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Date: Mon, 31 Aug 2009 16:09:37 -0400
From: Rik van Riel <riel@redhat.com>
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To: "Dike, Jeffrey G" <jeffrey.g.dike@intel.com>
CC: "balbir@linux.vnet.ibm.com" <balbir@linux.vnet.ibm.com>,
       "Wu, Fengguang" <fengguang.wu@intel.com>, Avi Kivity <avi@redhat.com>,
       Andrea Arcangeli <aarcange@redhat.com>,
       "Yu, Wilfred" <wilfred.yu@intel.com>,
       "Kleen, Andi" <andi.kleen@intel.com>,
       Hugh Dickins <hugh.dickins@tiscali.co.uk>,
       Andrew Morton <akpm@linux-foundation.org>,
       Christoph Lameter <cl@linux-foundation.org>,
       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       Mel Gorman <mel@csn.ul.ie>, LKML <linux-kernel@vger.kernel.org>,
       linux-mm <linux-mm@kvack.org>
Subject: Re: [RFC] respect the referenced bit of KVM guest pages?
References: <4A7AAE07.1010202@redhat.com> <20090806102057.GQ23385@random.random> <20090806105932.GA1569@localhost> <4A7AC201.4010202@redhat.com> <20090806130631.GB6162@localhost> <4A7AD79E.4020604@redhat.com> <20090816032822.GB6888@localhost> <4A878377.70502@redhat.com> <20090816045522.GA13740@localhost> <9EECC02A4CC333418C00A85D21E89326B6611F25@azsmsx502.amr.corp.intel.com> <20090821182439.GN29572@balbir.in.ibm.com> <9EECC02A4CC333418C00A85D21E8932601841832F9@azsmsx502.amr.corp.intel.com> <4A9C2A17.3080802@redhat.com> <9EECC02A4CC333418C00A85D21E893260184183339@azsmsx502.amr.corp.intel.com>
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Dike, Jeffrey G wrote:
>> This will be because the VM does not start aging pages
>> from the active to the inactive list unless there is
>> some memory pressure.
> 
> Which is the reason I gave the VM a puny amount of memory. 
 > We know the thing is under memory pressure because I've been
 > complaining about page discards.

Page discards by the host, which are invisible to the guest
OS.

The guest OS thinks it has enough pages.  The host disagrees
and swaps out some guest memory.