From: Jeff Moyer Subject: Re: [PATCH 0/6 v6][RFC] jbd[2]: enhance fsync performance when using CFQ Date: Fri, 02 Jul 2010 16:32:31 -0400 Message-ID: References: <1278100699-24132-1-git-send-email-jmoyer@redhat.com> <20100702202342.GH7001@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-ext4@vger.kernel.org, axboe@kernel.dk, linux-kernel@vger.kernel.org, tao.ma@oracle.com To: Vivek Goyal Return-path: Received: from mx1.redhat.com ([209.132.183.28]:6910 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758758Ab0GBUch (ORCPT ); Fri, 2 Jul 2010 16:32:37 -0400 In-Reply-To: <20100702202342.GH7001@redhat.com> (Vivek Goyal's message of "Fri, 2 Jul 2010 16:23:42 -0400") Sender: linux-ext4-owner@vger.kernel.org List-ID: Vivek Goyal writes: > On Fri, Jul 02, 2010 at 03:58:13PM -0400, Jeff Moyer wrote: > > [..] >> Changes from the last posting: >> - Yielding no longer expires the current queue. Instead, it sets up new >> requests from the target process so that they are issued in the yielding >> process' cfqq. This means that we don't need to worry about losing group >> or workload share. >> - Journal commits are now synchronous I/Os, which was required to get any >> sort of performance out of the fs_mark process in the presence of a >> competing reader. >> - WRITE_SYNC I/O no longer sets RQ_NOIDLE, for a similar reason. > > Hi Jeff, > > So this patchset relies on idling on WRITE_SYNC queues. Though in general > we don't have examples that why one should idle on processes doing WRITE_SYNC > IO because previous IO does not tell anything about the upcoming IO. I am > bringing up this point again to make sure that fundamentally we agree that > continue to idle on WRITE_SYNC is the right thing to do otherwise this patch > will fall apart. I think a mail server would be an example of an application that might do this. I'll see if I can get a real world test case (or perhaps some real world data) and verify that. I agree that if we choose not to idle on write's, then this approach can be thrown out the window. > I have yet to go through the patch in detail but allowing other queue to > dispatch requests in the same queue sounds like queue merging. So can > we use that semantics to say elv_merge_context() or elv_merge_queue() > instead of elv_yield(). In the code we can just merge the two queues when > the next request comes in and separate them out at the slice expiry I > guess. I considered that approach, but then you run into all of the questions about losing fairness across workloads and across groups. I believe the approach I've taken here is *significantly* simpler than merging and unmerging would be. Cheers, Jeff