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

On Mon, Apr 28, 2014 at 10:39 AM, Stepan Kasal [off-list ref] wrote:

From: theoleblond <redacted>
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.

compat/poll/poll.c comes from Gnulib, so it would be better to submit
the patch there and update.