From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from sc8-sf-mx1-b.sourceforge.net ([10.3.1.11] helo=sc8-sf-mx1.sourceforge.net) by sc8-sf-list1.sourceforge.net with esmtp (Exim 4.30) id 1BgJdd-0008I0-IS for user-mode-linux-devel@lists.sourceforge.net; Fri, 02 Jul 2004 01:43:17 -0700 Received: from letters.cs.ucsb.edu ([128.111.41.13]) by sc8-sf-mx1.sourceforge.net with esmtp (Exim 4.34) id 1BgJdd-0004nj-9K for user-mode-linux-devel@lists.sourceforge.net; Fri, 02 Jul 2004 01:43:17 -0700 Received: from cs.ucsb.edu (SY197111.resnet.ucsb.edu [128.111.197.111]) by letters.cs.ucsb.edu (8.11.7+Sun/8.11.7) with ESMTP id i628hGT02742 for ; Fri, 2 Jul 2004 01:43:16 -0700 (PDT) Message-ID: <40E51FF0.3020801@cs.ucsb.edu> From: "Lamia M.Youseff" MIME-Version: 1.0 Content-Type: text/html; charset=us-ascii Content-Transfer-Encoding: 7bit Subject: [uml-devel] does O_DIRECT hinder greatly file reading performance but not writing?? Sender: user-mode-linux-devel-admin@lists.sourceforge.net Errors-To: user-mode-linux-devel-admin@lists.sourceforge.net List-Unsubscribe: , List-Id: The user-mode Linux development list List-Post: List-Help: List-Subscribe: , List-Archive: Date: Fri, 02 Jul 2004 01:42:24 -0700 To: user-mode-linux-devel@lists.sourceforge.net Hi All,
I am kind of new to UML and I am experiencing a very strange performance hinder with it.
I am running two benchmarks; one that writes a 50Mbyte file and the other one reads a 50Mbyte file.
I tried the benchmarks with and without setting the O_DIRECT flag, in which cases the whole task is done in a matter of seconds(10-30 seconds) except when reading the file with O_DIRECT flag on. In this case, it takes 18-20 min!!!

I am running UML 2.4.19 on top of kernel 2.4.25.  I have time output like that (average of 20 runs)
Operation
 User Time (sec)
 Sys Time(sec)
 Wall Clock Time(sec)
 
Write Without O_DIRECT
 5.612000
 0.117500
 27.164500
 
Write With O_DIRECT
 10.007000
 0.017000
 22.199500
 
Read without O_DIRECT
3.035000
 0.091000
 7.760000
 
Read With O_DIRECT
 21.492000
 29.680000
 1133.054000
 
I would appreciate it very much if somebody would explain to me how to solve that strange performance problem, or even explain why does it happen for reading with O_DIRECT but not without !!??

Thanks allot,
Lamia Youseff
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