From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756379AbYEOMQT (ORCPT ); Thu, 15 May 2008 08:16:19 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752623AbYEOMQD (ORCPT ); Thu, 15 May 2008 08:16:03 -0400 Received: from smtp2.sssup.it ([193.205.80.100]:56226 "EHLO smtp2.sssup.it" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752457AbYEOMQB (ORCPT ); Thu, 15 May 2008 08:16:01 -0400 Date: Thu, 15 May 2008 14:21:56 +0200 From: Fabio Checconi To: Jens Axboe Cc: Matthew , Daniel J Blueman , Kasper Sandberg , Linux Kernel Subject: Re: performance "regression" in cfq compared to anticipatory, deadline and noop Message-ID: <20080515122156.GA11600@gandalf.sssup.it> References: <20080513130508.GQ16217@kernel.dk> <20080513180334.GS16217@kernel.dk> <20080513184057.GU16217@kernel.dk> <6278d2220805140105x27292033u6a97dcf13ab54263@mail.gmail.com> <20080514082622.GA16217@kernel.dk> <6278d2220805141352s3624d7b7qc90567f6b7a410dc@mail.gmail.com> <20080515070127.GH16217@kernel.dk> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20080515070127.GH16217@kernel.dk> User-Agent: Mutt/1.4.2.3i Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > From: Jens Axboe > Date: Thu, May 15, 2008 09:01:28AM +0200 > > I don't think it's 2.6.25 vs 2.6.26-rc2, I can still reproduce some > request size offsets with the patch. So still fumbling around with this, > I'll be sending out another test patch when I'm confident it's solved > the size issue. > IMO an interesting thing is how/why anticipatory doesn't show the issue. The device is not put into ANTIC_WAIT_NEXT if there is no dispatch returning no requests while the queue is not empty. This seems to be enough in the reported workloads. I don't think this behavior is the correct one (it is still racy WRT merges after breaking anticipation) anyway it should make things a little bit better. I fear that a complete solution would not involve only the scheduler. Introducing the very same behavior in cfq seems to be not so easy (i.e., start idling only if there was a dispatch round while the last request was being served) but an approximated version can be introduced quite easily. The patch below should do that, rescheduling the dispatch only if necessary; it is not tested at all, just posted for discussion. --- diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c index b399c62..41f1e0e 100644 --- a/block/cfq-iosched.c +++ b/block/cfq-iosched.c @@ -169,6 +169,7 @@ enum cfqq_state_flags { CFQ_CFQQ_FLAG_queue_new, /* queue never been serviced */ CFQ_CFQQ_FLAG_slice_new, /* no requests dispatched in slice */ CFQ_CFQQ_FLAG_sync, /* synchronous queue */ + CFQ_CFQQ_FLAG_dispatched, /* empty dispatch while idling */ }; #define CFQ_CFQQ_FNS(name) \ @@ -196,6 +197,7 @@ CFQ_CFQQ_FNS(prio_changed); CFQ_CFQQ_FNS(queue_new); CFQ_CFQQ_FNS(slice_new); CFQ_CFQQ_FNS(sync); +CFQ_CFQQ_FNS(dispatched); #undef CFQ_CFQQ_FNS static void cfq_dispatch_insert(struct request_queue *, struct request *); @@ -749,6 +751,7 @@ static void __cfq_set_active_queue(struct cfq_data *cfqd, cfqq->slice_end = 0; cfq_clear_cfqq_must_alloc_slice(cfqq); cfq_clear_cfqq_fifo_expire(cfqq); + cfq_clear_cfqq_dispatched(cfqq); cfq_mark_cfqq_slice_new(cfqq); cfq_clear_cfqq_queue_new(cfqq); } @@ -978,6 +981,7 @@ static struct cfq_queue *cfq_select_queue(struct cfq_data *cfqd) */ if (timer_pending(&cfqd->idle_slice_timer) || (cfqq->dispatched && cfq_cfqq_idle_window(cfqq))) { + cfq_mark_cfqq_dispatched(cfqq); cfqq = NULL; goto keep_queue; } @@ -1784,7 +1788,10 @@ cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq, if (cfq_cfqq_wait_request(cfqq)) { cfq_mark_cfqq_must_dispatch(cfqq); del_timer(&cfqd->idle_slice_timer); - blk_start_queueing(cfqd->queue); + if (cfq_cfqq_dispatched(cfqq)) { + cfq_clear_cfqq_dispatched(cfqq); + cfq_schedule_dispatch(cfqd); + } } } else if (cfq_should_preempt(cfqd, cfqq, rq)) { /*