linux-kernel - Re: [patch 3/3] vmscan: detect mapped file pages used only once

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Message-ID: <4B82EA4D.8040305@redhat.com>
Date:	Mon, 22 Feb 2010 15:34:21 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Johannes Weiner <hannes@...xchg.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Minchan Kim <minchan.kim@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 3/3] vmscan: detect mapped file pages used only once

On 02/22/2010 02:49 PM, Johannes Weiner wrote:
> The VM currently assumes that an inactive, mapped and referenced file
> page is in use and promotes it to the active list.
>
> However, every mapped file page starts out like this and thus a problem
> arises when workloads create a stream of such pages that are used only
> for a short time.  By flooding the active list with those pages, the VM
> quickly gets into trouble finding eligible reclaim canditates.  The
> result is long allocation latencies and eviction of the wrong pages.
>
> This patch reuses the PG_referenced page flag (used for unmapped file
> pages) to implement a usage detection that scales with the speed of
> LRU list cycling (i.e. memory pressure).
>
> If the scanner encounters those pages, the flag is set and the page
> cycled again on the inactive list.  Only if it returns with another
> page table reference it is activated.  Otherwise it is reclaimed as
> 'not recently used cache'.
>
> This effectively changes the minimum lifetime of a used-once mapped
> file page from a full memory cycle to an inactive list cycle, which
> allows it to occur in linear streams without affecting the stable
> working set of the system.
>
> Signed-off-by: Johannes Weiner<hannes@...xchg.org>

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

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