1087834 - Reduce heap churn involving |input_overflow_buf_|

Status: RESOLVED FIXED

(Core :: IPC, defect)

Description

[Nicholas Nethercote [:njn]]

Prior to bug 811596, when a large IPC message was received the
input_overflow_buf_ buffer would grow to fit the message. And it would never
shrunk, so 99.9% of the time the large buffer would be mostly unused space. So
in bug 811596 I added code to shrink the buffer after a message was dealt with.
This reduced the size down to 8 KiB, often from 500+ KiB, which is a big deal
on B2G.

Alas, I just discovered that I did it in a sub-optimal way: because C++ strings
don't provide a guaranteed way to shrink a string's capacity, I instead
replaced the string's buffer with a new, small one. But my code does this *for
every message*. And since most messages are small, that means that most of the
time we replace the existing buffer with a new one that is exactly the same
size. This causes many unnecessary allocations and lots of heap churn. Whoops.

Comment 1

[Nicholas Nethercote [:njn]]

Attachment: Reduce heap churn involving |input_overflow_buf_|

This patch changes things so that the buffer only gets replaced if it is bigger
than the standard (small) size. In some simple e10s-enabled browsing I found
this avoids the reallocation in 99.7% of cases.

Comment 2

[Nicholas Nethercote [:njn]]

Thank you for the fast review.

Comment 3

[Nicholas Nethercote [:njn]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/87123c85507b

Comment 4

[Ryan VanderMeulen [:RyanVM]]

https://hg.mozilla.org/mozilla-central/rev/87123c85507b