From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752752Ab2KGUsW (ORCPT ); Wed, 7 Nov 2012 15:48:22 -0500 Received: from mail-ee0-f46.google.com ([74.125.83.46]:38422 "EHLO mail-ee0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751149Ab2KGUsV (ORCPT ); Wed, 7 Nov 2012 15:48:21 -0500 Message-ID: <509AC911.1040700@gmail.com> Date: Wed, 07 Nov 2012 21:48:17 +0100 From: Erwan Velu User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.9) Gecko/20121014 Thunderbird/10.0.9 MIME-Version: 1.0 To: ak@linux.intel.com, axboe@kernel.dk CC: linux-kernel@vger.kernel.org Subject: Unexpected latencies on lseek() SEEK_SET on block devices Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi fellows, I'm been facing some lseek() troubles on a very light hardware (Atom E660) under heavy load (network + cpu + disk IOs). I'm using 3.2.32 on a 32bit Os with a local SSD as mass storage. If a do open a block device like sdb1 and lseek SEEK_SET in it, some unexpected latencies occurs. Using the same load, everything works perfectly by using contigous streams but once I do lseek it start to be laggy. I've been searching around for a while and finally found this message : https://lkml.org/lkml/2011/9/15/399 from Andy. The description was very similar to what I experienced but the patch from Andy was on to the fs layer. I've been looking the code for the block level layer and found the implementation is pretty different. http://lxr.linux.no/#linux+v3.2.33/fs/read_write.c#L69 vs http://lxr.linux.no/#linux+v3.2.33/fs/block_dev.c#L353 As I can see, we do first put the mutex, then i_size_read and then considering the kind of SEEK we want. The semantic changes from the read_write implementation where it does the locking only for SEEK_CUR and i_size_read isn't executed for SEEK_SET. So I really wonder if we shall rework this part to avoid the uncessary locking for all of them except SEEK_CUR and remove i_size_read from SEEK_SET. The i_size_read is also a matter as it does a memory barrier. On such low-end hardware I have, that could costs. I can work on it and validate its performances unless the experts you are told me this is a mandatory feature. Thanks for your attention and comments on this topic. Erwan,