1477997 - Add a button to trigger a full GC/CC

Status: NEW

(DevTools :: Memory, enhancement, P2)

Description

[Julien Wajsberg [:julienw]]

When debugging short term memory leaks, it's very common that we need to do a full GC before capturing a snapshot. Currently this is very cumbersome as we need to go to about:memory to do it. This would be very useful to have such a button directly in the memory tool UI.

Comment 1

[Julien Wajsberg [:julienw]]

I'm not very used to the code of this tool yet, but here is where I believe the change will be:

* https://searchfox.org/mozilla-central/source/devtools/client/memory/components/Toolbar.js => this is where the new button will be
* https://searchfox.org/mozilla-central/source/devtools/client/memory/app.js => this is where the code for the GC/CC will be
* https://searchfox.org/mozilla-central/source/toolkit/components/aboutmemory/content/aboutMemory.js#437 => This is how "Minimize memory usage" is implemented. I don't know if it's easy/possible to dispatch this message from the devtools panel though. Also I don't know if that's what we want, or if we'd rather want a GC+CC. I think "Minimize memory usage" additionally clears some cache (eg: decoded images).

Comment 2

[Alexandre Poirot [:ochameau]]

The GC has to be done in the actor, not in the frontend, otherwise it will be buggy when debugging remotely.
It is interesting to see that memory actor already contains GC and GC methods:
  https://searchfox.org/mozilla-central/source/devtools/server/actors/memory.js#63-65,69
But they are used only in tests.

The actual implementation is here:
  https://searchfox.org/mozilla-central/source/devtools/server/performance/memory.js#331-347
You may want to tweak it to do what about:memory is doing.

Comment 3

[Greg Tatum [:gregtatum]]

I recall a conversation back a few years ago that the memory tool actually does a full GC before taking the snapshot. You could ask fitzgen or jimb and they would know for sure.

Comment 4

[Julien Wajsberg [:julienw]]

Hey Jim, does that remind you something ?

Comment 5

[Jim Blandy :jimb]

I think what you want are these:

https://searchfox.org/mozilla-central/rev/3fa761ade83ed0b8ab463acb057c2cf0b104689e/js/xpconnect/idl/xpccomponents.idl#285-297

I don't see anything in the heap snapshot code that would request a GC. Here's the call that takes a heap snapshot:
https://searchfox.org/mozilla-central/rev/3fa761ade83ed0b8ab463acb057c2cf0b104689e/devtools/server/performance/memory.js#152

It calls this C++ function:
https://searchfox.org/mozilla-central/rev/3fa761ade83ed0b8ab463acb057c2cf0b104689e/devtools/shared/heapsnapshot/HeapSnapshot.cpp#1638

Nothing there jumps out at me.

Comment 6

[Zac Spitzer]

The memory tool doesn't do a GC before a snapshot https://bugzilla.mozilla.org/show_bug.cgi?id=1520597

Comment 7

[Julien Wajsberg [:julienw]]

Zac, I think you're mentioning about:memory in that bug, while this bug is about the memory panel in devtools.

That said I'm pretty sure none of them is doing a GC :)