From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail143.messagelabs.com (mail143.messagelabs.com [216.82.254.35]) by kanga.kvack.org (Postfix) with SMTP id 566956B01B0 for ; Mon, 24 May 2010 15:51:03 -0400 (EDT) Message-ID: <4BFAD899.4020909@redhat.com> Date: Mon, 24 May 2010 15:50:49 -0400 From: Ric Wheeler MIME-Version: 1.0 Subject: Re: RFC: dirty_ratio back to 40% References: <4BF51B0A.1050901@redhat.com> <20100521083408.1E36.A69D9226@jp.fujitsu.com> <4BF5D875.3030900@acm.org> In-Reply-To: <4BF5D875.3030900@acm.org> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Zan Lynx Cc: KOSAKI Motohiro , lwoodman@redhat.com, LKML , linux-mm , Nick Piggin , Jan Kara List-ID: On 05/20/2010 08:48 PM, Zan Lynx wrote: > On 5/20/10 5:48 PM, KOSAKI Motohiro wrote: >> Hi >> >> CC to Nick and Jan >> >>> We've seen multiple performance regressions linked to the lower(20%) >>> dirty_ratio. When performing enough IO to overwhelm the background >>> flush daemons the percent of dirty pagecache memory quickly climbs >>> to the new/lower dirty_ratio value of 20%. At that point all writing >>> processes are forced to stop and write dirty pagecache pages back to >>> disk. >>> This causes performance regressions in several benchmarks as well as >>> causing >>> a noticeable overall sluggishness. We all know that the dirty_ratio is >>> an integrity vs performance trade-off but the file system journaling >>> will cover any devastating effects in the event of a system crash. >>> >>> Increasing the dirty_ratio to 40% will regain the performance loss seen >>> in several benchmarks. Whats everyone think about this??? >> >> In past, Jan Kara also claim the exactly same thing. >> >> Subject: [LSF/VM TOPIC] Dynamic sizing of dirty_limit >> Date: Wed, 24 Feb 2010 15:34:42 +0100 >> >> > (*) We ended up increasing dirty_limit in SLES 11 to 40% as it >> used to be >> > with old kernels because customers running e.g. LDAP (using BerkelyDB >> > heavily) were complaining about performance problems. >> >> So, I'd prefer to restore the default rather than both Redhat and >> SUSE apply exactly >> same distro specific patch. because we can easily imazine other users >> will face the same >> issue in the future. > > On desktop systems the low dirty limits help maintain interactive > feel. Users expect applications that are saving data to be slow. They > do not like it when every application in the system randomly comes to > a halt because of one program stuffing data up to the dirty limit. > > The cause and effect for the system slowdown is clear when the dirty > limit is low. "I saved data and now the system is slow until it is > done." When the dirty page ratio is very high, the cause and effect is > disconnected. "I was just web surfing and the system came to a halt." > > I think we should expect server admins to do more tuning than desktop > users, so the default limits should stay low in my opinion. > Have you done any performance testing that shows this? A laptop the smaller default would spin up drives more often and greatly decrease your battery life. Note that both SLES and RHEL default away from the upstream default. Ric -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org