From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757311Ab0DAAh5 (ORCPT ); Wed, 31 Mar 2010 20:37:57 -0400 Received: from nat.nue.novell.com ([195.135.221.3]:54225 "EHLO emea5-mh.id5.novell.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1756776Ab0DAAhz (ORCPT ); Wed, 31 Mar 2010 20:37:55 -0400 X-Greylist: delayed 1200 seconds by postgrey-1.27 at vger.kernel.org; Wed, 31 Mar 2010 20:37:55 EDT Subject: Re: RFC: Ideal Adaptive Spinning Conditions From: "Peter W. Morreale" To: rostedt@goodmis.org Cc: Darren Hart , "lkml," , Peter Zijlstra , Gregory Haskins , Sven-Thorsten Dietrich , Chris Wright , Thomas Gleixner , Ingo Molnar , Eric Dumazet In-Reply-To: <1270078689.19685.8040.camel@gandalf.stny.rr.com> References: <4BB3D90C.3030108@us.ibm.com> <1270078689.19685.8040.camel@gandalf.stny.rr.com> Content-Type: text/plain; charset="UTF-8" Organization: Novell Inc. Date: Wed, 31 Mar 2010 18:17:46 -0600 Message-ID: <1270081066.2997.22.camel@hermosa.site> Mime-Version: 1.0 X-Mailer: Evolution 2.28.2 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2010-03-31 at 19:38 -0400, Steven Rostedt wrote: > On Wed, 2010-03-31 at 16:21 -0700, Darren Hart wrote: > > > o What type of lock hold times do we expect to benefit? > > 0 (that's a zero) :-p > > I haven't seen your patches but you are not doing a heuristic approach, > are you? That is, do not "spin" hoping the lock will suddenly become > free. I was against that for -rt and I would be against that for futex > too. > > > o How much contention is a good match for adaptive spinning? > > - this is related to the number of threads to run in the test > > o How many spinners should be allowed? > > > > I can share the kernel patches if people are interested, but they are > > really early, and I'm not sure they are of much value until I better > > understand the conditions where this is expected to be useful. > > Again, I don't know how you implemented your adaptive spinners, but the > trick to it in -rt was that it would only spin while the owner of the > lock was actually running. If it was not running, it would sleep. No > point waiting for a sleeping task to release its lock. Right. This was *critical* for the adaptive rtmutex. Note in the RT patch, everybody spins as long as the current owner is on CPU. FWIW, IIRC, Solaris has a heuristic approach where incoming tasks spin for a period of time before going to sleep. (Cray UINCOS did the same) > > Is this what you did? Because, IIRC, this only benefited spinlocks > converted to mutexes. It did not help with semaphores, because > semaphores could be held for a long time. Thus, it was good for short > held locks, but hurt performance on long held locks. > nod. The entire premise was based on the fact that we were converting spinlocks, which by definition were short held locks. What I found during early development was that the sleep/wakeup cycle was more intrusive for RT than spinning. IIRC, I measured something like 380k context switches/second prior to the adaptive patches for a dbench test and we cut this down to somewhere around 50k, with a corresponding increase in throughput. (I can't remember specific numbers any more, it was a while ago... ;-) When applied to semaphores, the benefit was in the noise range as I recall.. (dbench was chosen due to the heavy contention on the dcache spinlock) Best, -PWM > If userspace is going to do this, I guess the blocked task would need to > go into kernel, and spin there (with preempt enabled) if the task is > still active and holding the lock. > > Then the application would need to determine which to use. An adaptive > spinner for short held locks, and a normal futex for long held locks. > > -- Steve > >