From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761814AbZE2TRj (ORCPT ); Fri, 29 May 2009 15:17:39 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752205AbZE2TRb (ORCPT ); Fri, 29 May 2009 15:17:31 -0400 Received: from mx2.redhat.com ([66.187.237.31]:42715 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752054AbZE2TRa (ORCPT ); Fri, 29 May 2009 15:17:30 -0400 Date: Fri, 29 May 2009 15:16:49 -0400 From: Vivek Goyal To: Nauman Rafique Cc: Fabio Checconi , linux-kernel@vger.kernel.org, containers@lists.linux-foundation.org, dm-devel@redhat.com, jens.axboe@oracle.com, dpshah@google.com, lizf@cn.fujitsu.com, mikew@google.com, paolo.valente@unimore.it, ryov@valinux.co.jp, fernando@oss.ntt.co.jp, s-uchida@ap.jp.nec.com, taka@valinux.co.jp, guijianfeng@cn.fujitsu.com, jmoyer@redhat.com, dhaval@linux.vnet.ibm.com, balbir@linux.vnet.ibm.com, righi.andrea@gmail.com, m-ikeda@ds.jp.nec.com, jbaron@redhat.com, agk@redhat.com, snitzer@redhat.com, akpm@linux-foundation.org, peterz@infradead.org, Chad Talbott Subject: Re: [PATCH 02/20] io-controller: Common flat fair queuing code in elevaotor layer Message-ID: <20090529191649.GH26962@redhat.com> References: <1243377729-2176-1-git-send-email-vgoyal@redhat.com> <1243377729-2176-3-git-send-email-vgoyal@redhat.com> <20090528160003.GA4335@redhat.com> <20090529160610.GC26962@redhat.com> <20090529165716.GB18141@gandalf.sssup.it> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 29, 2009 at 12:06:03PM -0700, Nauman Rafique wrote: > On Fri, May 29, 2009 at 9:57 AM, Fabio Checconi wrote: > >> From: Vivek Goyal > >> Date: Fri, May 29, 2009 12:06:10PM -0400 > >> > >> On Thu, May 28, 2009 at 12:41:27PM -0700, Nauman Rafique wrote: > > ... > >> > I have some concerns about the new preemption logic. > >> > >> Actually we need a more proper definition of in-class preemption. Across > >> class preemption means that RT class always gets to run first. > >> > >> What does in-class preemption mean? If I look at the current CFQ code, > >> it does look like that preempting process will gain share. It is always > >> added to the front of the tree with "rb_key=0" and that means, this new > >> queue will get fresh time slice (even if it got time slice very recently). > >> > >> Currently I have just tried to make the behavior same as CFQ to reduce > >> the possiblility of regressions. That's a different thing that we can > >> discuss what should be the exact behavior in case of in-class preemption > >> and first it needs to be fixed in CFQ, if current behavior is an issue. > >> > >> On the other hand, I am not sure if previous bfq preemption logic was > >> working. We were checking if the new request belonged to the queue which > >> will be served next, then preempt the existing queue. While looking > >> for the next queue, I think we did not consider the current active > >> entity (as it was not on the tree). So after expiry of the current > >> queue, it might get selected next if it has not got its share. So there > >> was no point in preempting the queue. If queue already got its share, then > >> anyway the next queue will be selected next and there is no point in > >> preempting the current queue. > >> > > > > BFQ had no preemption logic, as far as I know; it simply was not > > preemptive, and the guarantees it provided took that into account. > > > > I don't know what is the best way to introduce a CFQ-like preemption logic > > into the wf2q+ code; for sure anything that does not schedule according > > to the algorithm's timestamps is a good candidate to break the guarantees > > the scheduler can provide, making it an extremely complex way to get > > the same worst-case delays of a (much simpler) round-robin scheduler. > > > > What you guys think of my suggestion of handling preemption? > Basically, we don't modify the start/finish tags, so overall the > fairness properties should not be broken. But in short term, we still > allow preemption and let one queue jump another. It sounded complicated from the description of it. I would prefer either we get rid of in-class preemtion thing completely or do in-class preemtption at the cost of gaining share, like cfq does. In fact, to begin with, I prefer to be as close as possible to CFQ and then change things selectively one piece at a time so that we can analyze the impact well. Thanks Vivek