Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1765332AbZAQRQW (ORCPT ); Sat, 17 Jan 2009 12:16:22 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1762729AbZAQRQO (ORCPT ); Sat, 17 Jan 2009 12:16:14 -0500 Received: from tomts16-srv.bellnexxia.net ([209.226.175.4]:63599 "EHLO tomts16-srv.bellnexxia.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1761658AbZAQRQN (ORCPT ); Sat, 17 Jan 2009 12:16:13 -0500 X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: ArcEAFGecUlMQWt2/2dsb2JhbACBbMpWhXM Date: Sat, 17 Jan 2009 12:15:56 -0500 From: Mathieu Desnoyers To: Leon Woestenberg Cc: Jens Axboe , Andrea Arcangeli , akpm@linux-foundation.org, Ingo Molnar , Linus Torvalds , linux-kernel@vger.kernel.org, ltt-dev@lists.casi.polymtl.ca, Thomas Gleixner Subject: Re: [RFC PATCH] block: Fix bio merge induced high I/O latency Message-ID: <20090117171556.GA32572@Krystal> References: <20090117004439.GA11492@Krystal> <20090117162657.GA31965@Krystal> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline In-Reply-To: X-Editor: vi X-Info: http://krystal.dyndns.org:8080 X-Operating-System: Linux/2.6.21.3-grsec (i686) X-Uptime: 12:11:28 up 16 days, 17:09, 3 users, load average: 0.72, 0.46, 0.41 User-Agent: Mutt/1.5.16 (2007-06-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1705 Lines: 48 * Leon Woestenberg (leon.woestenberg@gmail.com) wrote: > Hello Mathieu et al, > > On Sat, Jan 17, 2009 at 5:26 PM, Mathieu Desnoyers > wrote: > > A long standing I/O regression (since 2.6.18, still there today) has hit > > Slashdot recently : > > http://bugzilla.kernel.org/show_bug.cgi?id=12309 > > Are you sure you are solving the *actual* problem? > > The bugzilla entry shows a bisect attempt that leads to a patch > involving negative clock jumps. > http://bugzilla.kernel.org/show_bug.cgi?id=12309#c29 > > with a corrected link to the bisect patch: > http://bugzilla.kernel.org/show_bug.cgi?id=12309#c30 > > Wouldn't a negative clock jump be very influential to the > (time-driven) I/O schedulers and be a more probable cause? > When a merge is done, the lowest timestamp between the existing request and the new request to merge is kept as a start_time value for the merged request we end up with. In this case, that would probably make that request stay on top of the queue even if unrelated interactive I/O requests come. I suspect that this negative clock jump could have hidden the problem by making the start time of the interactive request lower than the start time of the merged request. Mathieu > Regards, > -- > Leon > > p.s. Added Thomas to the CC list as his name is on the patch Signed-off-by list. -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 -- 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/