compat/poll: sleep 1 millisecond to avoid busy wait

compat/poll: sleep 1 millisecond to avoid busy wait

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.

[jc: this was first accepted in msysgit tree in May 2012 via a pull
request and Paolo Bonzini has also accepted the same fix to Gnulib
around the same time; see $gmane/247518 for a bit more detail]

Signed-off-by: Stepan Kasal <kasal@ucw.cz>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Acked-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>