linux-kernel - Re: Over-eager swapping

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Message-ID: <alpine.DEB.2.00.1008191402050.1839@router.home>
Date:	Thu, 19 Aug 2010 14:03:42 -0500 (CDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Chris Webb <chris@...chsys.com>
cc:	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Minchan Kim <minchan.kim@...il.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: Over-eager swapping

On Thu, 19 Aug 2010, Chris Webb wrote:

> I tried this on a handful of the problem hosts before re-adding their swap.
> One of them now runs without dipping into swap. The other three I tried had
> the same behaviour of sitting at zero swap usage for a while, before
> suddenly spiralling up with %wait going through the roof. I had to swapoff
> on them to bring them back into a sane state. So it looks like it helps a
> bit, but doesn't cure the problem.
>
> I could definitely believe an explanation that we're swapping in preference
> to allocating remote zone pages somehow, given the imbalance in free memory
> between the nodes which we saw. However, I read the documentation for
> vm.zone_reclaim_mode, which suggests to me that when it was set to zero,
> pages from remote zones should be allocated automatically in preference to
> swap given that zone_reclaim_mode & 4 == 0?

If zone reclaim is off then pages from other nodes will be allocated if a
node is filled up with page cache.

zone reclaim typically only evicts clean page cache pages in order to keep
the additional overhead down. Enabling swapping allows a more aggressive
form of recovering memory in preference of going off line.

The VM should work fine even without zone reclaim.
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