Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752707Ab3F0AY7 (ORCPT ); Wed, 26 Jun 2013 20:24:59 -0400 Received: from mga09.intel.com ([134.134.136.24]:29243 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751581Ab3F0AY6 (ORCPT ); Wed, 26 Jun 2013 20:24:58 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.87,947,1363158000"; d="scan'208";a="360196013" Subject: Re: Performance regression from switching lock to rw-sem for anon-vma tree From: Tim Chen To: Ingo Molnar Cc: Ingo Molnar , Andrea Arcangeli , Mel Gorman , "Shi, Alex" , Andi Kleen , Andrew Morton , Michel Lespinasse , Davidlohr Bueso , "Wilcox, Matthew R" , Dave Hansen , Peter Zijlstra , Rik van Riel , linux-kernel@vger.kernel.org, linux-mm In-Reply-To: <1372282560.22432.139.camel@schen9-DESK> References: <1371165992.27102.573.camel@schen9-DESK> <20130619131611.GC24957@gmail.com> <1371660831.27102.663.camel@schen9-DESK> <1372205996.22432.119.camel@schen9-DESK> <20130626095108.GB29181@gmail.com> <1372282560.22432.139.camel@schen9-DESK> Content-Type: text/plain; charset="UTF-8" Date: Wed, 26 Jun 2013 17:25:01 -0700 Message-ID: <1372292701.22432.152.camel@schen9-DESK> Mime-Version: 1.0 X-Mailer: Evolution 2.32.3 (2.32.3-1.fc14) Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4222 Lines: 89 On Wed, 2013-06-26 at 14:36 -0700, Tim Chen wrote: > On Wed, 2013-06-26 at 11:51 +0200, Ingo Molnar wrote: > > * Tim Chen wrote: > > > > > On Wed, 2013-06-19 at 09:53 -0700, Tim Chen wrote: > > > > On Wed, 2013-06-19 at 15:16 +0200, Ingo Molnar wrote: > > > > > > > > > > vmstat for mutex implementation: > > > > > > procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu----- > > > > > > r b swpd free buff cache si so bi bo in cs us sy id wa st > > > > > > 38 0 0 130957920 47860 199956 0 0 0 56 236342 476975 14 72 14 0 0 > > > > > > 41 0 0 130938560 47860 219900 0 0 0 0 236816 479676 14 72 14 0 0 > > > > > > > > > > > > vmstat for rw-sem implementation (3.10-rc4) > > > > > > procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu----- > > > > > > r b swpd free buff cache si so bi bo in cs us sy id wa st > > > > > > 40 0 0 130933984 43232 202584 0 0 0 0 321817 690741 13 71 16 0 0 > > > > > > 39 0 0 130913904 43232 224812 0 0 0 0 322193 692949 13 71 16 0 0 > > > > > > > > > > It appears the main difference is that the rwsem variant context-switches > > > > > about 36% more than the mutex version, right? > > > > > > > > > > I'm wondering how that's possible - the lock is mostly write-locked, > > > > > correct? So the lock-stealing from Davidlohr Bueso and Michel Lespinasse > > > > > ought to have brought roughly the same lock-stealing behavior as mutexes > > > > > do, right? > > > > > > > > > > So the next analytical step would be to figure out why rwsem lock-stealing > > > > > is not behaving in an equivalent fashion on this workload. Do readers come > > > > > in frequently enough to disrupt write-lock-stealing perhaps? > > > > > > Ingo, > > > > > > I did some instrumentation on the write lock failure path. I found that > > > for the exim workload, there are no readers blocking for the rwsem when > > > write locking failed. The lock stealing is successful for 9.1% of the > > > time and the rest of the write lock failure caused the writer to go to > > > sleep. About 1.4% of the writers sleep more than once. Majority of the > > > writers sleep once. > > > > > > It is weird that lock stealing is not successful more often. > > > > For this to be comparable to the mutex scalability numbers you'd have to > > compare wlock-stealing _and_ adaptive spinning for failed-wlock rwsems. > > > > Are both techniques applied in the kernel you are running your tests on? > > > > Ingo, > > The previous experiment was done on a kernel without spinning. > I've redone the testing on two kernel for a 15 sec stretch of the > workload run. One with the adaptive (or optimistic) > spinning and the other without. Both have the patches from Alex to avoid > cmpxchg induced cache bouncing. > > With the spinning, I sleep much less for lock acquisition (18.6% vs 91.58%). > However, I've got doubling of write lock acquisition getting > blocked. So that offset the gain from spinning which may be why > I didn't see gain for this particular workload. > > No Opt Spin Opt Spin > Writer acquisition blocked count 3448946 7359040 > Blocked by reader 0.00% 0.55% > Lock acquired first attempt (lock stealing) 8.42% 16.92% > Lock acquired second attempt (1 sleep) 90.26% 17.60% > Lock acquired after more than 1 sleep 1.32% 1.00% > Lock acquired with optimistic spin N/A 64.48% > Adding also the mutex statistics for the 3.10-rc4 kernel with mutex implemenation of lock for anon_vma tree. Wonder if Ingo has any insight on why mutex performs better from these stats. Mutex acquisition blocked count 14380340 Lock acquired in slowpath (no sleep) 0.06% Lock acquired in slowpath (1 sleep) 0.24% Lock acquired in slowpath more than 1 sleep 0.98% Lock acquired with optimistic spin 99.6% Thanks. Tim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/