1744208 - Change the nursery's object/string fix-up lists to add to the front

Status: RESOLVED FIXED

(Core :: JavaScript: GC, task, P3)

Description

[Jan de Mooij [:jandem]]

We currently add objects/strings we tenure to the end of a list, but we pop objects/strings to fix up from the front of that list.

It seems better to add to the front instead, because:

• We can then usually fix up objects/strings shortly after moving them, so they're more likely to still be in the cache.
• Nursery objects are likely to refer to objects that are close to it in memory. By adding and removing at the front, we take more advantage of this which also improves cache locality.
• It's easier to implement.

Comment 1

[Jon Coppeard (:jonco)]

All of these are good reasons. I'm interested to see what difference this makes.

Comment 2

[Jan de Mooij [:jandem]]

Attachment: Bug 1744208 - Change the nursery's fix-up lists to add items to the front instead of back. r?jonco!

This seems to have better cache locality on typical workloads, because objects
that are tenured are now more likely to be fixed up shortly after. It's also easier
to implement.

Some benchmarks (Speedometer, ARES6, various Octane subtests like TypeScript) improve
a bit so this seems worth trying to see how it affects telemetry data in the browser.

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/a3c49e8fb9b8
Change the nursery's fix-up lists to add items to the front instead of back. r=jonco

Comment 4

[Iulian Moraru]

https://hg.mozilla.org/mozilla-central/rev/a3c49e8fb9b8

Comment 5

[Jan de Mooij [:jandem]]

Looking at the GLAM telemetry dashboard, this improved the gc_minor_us 99% percentile by 13.87%. The other percentiles didn't change.