linux-kernel - Re: [patch 10/10] mm: workingset: keep shadow entries in check

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Date:	Mon, 3 Jun 2013 19:15:58 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	linux-mm@...ck.org, Andi Kleen <andi@...stfloor.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Thelen <gthelen@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Hugh Dickins <hughd@...gle.com>, Jan Kara <jack@...e.cz>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mel Gorman <mgorman@...e.de>,
	Minchan Kim <minchan.kim@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Michel Lespinasse <walken@...gle.com>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Roman Gushchin <klamm@...dex-team.ru>,
	metin d <metdos@...oo.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [patch 10/10] mm: workingset: keep shadow entries in check

On Mon, Jun 03, 2013 at 11:20:32AM -0400, Johannes Weiner wrote:
> On Mon, Jun 03, 2013 at 10:25:33AM +0200, Peter Zijlstra wrote:
> > On Thu, May 30, 2013 at 02:04:06PM -0400, Johannes Weiner wrote:
> > > Previously, page cache radix tree nodes were freed after reclaim
> > > emptied out their page pointers.  But now reclaim stores shadow
> > > entries in their place, which are only reclaimed when the inodes
> > > themselves are reclaimed.  This is problematic for bigger files that
> > > are still in use after they have a significant amount of their cache
> > > reclaimed, without any of those pages actually refaulting.  The shadow
> > > entries will just sit there and waste memory.  In the worst case, the
> > > shadow entries will accumulate until the machine runs out of memory.
> > > 
> > 
> > Can't we simply prune all refault entries that have a distance larger
> > than the memory size? Then we must assume that no refault entry means
> > its too old, which I think is a fair assumption.
> 
> Two workloads bound to two nodes might not push pages through the LRUs
> at the same pace, so a distance might be bigger than memory due to the
> faster moving node, yet still be a hit in the slower moving one.  We
> can't really know until we evaluate it on a per-zone basis.

But wasn't patch 1 of this series about making sure each zone is scanned
proportionally to its size?

But given that, sure maybe 1 memory size is a bit strict, but surely we
can put a limit on things at about 2 memory sizes?



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