linux-ext4 - Re: [patch 1/4] mm: exclude reserved pages from dirtyable memory

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Date:	Wed, 21 Sep 2011 16:03:28 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Johannes Weiner <jweiner@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@...radead.org>,
	Dave Chinner <david@...morbit.com>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Rik van Riel <riel@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Chris Mason <chris.mason@...cle.com>,
	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>, xfs@....sgi.com,
	linux-btrfs@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 1/4] mm: exclude reserved pages from dirtyable memory

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 <jweiner@...hat.com>
> > ---
> >  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
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