New subject: Startup module size and ResourceLoader registry overhead

16 Sep 2019

Hello,
Startup module, is the Javascript code [1] that is being served at almost
every request to Wikimedia as part of <head> (non-blocking though) to load
other RL modules. It does important things for example it checks if the
requested modules already exist with the given hash in the local storage
cache.

Because of the given reasons, the startup module needs and includes list of
all registered modules with their version hash in the code. As the result
if you register a module, even if you don't load it, it adds a rather small
overhead (around 19 bytes after minification and compression) to every
request to any wiki the code is enabled. So if you register a new module in
core or one of the extensions that is deployed in all wikis (CX,
WikibaseClient, etc.), it's going to be added to every page request,
meaning 30 GBs more traffic every day to our users. Even if you don't use
it anywhere.

If you're adding a feature, 30GB/day is acceptable but sometimes developers
use Resource loader for dependency management or class loading (yours truly
used to do that) and introduce 20 modules instead of one and each one
causing an extra 600 GB/day. The big overhead for our users is bad for
three reasons: 1- RL has to handle the dependency management, making every
page view slightly slower and worse UX for our users 2- The extra network
is bad with places without access to broadband connection, where Wikipedia
is the most likely place that people learn and grow [2] 3- The scale we are
talking is so massive (petabytes a year) that It has environmental impact.

Let me give you an example, a couple of weeks ago we dropped 137 modules
from WikibaseClient. After deployment, it dropped 4TB/day from our network
(= 1.5 PB/year) and synthetic data shows, in an average case we drop 40 ms
from every response time [3].

We now have a dashboard to track size of size of RL registry [4] plus a
weekly metrics for changes [5][6]

If you're looking for ways to help. I wrote a tool [7] to do some graph
analysis and it gives list of extensions that has modules that can be
merged. The extensions that according to the analysis (that can have false
positives) can get better are TimedMediaHandler, PageTriage, Graph,
RevisionSlider, CodeMirror, Citoid, TemplateData, TwoColConflict,
Collection, CentralNotice, AdvancedSearch, 3D, MobileFrontend and many more
including some bits and pieces in core. I put the graphs of modules that
can be merged at [8] and I invite you to have fun with those modules.
Modules can be merged using package modules [9]

Most of the is work done by the performance team [10] and volunteers and
developers in lots of teams. I joined the party later as volunteer/WMDE
staff and I'm sharing mostly the results and writing long emails. Big kudos
to Krinkle, James Forrester, Santhosh Thottingal, Jon Robson, Alaa Sarhaan,
Alexandros Kosiaris, and so many others that helped and I forgot.

[1] An example from English Wikipedia:
https://en.wikipedia.org/w/load.php?lang=en&modules=startup&only=sc…
[2] https://arxiv.org/abs/1812.00474
[3] https://phabricator.wikimedia.org/T203696#5387672
[4] https://grafana.wikimedia.org/d/BvWJlaDWk/startup-module-size?orgId=1
[5] https://gist.github.com/Krinkle/f76229f512fead79fb4868824b5bee07
[6]
https://docs.google.com/document/d/1SESOADAH9phJTeLo4lqipAjYUMaLpGsQTAUqdgy…
[7] https://phabricator.wikimedia.org/T232728
[8] https://phabricator.wikimedia.org/T233048
[9] https://www.mediawiki.org/wiki/ResourceLoader/Package_modules
[10] https://phabricator.wikimedia.org/T202154

Best
-- 
Amir (he/him)