Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757894Ab1F1NGR (ORCPT ); Tue, 28 Jun 2011 09:06:17 -0400 Received: from mx1.redhat.com ([209.132.183.28]:61428 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757630Ab1F1NFB (ORCPT ); Tue, 28 Jun 2011 09:05:01 -0400 Date: Tue, 28 Jun 2011 09:04:57 -0400 From: Vivek Goyal To: Shaohua Li Cc: "linux-kernel@vger.kernel.org" , "jaxboe@fusionio.com" , "linux-fsdevel@vger.kernel.org" , "linux-ext4@vger.kernel.org" , "khlebnikov@openvz.org" , "jmoyer@redhat.com" Subject: Re: [RFC PATCH 0/3] block: Fix fsync slowness with CFQ cgroups Message-ID: <20110628130457.GA17552@redhat.com> References: <1309205864-13124-1-git-send-email-vgoyal@redhat.com> <1309223932.15392.186.camel@sli10-conroe> <20110628014039.GA15850@redhat.com> <1309226634.15392.197.camel@sli10-conroe> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1309226634.15392.197.camel@sli10-conroe> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2366 Lines: 50 On Tue, Jun 28, 2011 at 10:03:54AM +0800, Shaohua Li wrote: [..] > > > > Any feedback on how to solve this issue is appreciated. > > > Hi Vivek, > > > can we introduce a group think time check in cfq? say in a group the > > > last queue is backed for the group and the queue is a non-idle queue, if > > > the group think time is big, we don't allow the group idle and preempt > > > could happen. The fsync thread is a non-idle queue with Corrado's patch, > > > this allows fast group switch. > > > > In this case regular queue idle is hitting and not group idle. So some > > kind of think time stats probably might be useful for group idle check > > but not necessarily for queue idle. > I thought your problem is group idle issue. fsync uses WRITE_SYNC, which > will make the queue be sync-non-idle because REQ_NOIDLE is set. This is > exactly what Corrado's patch for. a fsync queue itself isn't idle unless > it's the last queue in a group. Am I missing anything? We idle on last queue on sync-noidle tree. So we idle on fysnc queue as it is last queue on sync-noidle tree. That's how we provide protection to all sync-noidle queues against sync-idle queues. Instead of idling on individual quues we do idling in group and that is on service tree. > > > Secondly, for this case think time will change. If you stop idling on > > fsync and jbd threads, both will be dispatching IOs fast and both will > > have small thinktime. We will think that thinktime is small so we > > will enable idle. Then there think time will increase as both will > > get blocked behind each other. And then we will removing idling. So > > looks like we will be oscillating between enabling and disabling > > think time. > That is possible, the think time check (even for queues) always has such > issue. Not sure how severe the issue is. Assume jbd will dispatch > several requests and this will make fsync thread think time big. > > > If we don't allow idling on sync-no-idle queues, then basic CFQ will > > be broken. > Hmm, CFQ only allows idling on sync queues, sync-no-idle queue isn't > allowed idling. See above. 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/