Hi all,
TL;DR: When using X-Wikimedia-Debug to profile web requests on Wikimedia
wikis, the generated profile information will now include details from
"w/index.php", and MWMultiVersion, and things like
wmf-config/CommonSettings.php. Details at
https://phabricator.wikimedia.org/T180183.
-
The debug profiler provided on Wikimedia production wikis[1] previously
could not cover the code that executes before MediaWiki core instantiates
ProfilerXhprof, which was in charge of calling `xhprof_enable`. This
normally happens within core's Setup.php.
While that point in Setup.php is before any important MediaWiki core logic,
it misses out on two other chunks of code:
1. Initialisation of MediaWiki core – This includes entry point code (eg.
index.php, PHPVersionCheck), but also the first steps of Setup before
Profiler. Such as AutoLoader, vendor, and LocalSettings.php. At WMF,
LocalSettings.php loads wmf-config/InitialiseSettings.php and
wmf-config/CommonSettings.php.
2. Wrapping of MediaWiki entrypoint – At Wikimedia, the index.php
entrypoint is itself further wrapped in something called "multiversion".
Multiversion is what determines the wiki ID (eg. "enwiki") and MediaWiki
branch (eg. "1.31.0-wmf.25") associated with the current domain (eg. "
en.wikipedia.org").
Over the past weeks, I've been refactoring MediaWiki core, wmf-config and
Wikimedia's HHVM settings to make we can instrument the above code as part
of our performance profiles.
This change happened in three phases:
## 1. Update wmf-config/StartProfiler to.. actually start the profiler!
The file name is somewhat deceptive because traditionally this is (and can)
only be used to *configure* the profiler, by assigning $wgProfiler. It
makes sense that we cannot instantiate the Profiler subclass from this
file, because the classes and run-time configuration are not and cannot be
available this early.
However, we don't the Profiler class to record data. The Profiler classes
typically obtain their data from native PHP. The one used at WMF is XHProf.
Previously, we would assign $wgProfiler['class'] = 'ProfilerXhprof', and
then later MediaWiki core instantiates ProfilerXhprof, which then calls
xhprof_enable. We now xhprof_enable directly from StartProfiler.php.
This change enabled coverage of code in Setup.php between 'include
StartProfiler' and 'Profiler::instance()'. – Mainly: vendor, LocalSettings,
wmf-config.
## 2. Update MediaWiki core to include StartProfiler earlier.
It is now the first thing included by Setup.php.
This change enabled coverage of code in Setup.php that previously was
before 'include StartProfiler'. – Namely: AutoLoader.php, Defines.php.
## 3. Configure WMF's PHP engine to use auto_prepend_file
This is the big one, and requires a PHP ini setting change. Third parties
can follow the same pattern
in order to get the same benefits:
* Put `xhprof_enable( $flags )`, along with any sampling/conditional
logic, in a separate file.
* Use it from two places:
** In StartProfiler.php, include using require_once.
** In php.ini, set auto_prepend_file=path/to/profiler.php.
This change enabled coverage of all remaining code. – Namely: multiversion,
w/index.php and things like PHPVersionCheck.
## Example
Using cURL:
$ curl -H 'X-Wikimedia-Debug: 1' 'https://en.wikipedia.beta.
wmflabs.org/w/load.php?debug=false&modules=startup&only=
scripts&forceprofile=1'
Output now includes:
- main() # resembles the wrapper at
operations/mediawiki-config.git:/w/load.php
-- run_init::/srv/mediawiki/multiversion/MWMultiVersion.php
-- MWMultiVersion::getMediaWiki
-- run_init:/srv/mediawiki/php-../load.php
-- run_init::/srv/mediawiki/php-../includes/Setup.php
-- run_init::/srv/mediawiki/php-../LocalSettings.php
-- run_init::/srv/mediawiki/wmf-config/CommonSettings.php
--
run_init::/srv/mediawiki/php-../extensions/Wikibase/client/WikibaseClient.php
Greetings,
-- Timo Tijhof
[0]
https://phabricator.wikimedia.org/T180183
[1]
https://wikitech.wikimedia.org/wiki/X-Wikimedia-Debug