From: Johannes Weiner Subject: Re: [patch 1/4] mm: exclude reserved pages from dirtyable memory Date: Thu, 22 Sep 2011 11:03:26 +0200 Message-ID: <20110922090326.GB29046@redhat.com> References: <1316526315-16801-1-git-send-email-jweiner@redhat.com> <1316526315-16801-2-git-send-email-jweiner@redhat.com> <20110921140423.GG4849@suse.de> <20110921150328.GJ4849@suse.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii 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: Mel Gorman Return-path: Content-Disposition: inline In-Reply-To: <20110921150328.GJ4849@suse.de> Sender: owner-linux-mm@kvack.org List-Id: linux-ext4.vger.kernel.org On Wed, Sep 21, 2011 at 04:03:28PM +0100, Mel Gorman wrote: > 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. Yes, that's much better, thanks. I assume this is meant the same for both the zone and the global level and we should not mess with totalreserve_pages in either case? -- 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