From: Mel Gorman Subject: Re: [patch 1/4] mm: exclude reserved pages from dirtyable memory Date: Wed, 21 Sep 2011 16:03:28 +0100 Message-ID: <20110921150328.GJ4849@suse.de> References: <1316526315-16801-1-git-send-email-jweiner@redhat.com> <1316526315-16801-2-git-send-email-jweiner@redhat.com> <20110921140423.GG4849@suse.de> 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 To: Johannes Weiner Return-path: Content-Disposition: inline In-Reply-To: <20110921140423.GG4849@suse.de> Sender: owner-linux-mm@kvack.org List-Id: linux-ext4.vger.kernel.org On Wed, Sep 21, 2011 at 03:04:23PM +0100, Mel Gorman wrote: > On Tue, Sep 20, 2011 at 03:45:12PM +0200, Johannes Weiner wrote: > > The amount of dirtyable pages should not include the total number of > > free pages: there is a number of reserved pages that the page > > allocator and kswapd always try to keep free. > > > > The closer (reclaimable pages - dirty pages) is to the number of > > reserved pages, the more likely it becomes for reclaim to run into > > dirty pages: > > > > +----------+ --- > > | anon | | > > +----------+ | > > | | | > > | | -- dirty limit new -- flusher new > > | file | | | > > | | | | > > | | -- dirty limit old -- flusher old > > | | | > > +----------+ --- reclaim > > | reserved | > > +----------+ > > | kernel | > > +----------+ > > > > Not treating reserved pages as dirtyable on a global level is only a > > conceptual fix. In reality, dirty pages are not distributed equally > > across zones and reclaim runs into dirty pages on a regular basis. > > > > But it is important to get this right before tackling the problem on a > > per-zone level, where the distance between reclaim and the dirty pages > > is mostly much smaller in absolute numbers. > > > > Signed-off-by: Johannes Weiner > > --- > > include/linux/mmzone.h | 1 + > > mm/page-writeback.c | 8 +++++--- > > mm/page_alloc.c | 1 + > > 3 files changed, 7 insertions(+), 3 deletions(-) > > > > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h > > index 1ed4116..e28f8e0 100644 > > --- a/include/linux/mmzone.h > > +++ b/include/linux/mmzone.h > > @@ -316,6 +316,7 @@ struct zone { > > * sysctl_lowmem_reserve_ratio sysctl changes. > > */ > > unsigned long lowmem_reserve[MAX_NR_ZONES]; > > + unsigned long totalreserve_pages; > > > > This is nit-picking but totalreserve_pages is a poor name because it's > a per-zone value that is one of the lowmem_reserve[] fields instead > of a total. After this patch, we have zone->totalreserve_pages and > totalreserve_pages but are not related to the same thing. > but they are not the same. > As you correctly pointed out to be on IRC, zone->totalreserve_pages is not the lowmem_reserve because it takes the high_wmark into account. Sorry about that, I should have kept thinking. The name is still poor though because it does not explain what the value is or what it means. zone->FOO value needs to be related to lowmem_reserve because this is related to balancing zone usage. zone->FOO value should also be related to the high_wmark because this is avoiding writeback from page reclaim err....... umm... this? /* * When allocating a new page that is expected to be * dirtied soon, the number of free pages and the * dirty_balance reserve are taken into account. The * objective is that the globally allowed number of dirty * pages should be distributed throughout the zones such * that it is very unlikely that page reclaim will call * ->writepage. * * dirty_balance_reserve takes both lowmem_reserve and * the high watermark into account. The lowmem_reserve * is taken into account because we don't want the * distribution of dirty pages to unnecessarily increase * lowmem pressure. The watermark is taken into account * because it's correlated with when kswapd wakes up * and how long it stays awake. */ unsigned long dirty_balance_reserve. -- Mel Gorman SUSE Labs -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org