From: Mike Snitzer Subject: Re: [patch,rfc v2] ext3/4: enhance fsync performance when using cfq Date: Wed, 21 Apr 2010 16:42:38 -0400 Message-ID: References: <20100407214631.GL3206@redhat.com> <20100408110442.GK10103@kernel.dk> <20100408140515.GB10879@redhat.com> <20100408140944.GQ10103@kernel.dk> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Vivek Goyal , "Theodore Ts'o" , linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org, dm-devel@redhat.com To: Jeff Moyer , Jens Axboe Return-path: Received: from mail-wy0-f174.google.com ([74.125.82.174]:58836 "EHLO mail-wy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756496Ab0DUUmk convert rfc822-to-8bit (ORCPT ); Wed, 21 Apr 2010 16:42:40 -0400 In-Reply-To: <20100408140944.GQ10103@kernel.dk> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Thu, Apr 8, 2010 at 10:09 AM, Jens Axboe wro= te: > On Thu, Apr 08 2010, Vivek Goyal wrote: >> On Thu, Apr 08, 2010 at 01:04:42PM +0200, Jens Axboe wrote: >> > On Wed, Apr 07 2010, Vivek Goyal wrote: >> > > On Wed, Apr 07, 2010 at 05:18:12PM -0400, Jeff Moyer wrote: >> > > > Hi again, >> > > > >> > > > So, here's another stab at fixing this. =A0This patch is very = much an RFC, >> > > > so do not pull it into anything bound for Linus. =A0;-) =A0For= those new to >> > > > this topic, here is the original posting: =A0http://lkml.org/l= kml/2010/4/1/344 >> > > > >> > > > The basic problem is that, when running iozone on smallish fil= es (up to >> > > > 8MB in size) and including fsync in the timings, deadline outp= erforms >> > > > CFQ by a factor of about 5 for 64KB files, and by about 10% fo= r 8MB >> > > > files. =A0From examining the blktrace data, it appears that io= zone will >> > > > issue an fsync() call, and will have to wait until it's CFQ ti= meslice >> > > > has expired before the journal thread can run to actually comm= it data to >> > > > disk. >> > > > >> > > > The approach below puts an explicit call into the filesystem-s= pecific >> > > > fsync code to yield the disk so that the jbd[2] process has a = chance to >> > > > issue I/O. =A0This bring performance of CFQ in line with deadl= ine. >> > > > >> > > > There is one outstanding issue with the patch that Vivek point= ed out. >> > > > Basically, this could starve out the sync-noidle workload if t= here is a >> > > > lot of fsync-ing going on. =A0I'll address that in a follow-on= patch. =A0For >> > > > now, I wanted to get the idea out there for others to comment = on. >> > > > >> > > > Thanks a ton to Vivek for spotting the problem with the initia= l >> > > > approach, and for his continued review. >> > > > =2E.. >> > > So we got to take care of two issues now. >> > > >> > > - Make it work with dm/md devices also. Somehow shall have to pr= opogate >> > > =A0 this yield semantic down the stack. >> > >> > The way that Jeff set it up, it's completely parallel to eg conges= tion >> > or unplugging. So that should be easily doable. >> > >> >> Ok, so various dm targets now need to define "yield_fn" and propogat= e the >> yield call to all the component devices. > > Exactly. To do so doesn't DM (and MD) need a blk_queue_yield() setter to establish its own yield_fn? The established dm_yield_fn would call blk_yield() for all real devices in a given DM target. Something like how blk_queue_merge_bvec() or blk_queue_make_request() allow DM to provide functional extensions. I'm not seeing such a yield_fn hook for stacking drivers to use. And as is, jbd and jbd2 just call blk_yield() directly and there is no way for the block layer to call into DM. What am I missing? Thanks, Mike -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html