Facebook's Fred Emmott works on benchmarking HHVM's performance when running various open-source PHP frameworks. This puts him in contact with MediaWiki's codebase. He wrote in to suggest that we experiment with using plain PHP files instead of CDB for the l10n cache. We should try that and see whether it improves performance.
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Deployment plan (see also T99740#5165753 by @Krinkle):
# [x] Have Scap (also) generate l10n cache in the array format whenever it calls `rebuildLocalisationCache`.
** See also T105683.
** (From 2015: [https://gerrit.wikimedia.org/r/224520](https://gerrit.wikimedia.org/r/#/c/mediawiki/tools/scap/+/224520/5/scap/tasks.py).)
** 2020: https://gerrit.wikimedia.org/r/c/mediawiki/tools/scap/+/558243
# [x] Enable `array` format for Beta Cluster wikis.
** (From 2015: <https://gerrit.wikimedia.org/r/217702>.)
** 2020: <https://gerrit.wikimedia.org/r/#/c/operations/mediawiki-config/+/508724/>
# [x] Package new `scap` and have it deployed to production (T245530); then run a full scap.
# [ ] Enable `array` format for mwdebug1001/mwdebug2001 for performance testing (establish baseline on x002, detect difference if any, confirm difference in other DC). This is temporary. Undo afterward to reduce differences between debug and prod.
# [ ] Enable `array` format for group0 (i.e. testwikis, closed, mw.org, office).
# [ ] Enable `array` format for wikidata.org in production.
# [ ] Enable `array` format for commons.wikimedia.org in production.
# [ ] Enable `array` format for group1 in production.
# [ ] Enable `array` format for group2 in production. (all wikis)
# [ ] Update Scap config to no longer generate the old cdb format via `rebuildLocalisationCache`.
# [ ] Remove config switch.