linux-kernel - Re: [patch] mm: vmscan: do not swap anon pages just because free+file is low

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Message-ID: <53232901.5030307@redhat.com>
Date:	Fri, 14 Mar 2014 12:06:25 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] mm: vmscan: do not swap anon pages just because free+file
 is low

On 03/14/2014 11:35 AM, Johannes Weiner wrote:
> Page reclaim force-scans / swaps anonymous pages when file cache drops
> below the high watermark of a zone in order to prevent what little
> cache remains from thrashing.
> 
> However, on bigger machines the high watermark value can be quite
> large and when the workload is dominated by a static anonymous/shmem
> set, the file set might just be a small window of used-once cache.  In
> such situations, the VM starts swapping heavily when instead it should
> be recycling the no longer used cache.
> 
> This is a longer-standing problem, but it's more likely to trigger
> after 81c0a2bb515f ("mm: page_alloc: fair zone allocator policy")
> because file pages can no longer accumulate in a single zone and are
> dispersed into smaller fractions among the available zones.
> 
> To resolve this, do not force scan anon when file pages are low but
> instead rely on the scan/rotation ratios to make the right prediction.

I am not entirely sure that the scan/rotation ratio will be
meaningful when the page cache has been essentially depleted,
but on larger systems the distance between the low and high
watermark is gigantic, and I have no better idea on how to
fix the bug you encountered, so ...

> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> Cc: <stable@...nel.org> [3.12+]

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

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