From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760035AbYB2PS5 (ORCPT ); Fri, 29 Feb 2008 10:18:57 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751794AbYB2PSq (ORCPT ); Fri, 29 Feb 2008 10:18:46 -0500 Received: from mail.syneticon.net ([213.239.212.131]:42727 "EHLO mail2.syneticon.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750918AbYB2PSp (ORCPT ); Fri, 29 Feb 2008 10:18:45 -0500 Message-ID: <47C82244.60307@wpkg.org> Date: Fri, 29 Feb 2008 16:18:28 +0100 From: Tomasz Chmielewski User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.0.8) Gecko/20061110 Mandriva/1.5.0.8-1mdv2007.1 (2007.1) Thunderbird/1.5.0.8 Mnenhy/0.7.4.666 MIME-Version: 1.0 To: LKML Cc: ml_lkml@thefluffyone.net Subject: Re: Very high IOWait during all disk activity Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > I recently noticed on a number of my Linux boxes that during disk > activity, CPU usage was consistently hitting 100%. A little digging > showed that the CPU was spending up to around 65% of its time in an > IOWait state. Checked this with kernels 2.6.22 and 2.6.25-rc3, and > also across SATA and PATA drives on three different machines, all with > the same results. I also checked back with an old Ubuntu 6.06 Live CD > and that also exhibits the problem. > > Having done some digging on the net, I can't get a definitive answer > as to whether this is considered "normal". Some people suggest that > IOWait is informational and doesn't indicate a problem, but based on > my admittedly limited understanding of such things, the CPU shouldn't > need to spend much time on disk I/O these days due to the use of DMA. > > Is it expected behaviour for the CPU to spend such a large amount of > time in the IOWait state during disk I/O? Unless you can write to the disk faster than fetch data from /dev/zero - yes, it is normal. BTW, it doesn't mean that your CPU's cycles are wasted. You will see big "wa" numbers when there are no other tasks to schedule at the same time. Try running: cat /dev/zero | bzip2 -c >/dev/null when your IOwait is big (because you write a big file), and then watch the numbers. -- Tomasz Chmielewski http://wpkg.org