1449346 - Investigate why emulated css flexbox is so much slower than xul flexbox on TART

Status: NEW

(Core :: Layout, enhancement, P3)

Description

[Brian Grinstead [:bgrins]]

Emulation mode is almost identical for ts_paint but is around 10x slower on tart (~22 vs ~2).

Tracking dashboards:
  tart - https://mzl.la/2Clo7R6
  ts_paint - https://mzl.la/2o1NnaL

Picking the latest push we have numbers for:

Without emulation:
- https://treeherder.mozilla.org/#/jobs?repo=mozilla-central&revision=b906009d875d1f5d29b0d1252cdb43a9b1a5889c&selectedJob=170541764&group_state=expanded
- Subtests: https://treeherder.mozilla.org/perf.html#/comparesubtest?originalProject=mozilla-central&originalRevision=b906009d875d1f5d29b0d1252cdb43a9b1a5889c&newProject=mozilla-central&newRevision=b906009d875d1f5d29b0d1252cdb43a9b1a5889c&originalSignature=0beffdc8b6c1bc5bc0a0292a2947ad260d39e876&newSignature=0beffdc8b6c1bc5bc0a0292a2947ad260d39e876&framework=1
- Raw data: https://taskcluster-artifacts.net/Bx6kvv7TRYiMb9hNLvG__A/0/public/test_info/perfherder-data.json

With emulation: 
- https://treeherder.mozilla.org/#/jobs?repo=mozilla-central&revision=b906009d875d1f5d29b0d1252cdb43a9b1a5889c&selectedJob=170541839&group_state=expanded
- Subtests: https://treeherder.mozilla.org/perf.html#/comparesubtest?originalProject=mozilla-central&originalRevision=b906009d875d1f5d29b0d1252cdb43a9b1a5889c&newProject=mozilla-central&newRevision=b906009d875d1f5d29b0d1252cdb43a9b1a5889c&originalSignature=664c274bfcf996b14aa7771b6f2779531c309868&newSignature=664c274bfcf996b14aa7771b6f2779531c309868&framework=1
- Raw data: https://taskcluster-artifacts.net/cdJuR1vHQlSHzuODhdEreQ/0/public/test_info/perfherder-data.json

Comment 1

[Brian Grinstead [:bgrins]]

Attachment: tart-data.json

data from normal run

Comment 2

[Brian Grinstead [:bgrins]]

Attachment: tart-flex-data.json

Comment 3

[Brian Grinstead [:bgrins]]

Xidorn mentioned that using attribute mapping for the [flex=N] selectors that we use in xul.css in emulation mode could be faster (https://bugzilla.mozilla.org/show_bug.cgi?id=1448555#c17). There may be some issues with that solution as per https://bugzilla.mozilla.org/show_bug.cgi?id=1448555#c19 but I'd be curious to see if it's having an effect.

The weird thing is that ts_paint is basically already parity so whatever perf cliff we are falling off of could definitely be something specific with the tab markup / css and not to do with these selectors.

Comment 4

[Xidorn Quan [:xidorn] UTC+11]

It would be good if you have profile for the tests so that people can analyze what is causing the slowness. e.g. if selectors is a problem, we would expect seeing spending lots of time in selectors module for matching. But as far as there isn't tons of elements, that's generally shouldn't be a big problem.

Comment 5

[Brian Grinstead [:bgrins]]

Here are some profiles gathered locally via: `./mach talos-test --geckoProfile -a tart --setpref layout.css.emulate-moz-box-with-flex=true` and `./mach talos-test --geckoProfile -a tart`

TART with emulation:
https://perfht.ml/2J5ZxrE

TART normally:
https://perfht.ml/2J2rqku

Comment 6

[Brian Grinstead [:bgrins]]

I did a try push with the WIP patch for Bug 1377253 and am seeing a 7% improvement on tart_flex: https://treeherder.mozilla.org/perf.html#/compare?originalProject=try&originalRevision=b36996c457fd14fd3e975c76793e4fb5200be6b6&newProject=try&newRevision=4e4e41c53b33854d60bf14ea283bdbead2094c3d&framework=1&showOnlyImportant=1