From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754206AbbFJHfw (ORCPT ); Wed, 10 Jun 2015 03:35:52 -0400 Received: from mail-wi0-f182.google.com ([209.85.212.182]:36268 "EHLO mail-wi0-f182.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753445AbbFJHfR (ORCPT ); Wed, 10 Jun 2015 03:35:17 -0400 Date: Wed, 10 Jun 2015 09:35:12 +0200 From: Ingo Molnar To: Waiman Long Cc: Peter Zijlstra , Ingo Molnar , Arnd Bergmann , linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Scott J Norton , Douglas Hatch Subject: Re: [PATCH v2 2/2] locking/qrwlock: Don't contend with readers when setting _QW_WAITING Message-ID: <20150610073512.GA17226@gmail.com> References: <1433863153-30722-1-git-send-email-Waiman.Long@hp.com> <1433863153-30722-3-git-send-email-Waiman.Long@hp.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1433863153-30722-3-git-send-email-Waiman.Long@hp.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Waiman Long wrote: > The current cmpxchg() loop in setting the _QW_WAITING flag for writers > in queue_write_lock_slowpath() will contend with incoming readers > causing possibly extra cmpxchg() operations that are wasteful. This > patch changes the code to do a byte cmpxchg() to eliminate contention > with new readers. > > A multithreaded microbenchmark running 5M read_lock/write_lock loop > on a 8-socket 80-core Westmere-EX machine running 4.0 based kernel > with the qspinlock patch have the following execution times (in ms) > with and without the patch: > > With R:W ratio = 5:1 > > Threads w/o patch with patch % change > ------- --------- ---------- -------- > 2 990 895 -9.6% > 3 2136 1912 -10.5% > 4 3166 2830 -10.6% > 5 3953 3629 -8.2% > 6 4628 4405 -4.8% > 7 5344 5197 -2.8% > 8 6065 6004 -1.0% > 9 6826 6811 -0.2% > 10 7599 7599 0.0% > 15 9757 9766 +0.1% > 20 13767 13817 +0.4% > > With small number of contending threads, this patch can improve > locking performance by up to 10%. With more contending threads, > however, the gain diminishes. Mind posting the microbenchmark? Thanks, Ingo