Date: Thu, 05 Nov 2015 11:04:13 -0800 From: John Baldwin <jhb@freebsd.org> To: freebsd-current@freebsd.org Cc: Konstantin Belousov <kostikbel@gmail.com>, Mateusz Guzik <mjguzik@gmail.com> Subject: Re: [PATCH] microoptimize by trying to avoid locking a locked mutex Message-ID: <13871467.CBcqGMncpJ@ralph.baldwin.cx> In-Reply-To: <20151105142628.GJ2257@kib.kiev.ua> References: <20151104233218.GA27709@dft-labs.eu> <20151105142628.GJ2257@kib.kiev.ua>
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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. -- John Baldwin
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