linux-kernel - Re: [PATCH 5/7] mm: vmscan: Do not writeback filesystem pages in kswapd except in high priority

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Date:	Thu, 11 Aug 2011 14:18:54 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mgorman@...e.de>
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>,
	Minchan Kim <minchan.kim@...il.com>
Subject: Re: [PATCH 5/7] mm: vmscan: Do not writeback filesystem pages in
 kswapd except in high priority

On 08/10/2011 06:47 AM, Mel Gorman wrote:
> It is preferable that no dirty pages are dispatched for cleaning from
> the page reclaim path. At normal priorities, this patch prevents kswapd
> writing pages.
>
> However, page reclaim does have a requirement that pages be freed
> in a particular zone. If it is failing to make sufficient progress
> (reclaiming<  SWAP_CLUSTER_MAX at any priority priority), the priority
> is raised to scan more pages. A priority of DEF_PRIORITY - 3 is
> considered to be the point where kswapd is getting into trouble
> reclaiming pages. If this priority is reached, kswapd will dispatch
> pages for writing.
>
> Signed-off-by: Mel Gorman<mgorman@...e.de>
> Reviewed-by: Minchan Kim<minchan.kim@...il.com>

My only worry with this patch is that maybe we'll burn too
much CPU time freeing pages from a zone.  However, chances
are we'll have freed pages from other zones when scanning
one zone multiple times (the page cache dirty limit is global,
the clean pages have to be _somewhere_).

Since the bulk of the allocators are not too picky about
which zone they get their pages from, I suspect this patch
will be an overall improvement pretty much all the time.

Acked-by: Rik van Riel <riel@...hat.com>
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