From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756555Ab0CKJ61 (ORCPT ); Thu, 11 Mar 2010 04:58:27 -0500 Received: from mtagate6.uk.ibm.com ([194.196.100.166]:35819 "EHLO mtagate6.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755724Ab0CKJ6Z (ORCPT ); Thu, 11 Mar 2010 04:58:25 -0500 Message-ID: <4B98BEB0.6020800@linux.vnet.ibm.com> Date: Thu, 11 Mar 2010 10:58:08 +0100 From: Christian Ehrhardt User-Agent: Thunderbird 2.0.0.23 (X11/20090817) MIME-Version: 1.0 To: Wu Fengguang CC: Jens Axboe , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , Martin Schwidefsky , Heiko Carstens , Hisashi Hifumi , KOSAKI Motohiro , Ronald , Bart Van Assche , Vladislav Bolkhovitin , Randy Dunlap , Nick Piggin Subject: Re: [RFC PATCH] Fix Readahead stalling by plugged device queues References: <4B979104.6010907@linux.vnet.ibm.com> <20100310130932.GB18509@localhost> <4B97AD52.7080201@linux.vnet.ibm.com> <20100311014542.GA8134@localhost> In-Reply-To: <20100311014542.GA8134@localhost> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Wu Fengguang wrote: > On Wed, Mar 10, 2010 at 10:31:46PM +0800, Christian Ehrhardt wrote: >> >> Wu Fengguang wrote: >> [...] >>> Christian, did you notice this commit for 2.6.33? >>> >>> commit 65a80b4c61f5b5f6eb0f5669c8fb120893bfb388 >> [...] >> >> I didn't see that particular one, due to the fact that whatever the >> result is it needs to work .32 >> >> Anyway I'll test it tomorrow and if that already accepted one fixes my >> issue as well I'll recommend distros older than 2.6.33 picking that one >> up in their on top patches. > > OK, thanks! That patch fixes my issue completely and is as we discussed less aggressive which is fine - thanks for pointing it out - Now I have something already upstream accepted to fix the issue, thats much better! >>> It should at least improve performance between .32 and .33, because >>> once two readahead requests are merged into one single IO request, >>> the PageUptodate() will be true at next readahead, and hence >>> blk_run_backing_dev() get called to break out of the suboptimal >>> situation. >> As you saw from my blktrace thats already the case without that patch. >> Once the second readahead comes in and merged it gets unplugged in >> 2.6.32 too - but still that is bad behavior as it denies my things like >> 68% throughput improvement :-). > > I mean, when readahead windows A and B are submitted in one IO -- > let's call it AB -- commit 65a80b4c61 will explicitly unplug on doing > readahead C. While in your trace, the unplug appears on AB. > > The 68% improvement is very impressive. Wondering if commit 65a80b4c61 > (the _conditional_ unplug) can achieve the same level of improvement :) Yep it can ! We can post update the patch description to bigger numbers :-) >>> Your patch does reduce the possible readahead submit latency to 0. >> yeah and I think/hope that is fine, because as I stated: >> - low utilized disk -> not an issue >> - high utilized disk -> unplug is an noop >> >> At least personally I consider a case where merging of a readahead >> window with anything except its own sibling very rare - and therefore >> fair to unplug after and RA is submitted. > > They are reasonable assumptions. However I'm not sure if this > unconditional unplug will defeat CFQ's anticipatory logic -- if there > are any. You know commit 65a80b4c61 is more about a *defensive* > protection against the rare case that two readahead windows get > merged. > >>> Is your workload a simple dd on a single disk? If so, it sounds like >>> something illogical hidden in the block layer. >> It might still be illogical hidden as e.g. 2.6.27 unplugged after the >> first readahead as well :-) >> But no my load is iozone running with different numbers of processes >> with one disk per process. >> That neatly resembles e.g. nightly backup jobs which tend to take longer >> and longer in all time increasing customer scenarios. Such an >> improvement might banish the backups back to the night were they belong :-) > > Exactly one process per disk? Are they doing sequential reads or more > complicated access patterns? Just sequential read where I see the win, but I also had sequential write, and random read/write as well as some mixed stuff like dbench. It improved sequential read and did not impact the others which is fine. Thank you for you quick replies! > Thanks, > Fengguang -- Grüsse / regards, Christian Ehrhardt IBM Linux Technology Center, System z Linux Performance