1597601 - Change EventQueue implementation to reduce allocation churn of Queue buffers

Status: RESOLVED FIXED

(Core :: XPCOM, enhancement)

Description

[Randell Jesup [:jesup] (needinfo me)]

EventQueue's internal use of mozilla::Queue can waste memory for most threads, and can cause intermittent allocations (every 256 events).

Switching to an implementation were the first buffer is treated as a circular buffer until it's full, at which point instead of allocating a solitary larger buffer we allocate another buffer, and release the first buffer when it drains. At that point, the second buffer ('page') -- if it's the last buffer -- becomes a semi-circular buffer.

This allows for the common case of secondary threads that rarely if ever have more than a few events queued can simply reuse a single (smaller) buffer, and just keep circular buffering. This avoids allocator traffic.

Comment 1

[Gerald Squelart (he/him) (not at Mozilla since 2022-09-15)]

Alternative idea I need to get out of my head, just in case the first-buffer-is-circular is a pain to implement:
Keep one emptied buffer and re-use it next time a new page is needed.
In the ideal case, it means the queue will only ever allocate two buffers and will keep swapping between them.
This gut-feels simpler, at the cost of one extra allocation.

Comment 2

[Randell Jesup [:jesup] (needinfo me)]

Circular queue wasn't hard. I did think about what you mention, but it has it's own housekeeping work. The initial allocation wouldn't be an issue.

Comment 3

[Randell Jesup [:jesup] (needinfo me)]

Attachment: Bug 1597601: Make event Queues use the first buffer as a circular buffer r=froydnj

This should avoid freeing and reallocating the buffer every N events, and
make it simpler to use smaller buffers, especially for non-MainThread queues.

Depends on D53910

Comment 4

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/69ebf8af42a5
Make event Queues use the first buffer as a circular buffer r=froydnj

Comment 5

[Cristina Coroiu [:ccoroiu]]

https://hg.mozilla.org/mozilla-central/rev/69ebf8af42a5