Date: Thu, 19 Aug 2010 14:03:42 -0500 (CDT)
From: Christoph Lameter <cl@linux-foundation.org>
To: Chris Webb <chris@arachsys.com>
cc: Lee Schermerhorn <Lee.Schermerhorn@hp.com>,
        Wu Fengguang <fengguang.wu@intel.com>,
        Minchan Kim <minchan.kim@gmail.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        Pekka Enberg <penberg@cs.helsinki.fi>,
        Andi Kleen <andi@firstfloor.org>
Subject: Re: Over-eager swapping
In-Reply-To: <20100819102055.GK2370@arachsys.com>
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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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