Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965769Ab0GPO4L (ORCPT ); Fri, 16 Jul 2010 10:56:11 -0400 Received: from mx1.redhat.com ([209.132.183.28]:41159 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965743Ab0GPO4J (ORCPT ); Fri, 16 Jul 2010 10:56:09 -0400 Date: Fri, 16 Jul 2010 10:56:00 -0400 From: Vivek Goyal To: Jeff Moyer Cc: Gui Jianfeng , Jens Axboe , Corrado Zoccolo , Shaohua Li , linux kernel mailing list Subject: Re: [PATCH] [RFC] CFQ: Make prio_trees per cfq group basis to improve IO performance Message-ID: <20100716145600.GF15382@redhat.com> References: <4C40247C.2010405@cn.fujitsu.com> <20100716140034.GC15382@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.20 (2009-12-10) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3012 Lines: 62 On Fri, Jul 16, 2010 at 10:21:46AM -0400, Jeff Moyer wrote: > Vivek Goyal writes: > > > On Fri, Jul 16, 2010 at 05:21:00PM +0800, Gui Jianfeng wrote: > >> Currently, prio_trees is global, and we rely on cfqq_close() to search > >> a coorperator. If the returned cfqq and the active cfqq don't belong to > >> the same group, coorperator searching fails. Actually, that's not the case. > >> Even if cfqq_close() returns a cfqq which belong to another cfq group, > >> it's still likely that a coorperator(same cfqg) resides in prio_trees. > >> This patch introduces per cfq group prio_trees that should solve the above > >> issue. > >> > > > > Hi Gui, > > > > I am not sure I understand the issue here. So are you saying that once > > we find a cfqq which is close but belongs to a different group we reject > > it. But there could be another cfqq in the same group which is not as > > close but still close enough. > > > > For example, assume there are two queues q1 and q2 and in group and third > > queue q3 in group B. Assume q1 is active queue and we are searching for > > cooperator. If cooperator code finds q3 as closest then we will not pick > > this queue as it belongs to a different group. But it could happen that > > q2 is also close enough and we never considered that possibility. > > > > If yes, then its a good theoritical concern but I am worried practically > > how often does it happen. Do you have any workload which suffers because > > of this? > > That was my reading. It also means that, in the case that we have > cgroups in use, each rb tree will be smaller. > > > I am not too inclined to push more complexity in CFQ until and unless we > > have a good use case. > > I don't think this adds complexity, does it? It simply moves the > priority trees up a level, which is arguably where they belong. What happens when cfqq moves to a different group. group_isolation=0. Then we also need to add code to change prio tree of the cfqq. Curretnly prio tree are global so we don't have to worry about it. I don't think this patch takes are of that issue. That's a different thing that I am beginning to not like group_isoation=0 because this additional variable that cfqq's can move dynamically across groups is making life hard while adding more code in CFQ. So if nobody is using it I was thinking of getting rid of group_isolation tunable. It does bring the issue of severe performance penalty for sync-noidle workloads across groups. I think that should be solved by a different tunable like don't worry about fairness if group is not driving a minimum queue depth and this should be adjustable by tunable so that system admin can decide the right balance between fairness/isolation and throughput. Thanks Vivek -- 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/