From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932624Ab0CJXpS (ORCPT ); Wed, 10 Mar 2010 18:45:18 -0500 Received: from mail-gw0-f46.google.com ([74.125.83.46]:37746 "EHLO mail-gw0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757439Ab0CJXpQ convert rfc822-to-8bit (ORCPT ); Wed, 10 Mar 2010 18:45:16 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:from:date:message-id:subject:to :cc:content-type:content-transfer-encoding; b=l/39ILhB5M41QoinXNDDrKZK4TDzDCeGkP7YoLGmNJ5T5EPPmpb/zlrXZIdhDJwuBD UklsqbV2A66EUrjrJqjnLp9QivPB3+5RThsgYXk3glRb0WaFHruZ1+xQR5E0149tXoDz 62TE74Q47rF+quhxsaNn16OiU06SV1P22irHI= MIME-Version: 1.0 In-Reply-To: <201003101817.42812.hpj@urpla.net> References: <201003101817.42812.hpj@urpla.net> From: David Rees Date: Wed, 10 Mar 2010 15:44:54 -0800 Message-ID: <72dbd3151003101544w18afc65ubbc85d5bfc435198@mail.gmail.com> Subject: Re: howto combat highly pathologic latencies on a server? To: Hans-Peter Jansen Cc: linux-kernel@vger.kernel.org Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 10, 2010 at 9:17 AM, Hans-Peter Jansen wrote: > While this system usually operates fine, it suffers from delays, that are > displayed in latencytop as: "Writing page to disk:     8425,5 ms": > ftp://urpla.net/lat-8.4sec.png, but we see them also in the 1.7-4.8 sec > range: ftp://urpla.net/lat-1.7sec.png, ftp://urpla.net/lat-2.9sec.png, > ftp://urpla.net/lat-4.6sec.png and ftp://urpla.net/lat-4.8sec.png. > > From other observations, this issue "feels" like it is induced by single > syncronisation points in the block layer, eg. if I create heavy IO load on > one RAID array, say resizing a VMware disk image, it can take up to a > minute to log in by ssh, although the ssh login does not touch this area at > all (different RAID arrays). Note, that the latencytop snapshots above are > made during normal operation, not this kind of load.. > > Might later kernels mitigate this problem? As this is a production system, > that is used 6.5 days a week, I cannot do dangerous experiments, also > switching to 64 bit is a problem due to the legacy stuff described above... > OTOH, my users suffer from this, and anything helping in this respect is > highly appreciated. Seems like a 2.6.32 based kernel which has per-BDI writeback and "CFQ low latency mode" changes might help a good deal. I know that on one of my bigger machines (similar in specs to yours) which has a lot of processes which do a decent amount of IO, latency and load average has gone down after going to a 2.6.32 kernel from a 2.6.31 kernel (Fedora 11 system). Like Chris suggested, I've also heard that using the noop IO scheduler can work well on Areca controllers on some kernels and workloads. It's worth a shot and you can even try changing it at run-time. -Dave