905221 - nsEventQueue slop because NewPage() allocates non-power-of-two sized event queue objects

Status: RESOLVED FIXED

(Core :: XPCOM, defect)

Description

[Chris Peterson [:cpeterson]]

nsEventQueue uses a linked list of Page objects to pass pointers to nsThreads. A Page is a struct containing 251 pointers. Depending on sizeof(void*), a Page uses only 1004 or 2008 bytes of its heap block's actual 1024 or 2048 bytes before allocating the next Page.

Also, one might ask whether 250 is a reasonable value for EVENTS_PER_PAGE. In my testing, most nsEventQueues have a high-water mark less than ~50 events.

https://hg.mozilla.org/mozilla-central/file/4930fdea3efa/xpcom/threads/nsEventQueue.h#l63

Comment 1

[Nicholas Nethercote [inactive]]

If you change it to 255, a static assertion that sizeof(Page) is a power of two would be great!

Comment 2

[Chris Peterson [:cpeterson]]

Attachment: EVENTS_PER_PAGE.patch

Increase nsEventQueue::EVENTS_PER_PAGE from 250 to 255 so sizeof(nsEventQueue::Page) will be a power of two to avoid heap allocation slop.

Comment 3

[Chris Peterson [:cpeterson]]

bsmedberg: btw, here are your original comments on EVENTS_PER_PAGE from 2006:

https://bugzilla.mozilla.org/show_bug.cgi?id=326273#c54

Comment 4

[Chris Peterson [:cpeterson]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/f43fed6391f7

Comment 5

[Ed Morley [:emorley]]

https://hg.mozilla.org/mozilla-central/rev/f43fed6391f7