Date: Thu, 5 Nov 2015 13:45:19 -0800 From: Adrian Chadd <adrian.chadd@gmail.com> To: Mateusz Guzik <mjguzik@gmail.com>, John Baldwin <jhb@freebsd.org>, 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: <CAJ-VmonnH4JJg0XqX1SoBXBa%2B9Xfmk%2BHFv58ETaQ9v1-uAAhdQ@mail.gmail.com> In-Reply-To: <20151105192623.GB27709@dft-labs.eu> References: <20151104233218.GA27709@dft-labs.eu> <20151105142628.GJ2257@kib.kiev.ua> <13871467.CBcqGMncpJ@ralph.baldwin.cx> <20151105192623.GB27709@dft-labs.eu>
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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? -adrian
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