Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759008AbZAOAu6 (ORCPT ); Wed, 14 Jan 2009 19:50:58 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753907AbZAOAur (ORCPT ); Wed, 14 Jan 2009 19:50:47 -0500 Received: from mx1.suse.de ([195.135.220.2]:42977 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753544AbZAOAup (ORCPT ); Wed, 14 Jan 2009 19:50:45 -0500 Date: Thu, 15 Jan 2009 01:50:42 +0100 From: Nick Piggin To: Avi Kivity Cc: Peter Zijlstra , Linus Torvalds , Ingo Molnar , "Paul E. McKenney" , Gregory Haskins , Matthew Wilcox , Andi Kleen , Chris Mason , Andrew Morton , Linux Kernel Mailing List , linux-fsdevel , linux-btrfs , Thomas Gleixner , Peter Morreale , Sven Dietrich , Dmitry Adamushko Subject: Re: [PATCH -v8][RFC] mutex: implement adaptive spinning Message-ID: <20090115005042.GC32044@wotan.suse.de> References: <1231774622.4371.96.camel@laptop> <496B6C23.8000808@redhat.com> <1231780388.4371.185.camel@laptop> <496B7EBC.6020208@redhat.com> <1231951599.14825.18.camel@laptop> <20090114170445.GA18964@wotan.suse.de> <496E1F80.20009@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <496E1F80.20009@redhat.com> User-Agent: Mutt/1.5.9i Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1964 Lines: 44 On Wed, Jan 14, 2009 at 07:23:12PM +0200, Avi Kivity wrote: > Nick Piggin wrote: > >>(no they're not, Nick's ticket locks still spin on a shared cacheline > >>IIRC -- the MCS locks mentioned could fix this) > >> > > > >It reminds me. I wrote a basic variation of MCS spinlocks a while back. And > >converted dcache lock to use it, which showed large dbench improvements on > >a big machine (of course for different reasons than the dbench improvements > >in this threaed). > > > >http://lkml.org/lkml/2008/8/28/24 > > > >Each "lock" object is sane in size because given set of spin-local queues > >may > >only be used once per lock stack. But any spinlocks within a mutex > >acquisition > >will always be at the bottom of such a stack anyway, by definition. > > > >If you can use any code or concept for your code, that would be great. > > > > Does it make sense to replace 'nest' with a per-cpu counter that's > incremented on each lock? I guest you'd have to search for the value of > nest on unlock, but it would a very short search (typically length 1, 2 > if lock sorting is used to avoid deadlocks). > > I think you'd need to make the lock store the actual node pointer, not > the cpu number, since the values of nest would be different on each cpu. > > That would allow you to replace spinlocks with mcs_locks wholesale. nest could get quite large (and basically is unbounded), though. I think there would definitely be variations (NMCS locks is interesting). But OTOH I think that for _most_ spinlocks, optimising for spinning case is wrong. For some really tricky global spinlocks, yes I think MCS is a good idea. But moreso as a stopgap until hopefully more scalable algorithms are written. -- 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/