benchHtmlOpenElement |     | 50   | 5   | 2.782mb  | 18.514μs | ±1.94% |
+-----------+----------------------+-----+------+-----+----------+----------+--------+

[3]

> phpbench run --config=tests/Benchmark/phpbench.json --report=aggregate 'tests/Benchmark/includes/HtmlBench.php' '--ref=original' '--report=aggregate'
PHPBench (1.1.2) running benchmarks...
with configuration file: /Users/kostajh/src/mediawiki/w/tests/Benchmark/phpbench.json
with PHP version 7.4.24, xdebug ✔, opcache ❌
comparing [actual vs. original]

\MediaWiki\Tests\Benchmark\HtmlBench

    benchHtmlOpenElement....................R5 I4 ✔ [Mo8.194μs vs. Mo18.514μs] -55.74% (±0.50%)

Subjects: 1, Assertions: 1, Failures: 0, Errors: 0
+-----------+----------------------+-----+------+-----+---------------+-----------------+----------------+
| benchmark | subject              | set | revs | its | mem_peak      | mode            | rstdev         |
+-----------+----------------------+-----+------+-----+---------------+-----------------+----------------+
| HtmlBench | benchHtmlOpenElement |     | 50   | 5   | 2.782mb 0.00% | 8.194μs -55.74% | ±0.50% -74.03% |
+-----------+----------------------+-----+------+-----+---------------+-----------------+----------------+

[4] Thanks to @zeljkofilipin for reviewing a draft of this post.

Written by kostajh on Oct 28 2021, 1:54 PM.
Staff Software Engineer, Growth Team
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My personal opinion is that a tool like this would be great. Replacing Benchmarker with it also sounds good, and having it in CI would also be nice. It should use relative assertions though, since the absolute times are affected by the environment where phpbench is executed.

The only concern I have is that phpbench uses microtime for time measurements instead of hrtime, which would be more accurate and have higher resolution. There's an issue for that but it was closed as wontfix :-/