1384714 - Use subscript loader to load browser.xul scripts

Status: RESOLVED FIXED

(Firefox :: General, enhancement)

Description

[Kris Maglione [:kmag]]

There's a lot of overhead to loading scripts from <script> tags in browser.xul. They're loaded asynchronously, and can't begin loading until after we begin loading browser.xul. They're loaded using off-main-thread compilation, which means we need to create and merge a new JS zone for each script we load that way. They don't benefit from the startup cache at all.

If we load those scripts using the subscript loader, they're loaded from the preloader cache, and begin loading as early as possible. And they're all loaded into a single off-thread zone, which means there are less zones to create and merge. From a quick test, this seems to save about 20-40ms on Windows ts_paint talos runs.

The main downside is that subscript loader scripts don't benefit from lazy parsing, so we might wind up increasing memory usage if most of the functions in those scripts aren't used very often. But we should hopefully be able to fix that by migrating more browser JS to lazy loading.

Comment 1

[Kris Maglione [:kmag]]

Attachment: Bug 1384714: Use subscript loader to load browser.xul scripts.

There's a lot of overhead to loading scripts from <script> tags in
browser.xul. They're loaded asynchronously, and can't begin loading until
after we begin loading browser.xul. They're loaded using off-main-thread
compilation, which means we need to create and merge a new JS zone for each
script we load that way. They don't benefit from the startup cache at all.

If we load those scripts using the subscript loader, they're loaded from the
preloader cache, and begin loading as early as possible. And they're all
loaded into a single off-thread zone, which means there are less zones to
create and merge. From a quick test, this seems to save about 20-40ms on
Windows ts_paint talos runs.

The main downside is that subscript loader scripts don't benefit from lazy
parsing, so we might wind up increasing memory usage if most of the functions
in those scripts aren't used very often. But we should hopefully be able to
fix that by migrating more browser JS to lazy loading.

Review commit: https://reviewboard.mozilla.org/r/161676/diff/#index_header
See other reviews: https://reviewboard.mozilla.org/r/161676/

Comment 2

[Dave Townsend [:mossop]]

Comment on attachment 8890539 [details]
Bug 1384714: Use subscript loader to load browser.xul scripts.

https://reviewboard.mozilla.org/r/161676/#review167598

Comment 3

[Kan-Ru Chen [:kanru] (UTC+9)]

ping, should we land this?

Comment 4

[Kris Maglione [:kmag]]

I'll land. I just decided to wait until after the merge, and it's ended up in my backlog in the mean time.

Comment 5

[Kris Maglione [:kmag]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/b80466887686fbefe5a67aebc86f6f4aaf5feac8
Bug 1384714: Use subscript loader to load browser.xul scripts. r=Mossop

Comment 6

[Phil Ringnalda (:philor)]

https://hg.mozilla.org/mozilla-central/rev/b80466887686

Comment 7

[Joel Maher ( :jmaher ) (UTC -8)]

I see a perf improvement:
== Change summary for alert #9098 (as of August 26 2017 19:16 UTC) ==

Improvements:

  5%  tart summary windows10-64 opt e10s     6.17 -> 5.84

For up to date results, see: https://treeherder.mozilla.org/perf.html#/alerts?id=9098