From owner-freebsd-current@freebsd.org Thu Nov 5 23:26:44 2015 Return-Path: Delivered-To: freebsd-current@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id 324FCA276E9 for ; Thu, 5 Nov 2015 23:26:44 +0000 (UTC) (envelope-from jhb@freebsd.org) Received: from bigwig.baldwin.cx (bigwig.baldwin.cx [IPv6:2001:470:1f11:75::1]) (using TLSv1 with cipher DHE-RSA-CAMELLIA256-SHA (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 0E3FC1A23 for ; Thu, 5 Nov 2015 23:26:44 +0000 (UTC) (envelope-from jhb@freebsd.org) Received: from ralph.baldwin.cx (c-73-231-226-104.hsd1.ca.comcast.net [73.231.226.104]) by bigwig.baldwin.cx (Postfix) with ESMTPSA id 09A79B97D; Thu, 5 Nov 2015 18:26:42 -0500 (EST) From: John Baldwin To: Adrian Chadd Cc: Mateusz Guzik , freebsd-current , Konstantin Belousov Subject: Re: [PATCH] microoptimize by trying to avoid locking a locked mutex Date: Thu, 05 Nov 2015 14:19:11 -0800 Message-ID: <1563180.x0Z3Ou4xid@ralph.baldwin.cx> User-Agent: KMail/4.14.3 (FreeBSD/10.2-STABLE; KDE/4.14.3; amd64; ; ) In-Reply-To: References: <20151104233218.GA27709@dft-labs.eu> <20151105192623.GB27709@dft-labs.eu> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.2.7 (bigwig.baldwin.cx); Thu, 05 Nov 2015 18:26:42 -0500 (EST) X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.20 Precedence: list List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 05 Nov 2015 23:26:44 -0000 On Thursday, November 05, 2015 01:45:19 PM Adrian Chadd wrote: > On 5 November 2015 at 11:26, Mateusz Guzik 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. -- John Baldwin