1672595 - Why are there so many <8-byte allocations?

Status: RESOLVED FIXED

(Core :: Memory Allocator, task)

Description

[Paul Bone [:pbone]]

In jemalloc-replay stats I see that there is 19KiB used by objects 8-bytes or smaller.

8-bin Distribution:
request   :  count percent
1 -     1:     58      3%
2 -     2:     29      1%
3 -     3:    177      8%
4 -     4:    134      6%
5 -     5:    182      8%
6 -     6:    281     13%
7 -     7:    156      7%
8 -     8:   1127     52%

Ideally we should not be allocating such small objects at all, since they fit within a pointer anyway. I expect some of these are things like strings that are sometimes long and sometimes short, but I'd like to know for sure.

Comment 1

[Mike Hommey [:glandium]]

You should probably look at = 8 bytes allocations too, we might have the surprise that it's for small structs. It might be useful to instrument operator new.

Comment 2

[Paul Bone [:pbone]]

I've checked out all the cases of 8 byte allocations.

• The majority are short strings allocated from third-party code like libfontconfig.
• Bug 1736764 shows a case where a small object can be inlined.
• Bug 1736809 is a case of short strings that we could inline, but is hardly worth it for only 128 bytes.
• The rest are two small to even look at.

Marking as fixed since this concludes the investigation.