Date: Thu, 05 Nov 2015 16:41:16 -0800 From: John Baldwin <jhb@freebsd.org> To: Ian Lepore <ian@freebsd.org> Cc: Adrian Chadd <adrian.chadd@gmail.com>, Mateusz Guzik <mjguzik@gmail.com>, freebsd-current <freebsd-current@freebsd.org>, Konstantin Belousov <kostikbel@gmail.com> Subject: Re: [PATCH] microoptimize by trying to avoid locking a locked mutex Message-ID: <3595970.jWQn08DLd8@ralph.baldwin.cx> In-Reply-To: <1446766522.91534.412.camel@freebsd.org> References: <20151104233218.GA27709@dft-labs.eu> <1563180.x0Z3Ou4xid@ralph.baldwin.cx> <1446766522.91534.412.camel@freebsd.org>
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On Thursday, November 05, 2015 04:35:22 PM Ian Lepore wrote: > On Thu, 2015-11-05 at 14:19 -0800, John Baldwin wrote: > > On Thursday, November 05, 2015 01:45:19 PM Adrian Chadd wrote: > > > On 5 November 2015 at 11:26, Mateusz Guzik <mjguzik@gmail.com> > > > wrote: > > > > On Thu, Nov 05, 2015 at 11:04:13AM -0800, John Baldwin wrote: > > > > > On Thursday, November 05, 2015 04:26:28 PM Konstantin Belousov > > > > > wrote: > > > > > > On Thu, Nov 05, 2015 at 12:32:18AM +0100, Mateusz Guzik > > > > > > wrote: > > > > > > > mtx_lock will unconditionally try to grab the lock and if > > > > > > > that fails, > > > > > > > will call __mtx_lock_sleep which will immediately try to do > > > > > > > the same > > > > > > > atomic op again. > > > > > > > > > > > > > > So, the obvious microoptimization is to check the state in > > > > > > > __mtx_lock_sleep and avoid the operation if the lock is not > > > > > > > free. > > > > > > > > > > > > > > This gives me ~40% speedup in a microbenchmark of 40 find > > > > > > > processes > > > > > > > traversing tmpfs and contending on mount mtx (only used as > > > > > > > an easy > > > > > > > benchmark, I have WIP patches to get rid of it). > > > > > > > > > > > > > > Second part of the patch is optional and just checks the > > > > > > > state of the > > > > > > > lock prior to doing any atomic operations, but it gives a > > > > > > > very modest > > > > > > > speed up when applied on top of the __mtx_lock_sleep > > > > > > > change. As such, > > > > > > > I'm not going to defend this part. > > > > > > Shouldn't the same consideration applied to all spinning > > > > > > loops, i.e. > > > > > > also to the spin/thread mutexes, and to the spinning parts of > > > > > > sx and > > > > > > lockmgr ? > > > > > > > > > > I agree. I think both changes are good and worth doing in our > > > > > other > > > > > primitives. > > > > > > > > > > > > > I glanced over e.g. rw_rlock and it did not have the issue, now > > > > that I > > > > see _sx_xlock_hard it wuld indeed use fixing. > > > > > > > > Expect a patch in few h for all primitives I'll find. I'll stress > > > > test > > > > the kernel, but it is unlikely I'll do microbenchmarks for > > > > remaining > > > > primitives. > > > > > > Is this stuff you're proposing still valid for non-x86 platforms? > > > > Yes. It just does a read before trying the atomic compare and swap > > and > > falls through to the hard case as if the atomic op failed if the > > result > > of the read would result in a compare failure. > > > > The atomic ops include barriers, the new pre-read of the variable > doesn't. Will that cause problems, especially for code inside a loop > where the compiler may decide to shuffle things around? I do not believe so. Eventually you have to go through a barrier to break out of the loop. > I suspect the performance gain will be biggest on the platforms where > atomic ops are expensive (I gather from various code comments that's > the case on x86). Yes, and where you have contention. :-/ -- John Baldwin
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