1473414 - Use special-purpose allocator for CallbackNode objects

Status: RESOLVED WONTFIX

(Core :: Preferences: Backend, enhancement)

Description

[Kris Maglione [:kmag]]

We allocate thousands of CallbackNode objects in every process, and the number of live objects doesn't go down much.

Prior to bug 1472523, the general-purpose allocator worked pretty well, since the 32 byte objects fit neatly into a jemalloc bucket size. But bug 1472523 increased the size to 40 bytes, which bumps them up to the 64 byte allocation size, and wastes 24 bytes per object. It's still a net win, but we can do better.

If we use an arena allocator and a free list for unused entries, wasted memory goes down considerably, and we save 10-15K per base content process.

Comment 1

[Mike Hommey [:glandium]]

For 40 bytes, it should be 48, not 64.

Comment 2

[Kris Maglione [:kmag]]

(In reply to Mike Hommey [:glandium] from comment #1)
> For 40 bytes, it should be 48, not 64.

I'll assume you're right. I did check the value MallocSizeOf reported, but that was a week or so ago, and I probably misremembered.

Either way, I did test this with a special-purpose allocator, and it did save a considerable amount of memory.

Comment 3

[Kris Maglione [:kmag]]

If it's only an extra 8 bytes, I think we're actually better off using that space for a Variant so we can support matching multiple preferences in the same callback when callers use RegisterCallbacks.

Comment 4

[Nicholas Nethercote [inactive]]

My favourite comment in the entire codebase is this one:
https://searchfox.org/mozilla-central/rev/6ef785903fee6c0b16a1eab79d722373d940fd78/memory/build/mozjemalloc.cpp#53-88

:)

Comment 5

[Kris Maglione [:kmag]]

(In reply to Nicholas Nethercote [:njn] from comment #4)
> My favourite comment in the entire codebase is this one:
> https://searchfox.org/mozilla-central/rev/
> 6ef785903fee6c0b16a1eab79d722373d940fd78/memory/build/mozjemalloc.cpp#53-88
> 
> :)

Handy.