From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S261439AbUE3AsN (ORCPT ); Sat, 29 May 2004 20:48:13 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S261418AbUE3AsM (ORCPT ); Sat, 29 May 2004 20:48:12 -0400 Received: from mx1.redhat.com ([66.187.233.31]:17814 "EHLO mx1.redhat.com") by vger.kernel.org with ESMTP id S261358AbUE3AsF (ORCPT ); Sat, 29 May 2004 20:48:05 -0400 Subject: Re: iowait problems on 2.6, not on 2.4 From: Doug Ledford To: Andrew Morton Cc: Antonio Larrosa =?ISO-8859-1?Q?Jim=E9nez?= , linux-kernel@vger.kernel.org, linux-scsi mailing list In-Reply-To: <20040528154525.6ed5f7b9.akpm@osdl.org> References: <200405261743.28111.antlarr@tedial.com> <20040526205225.7a0866aa.akpm@osdl.org> <200405281516.41901.antlarr@tedial.com> <20040528154525.6ed5f7b9.akpm@osdl.org> Content-Type: text/plain; charset=UTF-8 Message-Id: <1085877991.18642.336.camel@compaq.xsintricity.com> Mime-Version: 1.0 X-Mailer: Ximian Evolution 1.4.5 (1.4.5-1) Date: Sat, 29 May 2004 20:46:31 -0400 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2004-05-28 at 18:45, Andrew Morton wrote: > Antonio Larrosa Jiménez wrote: > > > > On Thursday 27 May 2004 05:52, you wrote: > > > Antonio Larrosa Jiménez wrote: > > > > My next test will be to do the "dd tests" on one of the internal hard > > > > disks and use it for the data instead of the external raid. > > > > > > That's a logical next step. The reduced read bandwith on the raid array > > > should be fixed up before we can go any further. I don't recall any > > > reports of qlogic fc-scsi performance regressions though. > > > > Ok, let's analyze that first. > > > > The dd tests gave the following results: > > Let me cc linux-scsi. > > Guys: poke. Does anyone know why this: > > The machine is a 4 cpu Pentium III (Cascades) system with four SCSI > SEAGATE ST336704 hard disks connected to an Adaptec AIC-7899P U160/m, and > a external RAID connected to a QLA2200/QLA2xxx FC-SCSI Host Bus Adapter. > The machine has 1Gb RAM. > > got all slow at reads? My first guess would be read ahead values. Try poking around with those. When using a hard disk vs. a raid array, it's easier to trigger firmware read ahead since all reads go to a single physical device and that in turn compensates for lack of reasonable OS read ahead. On a raid array, depending on the vendor, there may be next to no firmware initiated read ahead and that can drastically reduce read performance on sequential reads. > > So yes, I suppose there's a regression on the qlogic fc-scsi module. A regression, yes. In the qlogic-fc driver? I doubt it. Performance tuning is what I think this basically boils down to. But, I could be wrong. Give it a try and see what happens. In the 2.4 kernels I would tell you to tweak /proc/sys/vm/{min,max}-readahead, don't know if those two knobs still exist in 2.6 and if they have the same effect. Andrew? -- Doug Ledford 919-754-3700 x44233 Red Hat, Inc. 1801 Varsity Dr. Raleigh, NC 27606