Date: Sat, 29 Aug 2015 21:38:30 +0200 From: Jens Gustedt <jens.gustedt@...ia.fr> To: musl@...ts.openwall.com Subject: Re: [PATCH 1/2] let them spin Rich, I hope with the graphics at hand, you'll understand a bit better what I was up to. Again my apologies. Still I try to answer to some of the arguments that you give below: Am Samstag, den 29.08.2015, 13:16 -0400 schrieb Rich Felker: > On Sat, Aug 29, 2015 at 10:50:44AM +0200, Jens Gustedt wrote: > > Remove a test in __wait that looked if other threads already attempted to > > go to sleep in futex_wait. > > > > This has no impact on the fast path. But other than one might think at a > > first glance, this slows down if there is congestion. > > > > Applying this patch shows no difference in behavior in a mono-core > > setting, so it seems that this shortcut is just superfluous. > > The purpose of this code is is twofold: improving fairness of the lock > and avoiding burning cpu time that's _known_ to be a waste. > > If you spin on a lock that already has waiters, the thread that spins > has a much better chance to get the lock than any of the existing > waiters which are in futex_wait. Assuming sufficiently many cores that > all threads that are not sleeping don't get preempted, the spinning > thread is basically guaranteed to get the lock unless it spins so long > to make it futex_wait. This is simply because returning from > futex_wake (which all the other waiters have to do) takes a lot more > time than one spin. I suspect there are common loads under which many > of the waiters will NEVER get the lock. Yes and no. I benched the things to know a bit more. On my machine one loop in a spin lock is just about 10 times faster than a failed call to futex_wait. Even for the current strategy, one of the futex_waiting threads gets woken up and gets his chance with a new spinning phase. So the difference isn't dramatic, just one order of magnitude and everybody gets his chance. These chances are not equal, sure, but NEVER in capitals is certainly a big word. On the other hands the difference in throughput on the multi-core setting between the different spin versions is dramatic for malloc, I find. > The other issue is simply wasted cpu time. Unlike in the no-waiters > case where spinning _can_ save a lot of cpu time (no futex_wait or > futex_wake syscall), spinning in the with-waiters case always wastes > cpu time. If the spinning thread gets the lock when the old owner > unlocks it, the old owner has already determined that it has to send a > futex_wake. So now you have (1) the wasted futex_wake syscall in the > old owner thread and (2) a "spurious" wake in one of the waiters, > which wakes up only to find that the lock was already stolen, and > therefore has to call futex_wait again and start the whole process > over. This wasted CPU time was one of my worries at first, this is why I made the mono core test. Basically on mono core spinning is *always* a waste, and going into futex_wait immediately should always be a win. (BTW, the current implementation should have a performance gap for exactly 2 threads that alternate on the lock.) But, there is almost no difference between the two strategies, the bias even here goes a tiny bit towards doing some spin, even under very high load. So, no, I don't think there is much wasted CPU time. At least performance seems to be largely dominated by other things, so "spinning" shouldn't be a worry for that in the current state of things. Jens -- :: INRIA Nancy Grand Est ::: Camus ::::::: ICube/ICPS ::: :: ::::::::::::::: office Strasbourg : +33 368854536 :: :: :::::::::::::::::::::: gsm France : +33 651400183 :: :: ::::::::::::::: gsm international : +49 15737185122 :: :: http://icube-icps.unistra.fr/index.php/Jens_Gustedt :: Download attachment "signature.asc" of type "application/pgp-signature" (182 bytes)
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