linux-kernel - Re: [PATCH 8/8] mm: vmscan: Do not writeback filesystem pages from kswapd

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Date:	Fri, 22 Jul 2011 14:31:38 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
	XFS <xfs@....sgi.com>, Dave Chinner <david@...morbit.com>,
	Christoph Hellwig <hch@...radead.org>,
	Johannes Weiner <jweiner@...hat.com>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Rik van Riel <riel@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>
Subject: Re: [PATCH 8/8] mm: vmscan: Do not writeback filesystem pages from
 kswapd

On Fri, Jul 22, 2011 at 02:57:12PM +0200, Peter Zijlstra wrote:
> On Thu, 2011-07-21 at 17:28 +0100, Mel Gorman wrote:
> > Assuming that flusher threads will always write back dirty pages promptly
> > then it is always faster for reclaimers to wait for flushers. This patch
> > prevents kswapd writing back any filesystem pages. 
> 
> That is a somewhat sort changelog for such a big assumption ;-)
> 

That is an understatement but the impact of the patch is discussed in
detail in the leader. On NUMA, this patch has a negative impact so
I put no effort into the changelog. The patch is part of the series
because it was specifically asked for.

> I think it can use a few extra words to explain the need to clean pages
> from @zone vs writeback picks whatever fits best on disk and how that
> works out wrt the assumption.
> 

At the time of writing the changelog, I knew that flushers were
not finding pages from the correct zones quickly enough in the NUMA
usecase. The changelog documents the assumptions testing shows them to
be false.

> What requirements does this place on writeback and how does it meet
> them.

It places a requirement on writeback to prioritise pages from zones
under memory pressure. It doesn't meet them. I mention in the leader
that I think patch 8 should be dropped which is why the changelog
sucks.

-- 
Mel Gorman
SUSE Labs
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