2009/1/14 Chris Mason <chris.mason@xxxxxxxxxx>: > On Tue, 2009-01-13 at 18:21 +0100, Peter Zijlstra wrote: >> On Tue, 2009-01-13 at 08:49 -0800, Linus Torvalds wrote: >> > >> > So do a v10, and ask people to test. >> >> --- >> Subject: mutex: implement adaptive spinning >> From: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> >> Date: Mon Jan 12 14:01:47 CET 2009 >> >> Change mutex contention behaviour such that it will sometimes busy wait on >> acquisition - moving its behaviour closer to that of spinlocks. >> > > I've spent a bunch of time on this one, and noticed earlier today that I > still had bits of CONFIG_FTRACE compiling. I wasn't actually tracing > anything, but it seems to have had a big performance hit. > > The bad news is the simple spin got much much faster, dbench 50 coming > in at 1282MB/s instead of 580MB/s. (other benchmarks give similar > results) > > v10 is better that not spinning, but its in the 5-10% range. So, I've > been trying to find ways to close the gap, just to understand exactly > where it is different. > > If I take out: > /* > * If there are pending waiters, join them. > */ > if (!list_empty(&lock->wait_list)) > break; > > > v10 pops dbench 50 up to 1800MB/s. The other tests soundly beat my > spinning and aren't less fair. But clearly this isn't a good solution. > > I tried a few variations, like only checking the wait list once before > looping, which helps some. Are there other suggestions on better tuning > options? (some thoughts/speculations) Perhaps for highly-contanded mutexes the spinning implementation may quickly degrade [*] to the non-spinning one (i.e. the current sleep-wait mutex) and then just stay in this state until a moment of time when there are no waiters [**] -- i.e. list_empty(&lock->wait_list) == 1 and waiters can start spinning again. what may trigger [*]: (1) obviously, an owner scheduling out. Even if it happens rarely (otherwise, it's not a target scenario for our optimization), due to the [**] it may take quite some time until waiters are able to spin again. let's say, waiters (almost) never block (and possibly, such cases would be better off just using a spinlock after some refactoring, if possible) (2) need_resched() is triggered for one of the waiters. (3) !owner && rt_task(p) quite unlikely, but possible (there are 2 race windows). Of course, the question is whether it really takes a noticeable amount of time to get out of the [**] state. I'd imagine it can be a case for highly-contended locks. If this is the case indeed, then which of 1,2,3 gets triggered the most. Have you tried removing need_resched() checks? So we kind of emulate real spinlocks here. > > -chris > -- Best regards, Dmitry Adamushko -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
