Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755732Ab0DFN3R (ORCPT ); Tue, 6 Apr 2010 09:29:17 -0400 Received: from mx1.redhat.com ([209.132.183.28]:14769 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754644Ab0DFN3G (ORCPT ); Tue, 6 Apr 2010 09:29:06 -0400 Message-ID: <4BBB36FA.4020008@redhat.com> Date: Tue, 06 Apr 2010 16:28:26 +0300 From: Avi Kivity User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.9) Gecko/20100330 Fedora/3.0.4-1.fc12 Thunderbird/3.0.4 MIME-Version: 1.0 To: Darren Hart CC: linux-kernel@vger.kernel.org, Thomas Gleixner , Peter Zijlstra , Ingo Molnar , Eric Dumazet , "Peter W. Morreale" , Rik van Riel , Steven Rostedt , Gregory Haskins , Sven-Thorsten Dietrich , Chris Mason , John Cooper , Chris Wright Subject: Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive spinning References: <1270499039-23728-1-git-send-email-dvhltc@us.ibm.com> <4BBA5305.7010002@redhat.com> <4BBA5C00.4090703@us.ibm.com> <4BBA6279.20802@redhat.com> <4BBA6B6F.7040201@us.ibm.com> In-Reply-To: <4BBA6B6F.7040201@us.ibm.com> Content-Type: text/plain; charset=UTF-8; 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: 1852 Lines: 50 On 04/06/2010 01:59 AM, Darren Hart wrote: > > >>>> I'd be interested in seeing runs where the average number of >>>> waiters is 0.2, 0.5, 1, and 2, corresponding to moderate-to-bad >>>> contention. >>>> 25 average waiters on compute bound code means the application >>>> needs to be rewritten, no amount of mutex tweaking will help it. >>> >>> Perhaps something NR_CPUS threads would be of more interest? >> >> That seems artificial. > > How so? Several real world applications use one thread per CPU to > dispatch work to, wait for events, etc. Yes, but that's the best case for spinning. You could simply use a userspace spinlock in this case. > >>>> Does the wakeup code select the spinning waiter, or just a random >>>> waiter? >>> >>> The wakeup code selects the highest priority task in fifo order to >>> wake-up - however, under contention it is most likely going to go >>> back to sleep as another waiter will steal the lock out from under >>> it. This locking strategy is unashamedly about as "unfair" as it gets. >> >> Best to avoid the wakeup if we notice the lock was stolen. > > You really can't do this precisely. You can read the futex value at > various points along the wakeup path, but at some point you have to > commit to waking a task, and you still have a race between the time > you wake_up_task() and when it is scheduled and attempts the cmpxchg > itself. > The race is not a problem as it's just an optimization. If you lose it you get a spurious wakeup, if you win you save some cpu cycles. -- error compiling committee.c: too many arguments to function -- 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/