On Mon, Apr 28, 2014 at 5:35 PM, Stepan Kasal <ka...@ucw.cz> wrote: > From: theoleblond <theodore.lebl...@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 I very much prefer the commit message of the earlier post. > > ... 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. >
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