Today we recognise Thiemo’s broad impact in improving performance of Wikimedia software. From optimizing code across the MediaWiki stack as felt on Wikipedia.org, to speeding up CI for faster developer feedback; this work benefits us every day!

Thiemo Kreuz works in the Technical Wishes team at Wikimedia Deutschland. He did most of this performance work as a paid software developer. “We are free to spend a portion of our time on side projects like these”, Thiemo wrote to us.

Performance as part of a routine

The tools on performance.wikimedia.org are part of building a culture of performance. These tools help you understand how code performs in production and on real devices. These tools empower developers to maintain performance through regular assessment and incremental improvement. Perf matters, because improving performance is an essential step toward equity of access!

We celebrate Thiemo’s tireless efforts with a story about performance as part of a routine, rather than one specific change. We’ll look at a few examples, but there are many other interesting Git commits if you’re curious for more.

Wikitext editor

The CodeMirror extension for MediaWiki provides syntax highlighting, for example, when editing template pages.

“I found a nasty performance issue in CodeMirror’s syntax highlighter for wikitext that was sitting there for a really, really long time”, Thiemo wrote about T270317 and T270237, which would cause your browser to freeze on long articles. “But nobody could figure out why. Answer: Bad regexes with missing boundary assertions.”

VisualEditor template editor

With the WMDE Technical Wishes team, Thiemo worked on VisualEditor’s template dialog and dramatically improved its performance. “This is mostly about lazy-loading parts of the UI”, Thiemo wrote. This matters because the community maintains templates that sometimes define several hundred parameters.

Faster stylesheet compilation

ResourceLoader is the MediaWiki delivery system for frontend styles, scripts, and localisation. It uses the Less.php library for stylesheet compilation. Thiemo heavily optimized the stylesheet parser through native function calls, inlining, and other techniques. This resulted in a 15% reduction in this change, 8% in this change, 5% in another change, and several more changes after that.

The motivation for this work was faster feedback from CI. While we compile only a handful of Less stylesheets during a page view, we have several hundred Less stylesheet files in our codebase. Our CI automatically checks all frontend assets for compilation errors, without needing dedicated unit tests. This speed-up brought us one step closer to realising the 5-minute pipeline.

Codesniffer rules

MediaWiki has extensive static analysis rules that automate and codify things we learned over two decades. Many such rules are implemented using PHP_CodeSniffer and run both locally and in CI via the composer test command. New rules are developed all the time and discussed in Phabricator. These new rules come at a cost.

“I keep coming back to our MediaWiki ruleset for PHPCS to check if it still runs as fast as it used to”, Thiemo wrote. “I find this particularly interesting because it requires a very specific ‘unfair’ type of optimization: We don’t care how slow the unhappy path is when it finds errors, because that’s the exceptional case that typically never happens. But we care a lot about the happy path, because that gets executed over and over again with every CI run.”

Example changes: 3X faster MultipleEmptyLines rule, 10X faster EmptyTag documentation rule.

Back to basics

Thiemo likes improving low-level libraries and frameworks, such as wikimedia/services and OOUI. “The idea is that even the tiniest of optimizations can make a notable difference, because a piece of library code is executed so often”, Thiemo wrote.

Web Perf Hero award

The Web Perf Hero award is given to individuals who have gone above and beyond to improve the web performance of Wikimedia projects. The initiative started in 2020 and takes the form of a Phabricator badge. You can find past recipients at the Web Perf Hero award page on Wikitech.