From owner-freebsd-current@freebsd.org Thu Nov 5 21:45:20 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 A3B54A27D3E for ; Thu, 5 Nov 2015 21:45:20 +0000 (UTC) (envelope-from adrian.chadd@gmail.com) Received: from mail-io0-x235.google.com (mail-io0-x235.google.com [IPv6:2607:f8b0:4001:c06::235]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (Client CN "smtp.gmail.com", Issuer "Google Internet Authority G2" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id 719C81CFB; Thu, 5 Nov 2015 21:45:20 +0000 (UTC) (envelope-from adrian.chadd@gmail.com) Received: by iodd200 with SMTP id d200so104919273iod.0; Thu, 05 Nov 2015 13:45:20 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :content-type; bh=0uMMrl84kY48vuSIhw4I/hc6HEO5fhTXcSll8dwm4OQ=; b=AFMeuicFQCbWUT5USSplkJ2za1FBWvGn6OKy7Jjh8Z1LFNJqHHP4e7Xp1GnTxqVl0h KwrglkOwVxkvkSHHuWqrqzjdc2emgJFaEmG/095BEpBRVXctLtY/Wk8DUYHJQiwFmztZ HtZTOljFNXZy/Mbv03AFhwA3rCahdlILqLm8VOxOsIaWb5aXYJjON0CS/W4Ql96pQVKJ Px7vqGq6vCB2ZqJclQXGLewXam1LKAmiiaTCF/kAYnlSI7z1FCms0yWnIn5v8yty/dJQ L2NVFNhaGmJTCZGTahOK6BQHGZAODTv17AS4aAxTwCBF/4UzH5xC2ecbbJyVmJhU9HFE cs1g== MIME-Version: 1.0 X-Received: by 10.107.3.72 with SMTP id 69mr11810210iod.75.1446759919983; Thu, 05 Nov 2015 13:45:19 -0800 (PST) Received: by 10.36.217.196 with HTTP; Thu, 5 Nov 2015 13:45:19 -0800 (PST) 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> Date: Thu, 5 Nov 2015 13:45:19 -0800 Message-ID: Subject: Re: [PATCH] microoptimize by trying to avoid locking a locked mutex From: Adrian Chadd To: Mateusz Guzik , John Baldwin , freebsd-current , Konstantin Belousov Content-Type: text/plain; charset=UTF-8 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 21:45:20 -0000 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? -adrian