[PATCH] Sleep 1 millisecond in poll() to avoid busy wait

From: theoleblond <theodore.leblond@gmail.com>
Date: Wed, 16 May 2012 06:52:49 -0700

I played around with this quite a bit. After trying some more complex schemes, I found that what worked best is to just sleep 1 millisecond between iterations. Though it's a very short time, it still completely eliminates the busy wait condition, without hurting perf.

There code uses SleepEx(1, TRUE) to sleep. See this page for a good discussion of why that is better than calling SwitchToThread, which is what was used previously: http://stackoverflow.com/questions/1383943/switchtothread-vs-sleep1

Note that calling SleepEx(0, TRUE) does *not* solve the busy wait.

The most striking case was when testing on a UNC share with a large repo, on a single CPU machine. Without the fix, it took 4 minutes 15 seconds, and with the fix it took just 1:08! I think it's because git-upload-pack's busy wait was eating the CPU away from the git process that's doing the real work. With multi-proc, the timing is not much different, but tons of CPU time is still wasted, which can be a killer on a server that needs to do bunch of other things.

I also tested the very fast local case, and didn't see any measurable difference. On a big repo with 4500 files, the upload-pack took about 2 seconds with and without the fix. ---

This is one of the patches that lives in msysGit, could it be accepted upstream? It modifies the Windows compat function only.

Have a nice day,
	Stepan

 compat/poll/poll.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)