[webkit-reviews] review requested: [Bug 27236] Memory is not released back to the system from TCMalloc on Windows : [Attachment 32677] This patch implements TCMalloc_SystemRelease() on windows and adds a new mechanism to release memory back to the system via a background thread.

[bugzilla-daemon at webkit.org]

Ada Chan <adachan at apple.com> has asked	for review:
Bug 27236: Memory is not released back to the system from TCMalloc on Windows
https://bugs.webkit.org/show_bug.cgi?id=27236

Attachment 32677: This patch implements TCMalloc_SystemRelease() on windows and
adds a new mechanism to release memory back to the system via a background
thread.
https://bugs.webkit.org/attachment.cgi?id=32677&action=review

------- Additional Comments from Ada Chan <adachan at apple.com>
This patch implements TCMalloc_SystemRelease() on windows and adds a new
mechanism to release memory back to the system via a background thread.  That
new mechanism is turned on by default.	

Every 5 seconds, the background thread wakes up and does the following:
- Check if the number of pages allocated in the last 5 seconds is greater than
1/3 of the free committed pages.  If so, skip this scavenge because it's a sign
that there is a lot of activity going on right now and thrashing may happen if
we release memory back to the system now.
- Otherwise, go through the list of free spans (from largest to smallest) and
release up to 1/3 of the free committed pages back to the system.  We can
release more if there's absolutely no activity going on in the last 5 seconds.
- If the number of free committed pages reaches kMinimumFreeCommittedPageCount,
we can stop the scavenging and block the scavenging thread until the number of
free committed pages goes above kMinimumFreeCommittedPageCount.