From: Mel Gorman Subject: Re: [patch 2/2/4] mm: try to distribute dirty pages fairly across zones Date: Wed, 28 Sep 2011 10:36:29 +0100 Message-ID: <20110928093629.GE11313@suse.de> References: <1316526315-16801-1-git-send-email-jweiner@redhat.com> <1316526315-16801-3-git-send-email-jweiner@redhat.com> <20110921160226.1bf74494.akpm@google.com> <20110922085242.GA29046@redhat.com> <20110923144248.GC2606@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Cc: Andrew Morton , Christoph Hellwig , Dave Chinner , Wu Fengguang , Jan Kara , Rik van Riel , Minchan Kim , Chris Mason , Theodore Ts'o , Andreas Dilger , xfs@oss.sgi.com, linux-btrfs@vger.kernel.org, linux-ext4@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Andrew Morton To: Johannes Weiner Return-path: Received: from cantor2.suse.de ([195.135.220.15]:45083 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751434Ab1I1Jge (ORCPT ); Wed, 28 Sep 2011 05:36:34 -0400 Content-Disposition: inline In-Reply-To: <20110923144248.GC2606@redhat.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Fri, Sep 23, 2011 at 04:42:48PM +0200, Johannes Weiner wrote: > The maximum number of dirty pages that exist in the system at any time > is determined by a number of pages considered dirtyable and a > user-configured percentage of those, or an absolute number in bytes. > > This number of dirtyable pages is the sum of memory provided by all > the zones in the system minus their lowmem reserves and high > watermarks, so that the system can retain a healthy number of free > pages without having to reclaim dirty pages. > > But there is a flaw in that we have a zoned page allocator which does > not care about the global state but rather the state of individual > memory zones. And right now there is nothing that prevents one zone > from filling up with dirty pages while other zones are spared, which > frequently leads to situations where kswapd, in order to restore the > watermark of free pages, does indeed have to write pages from that > zone's LRU list. This can interfere so badly with IO from the flusher > threads that major filesystems (btrfs, xfs, ext4) mostly ignore write > requests from reclaim already, taking away the VM's only possibility > to keep such a zone balanced, aside from hoping the flushers will soon > clean pages from that zone. > > Enter per-zone dirty limits. They are to a zone's dirtyable memory > what the global limit is to the global amount of dirtyable memory, and > try to make sure that no single zone receives more than its fair share > of the globally allowed dirty pages in the first place. As the number > of pages considered dirtyable exclude the zones' lowmem reserves and > high watermarks, the maximum number of dirty pages in a zone is such > that the zone can always be balanced without requiring page cleaning. > > As this is a placement decision in the page allocator and pages are > dirtied only after the allocation, this patch allows allocators to > pass __GFP_WRITE when they know in advance that the page will be > written to and become dirty soon. The page allocator will then > attempt to allocate from the first zone of the zonelist - which on > NUMA is determined by the task's NUMA memory policy - that has not > exceeded its dirty limit. > > At first glance, it would appear that the diversion to lower zones can > increase pressure on them, but this is not the case. With a full high > zone, allocations will be diverted to lower zones eventually, so it is > more of a shift in timing of the lower zone allocations. Workloads > that previously could fit their dirty pages completely in the higher > zone may be forced to allocate from lower zones, but the amount of > pages that 'spill over' are limited themselves by the lower zones' > dirty constraints, and thus unlikely to become a problem. > > For now, the problem of unfair dirty page distribution remains for > NUMA configurations where the zones allowed for allocation are in sum > not big enough to trigger the global dirty limits, wake up the flusher > threads and remedy the situation. Because of this, an allocation that > could not succeed on any of the considered zones is allowed to ignore > the dirty limits before going into direct reclaim or even failing the > allocation, until a future patch changes the global dirty throttling > and flusher thread activation so that they take individual zone states > into account. > > Signed-off-by: Johannes Weiner Acked-by: Mel Gorman -- Mel Gorman SUSE Labs