[PATCH v2] Sleep 1 millisecond in poll() to avoid busy wait

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

SwitchToThread() only gives away the rest of the current time slice to another thread in the current process. So if the thread that feeds the file decscriptor we're polling is not in the current process, we get busy-waiting.

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. ---

On Mon, Apr 28, 2014 at 05:05:47PM +0200, Johannes Sixt wrote:

... but Paolo had a nice short description of the issue there; I inserted that to the top of the earlier commit message.

The latter diff (without the comment), gets us closer to gnulib's poll.c.

Have a nice day,
	Stepan

 compat/poll/poll.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)