Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752796Ab0DAFP7 (ORCPT ); Thu, 1 Apr 2010 01:15:59 -0400 Received: from e8.ny.us.ibm.com ([32.97.182.138]:46002 "EHLO e8.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752632Ab0DAFPx (ORCPT ); Thu, 1 Apr 2010 01:15:53 -0400 Message-ID: <4BB42C04.7090308@us.ibm.com> Date: Wed, 31 Mar 2010 22:15:48 -0700 From: Darren Hart User-Agent: Thunderbird 2.0.0.24 (X11/20100317) MIME-Version: 1.0 To: rostedt@goodmis.org CC: "lkml," , Peter Zijlstra , Gregory Haskins , Sven-Thorsten Dietrich , Peter Morreale , Thomas Gleixner , Ingo Molnar , Eric Dumazet , Chris Mason Subject: Re: RFC: Ideal Adaptive Spinning Conditions References: <4BB3D90C.3030108@us.ibm.com> <1270078689.19685.8040.camel@gandalf.stny.rr.com> <4BB40140.20109@us.ibm.com> <1270088753.19685.8218.camel@gandalf.stny.rr.com> In-Reply-To: <1270088753.19685.8218.camel@gandalf.stny.rr.com> Content-Type: text/plain; charset=ISO-8859-15; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2719 Lines: 65 Steven Rostedt wrote: > On Wed, 2010-03-31 at 19:13 -0700, Darren Hart wrote: >> 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. >> I'm not sure what you're getting at here. Adaptive spinning is indeed >> hoping the lock will become free while you are spinning and checking >> it's owner... > > I'm talking about the original idea people had of "lets spin for 50us > and hope it is unlocked before then", which I thought was not a good > idea. > > >>>> 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. >> It does exactly this. > > OK, that's good. > >>> 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. >> Trouble is, I'm still seeing performance penalties even on the shortest >> critical section possible (lock();unlock();) > > performance penalties compared to what? not having adaptive at all? Right. See the data in the original mail: futex_lock: Result: 635 Kiter/s futex_lock_adaptive: Result: 542 Kiter/s So 15% fewer lock/unlock iterations per second with in kernel adaptive spinning enabled for a critical section approaching 0 in length. But If we agree I'm taking the right approach, then it's time for me to polish things up a bit and send them out for review. -- Darren Hart IBM Linux Technology Center Real-Time Linux Team -- 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/