1574986 - GC-ubench incorrectly displays minor GCs as major GC slices

Status: RESOLVED FIXED

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

Description

[Steve Fink [:sfink] [:s:]]

Once upon a time, we had a minor GC number, a major GC number, and a general GC number that was pretty much a major GC slice count. We use the latter GC number to detect when GC activity happened (so we might need to update pointers or invalidate some state or whatever.) But we quickly found that the general GC number needed to be bumped for minor GCs, since that is the most likely way that things would get invalidated anyway.

GC ubench still assumes that an incremented GC number means you did a GC slice, and draws a line represnting that. I've been staring at the graph recently, and I couldn't figure out what these weird standalone GCs were that never seemed to free any memory. It turns out they're just the minor GCs, but we have no way to distinguish those from GC slices that trigger a minor GC. (And I guess that means we also can't distinguish a GC slice without a minor GC from not doing anything at all!)

I will also note here that the memory display on GC ubench is lacking zone-specific triggers, and I suspect that's a large part of why the triggers aren't matching up all that well with observed behavior.

Comment 1

[Steve Fink [:sfink] [:s:]]

Attachment: Bug 1574986 - Comment on how GCs are determined in the GC-ubench display, and mention the missing piece of information r=jonco

Comment 2

[Steve Fink [:sfink] [:s:]]

Attachment: Bug 1574986 - Report count of GC slices r=jonco

Depends on D42534

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/6cabe3bdd6ee
Comment on how GCs are determined in the GC-ubench display, and mention the missing piece of information r=jonco

Comment 4

[Daniel Varga [:dvarga]]

https://hg.mozilla.org/mozilla-central/rev/6cabe3bdd6ee

Comment 5

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/883e6437a6ea
Report count of GC slices r=jonco

Comment 6

[Andrei Ciure[:aciure]]

https://hg.mozilla.org/mozilla-central/rev/883e6437a6ea