linux-kernel - Re: Over-eager swapping

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Message-ID: <20100819102055.GK2370@arachsys.com>
Date:	Thu, 19 Aug 2010 11:20:55 +0100
From:	Chris Webb <chris@...chsys.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
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

Christoph Lameter <cl@...ux-foundation.org> writes:

> On Wed, 18 Aug 2010, Chris Webb wrote:
> 
> > > != 0.  And even then, zone reclaim should only reclaim file pages, not
> > > anon.  In theory...
> >
> > Hi. This is zero on all our machines:
> >
> > # sysctl vm.zone_reclaim_mode
> > vm.zone_reclaim_mode = 0
> 
> Set it to 1.

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?

Cheers,

Chris.
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