From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752307Ab0CBKEU (ORCPT ); Tue, 2 Mar 2010 05:04:20 -0500 Received: from gir.skynet.ie ([193.1.99.77]:52891 "EHLO gir.skynet.ie" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751563Ab0CBKES (ORCPT ); Tue, 2 Mar 2010 05:04:18 -0500 Date: Tue, 2 Mar 2010 10:04:02 +0000 From: Mel Gorman To: Nick Piggin Cc: Christian Ehrhardt , Andrew Morton , "linux-kernel@vger.kernel.org" , epasch@de.ibm.com, SCHILLIG@de.ibm.com, Martin Schwidefsky , Heiko Carstens , christof.schmitt@de.ibm.com, thoss@de.ibm.com, hare@suse.de, gregkh@novell.com Subject: Re: Performance regression in scsi sequential throughput (iozone) due to "e084b - page-allocator: preserve PFN ordering when __GFP_COLD is set" Message-ID: <20100302100402.GH3852@csn.ul.ie> References: <20100212100519.GA29085@laptop> <4B796C6D.80800@linux.vnet.ibm.com> <20100216112517.GE1194@csn.ul.ie> <4B7ACC1E.9080205@linux.vnet.ibm.com> <4B7BBCFC.4090101@linux.vnet.ibm.com> <20100218114310.GC32626@csn.ul.ie> <4B7D664C.20507@linux.vnet.ibm.com> <4B7E73BF.5030901@linux.vnet.ibm.com> <20100219151934.GA1445@csn.ul.ie> <20100302065225.GC8653@laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20100302065225.GC8653@laptop> User-Agent: Mutt/1.5.17+20080114 (2008-01-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 02, 2010 at 05:52:25PM +1100, Nick Piggin wrote: > On Fri, Feb 19, 2010 at 03:19:34PM +0000, Mel Gorman wrote: > > On Fri, Feb 19, 2010 at 12:19:27PM +0100, Christian Ehrhardt wrote: > > > Eventually it might come down to a discussion of allocation priorities and > > > we might even keep them as is and accept this issue - I still would prefer > > > a good second chance implementation, other page cache allocation flags or > > > something else that explicitly solves this issue. > > > > > > > In that line, the patch that replaced congestion_wait() with a waitqueue > > makes some sense. > > > > > Mel's patch that replaces congestion_wait with a wait for the zone watermarks > > > becoming available again is definitely a step in the right direction and > > > should go into upstream and the long term support branches. > > > > I'll need to do a number of tests before I can move that upstream but I > > don't think it's a merge candidate. Unfortunately, I'll be offline for a > > week starting tomorrow so I won't be able to do the testing. > > > > When I get back, I'll revisit those patches with the view to pushing > > them upstream. I hate to treat symptoms here without knowing the > > underlying problem but this has been spinning in circles for ages with > > little forward progress :( > > The zone pressure waitqueue patch makes sense. I've just started the rebase and considering what sort of test is best for it. > We may even want to make > it more strictly FIFO (eg. check upfront if there are waiters on the > queue before allocating a page, and if yes then add ourself to the back > of the waitqueue). To be really strict about this, we'd have to check in the hot-path of the per-cpu allocator which would be undesirable. We could check further in the slow-path but I bet it'd be very rare that the logic would be triggered. For a process to enter the FIFO due to waiters that were not yet woken up, the system would have to be a) under heavy memory pressure b) reclaim taking such a long time that check_zone_pressure() is not being called in time and c) a process exiting or otherwise freeing memory such that the watermarks are cleared without reclaim being involved. This seems overkill but maybe you have a simplier case in mind? > And also possibly even look at doing the wakeups in > the page-freeing path. Although that might start adding too much > overhead, so it's quite possible your sloppy-but-lighter timeout > approach is preferable. > That's how I felt about it. I was going to put another check_zone_pressure() check after a pcp drain but thought it was too expensive. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab