From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Mon, 12 Mar 2001 11:51:41 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Mon, 12 Mar 2001 11:51:32 -0500 Received: from f197.law11.hotmail.com ([64.4.17.197]:64778 "EHLO hotmail.com") by vger.kernel.org with ESMTP id ; Mon, 12 Mar 2001 11:51:14 -0500 X-Originating-IP: [204.69.198.2] From: "Ying Chen" To: linux-kernel@vger.kernel.org Subject: raw device and linux scheduling performance weirdness Date: Mon, 12 Mar 2001 08:50:42 -0800 Mime-Version: 1.0 Content-Type: text/plain; format=flowed Message-ID: X-OriginalArrivalTime: 12 Mar 2001 16:50:43.0109 (UTC) FILETIME=[93A45550:01C0AB14] Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Hi, I ran some really trivial raw disk performance tests on 2.4.0 using the raw disk support in it. I seem to be getting some really strange performance results. My program opens up a raw device, then does a sequence of sequential/random reads/writes on the raw device using pread/pwrite. I put timing around both the sequence and the individual requests. I noticed that in some of the runs the elapsed time for the whole sequence of I/O requests is significantly longer than the sum of the individual I/O request response times (like 100 times longer say), yet my program does nothing in between the requests but a gettimeofday call to record the request starting time. The system has nothing else running when the tests were run, so the process should not be contenting with other things. This seems to me that somehow the raw device I/O process is either stuck or the linux scheduler is skewing things up somewhere. I tried to nice the process to higher priority values, it didn't seem to help. Any ideas? Thanks, Ying _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com