linux-kernel - Re: [patch 0/6] Guest page hinting version 7.

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Date:	Thu, 02 Apr 2009 13:34:37 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Rik van Riel <riel@...hat.com>
CC:	Martin Schwidefsky <schwidefsky@...ibm.com>, akpm@...l.org,
	Nick Piggin <nickpiggin@...oo.com.au>, frankeh@...son.ibm.com,
	virtualization@...ts.osdl.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, linux-mm@...ck.org,
	hugh@...itas.com, Xen-devel <xen-devel@...ts.xensource.com>
Subject: Re: [patch 0/6] Guest page hinting version 7.

Rik van Riel wrote:
> Jeremy Fitzhardinge wrote:
>> The more complex host policy decisions of how to balance overall 
>> memory use system-wide are much in the same for both mechanisms.
> Not at all.  Page hinting is just an optimization to host swapping, where
> IO can be avoided on many of the pages that hit the end of the LRU.
>
> No decisions have to be made at all about balancing memory use
> between guests, it just happens through regular host LRU aging.

When the host pages out a page belonging to guest A, then its making a 
policy decision on how large guest A should be compared to B.  If the 
policy is a global LRU on all guest pages, then that's still a policy on 
guest sizes: the target size is a function of its working set, assuming 
that the working set is well modelled by LRU.  I imagine that if the 
guest and host are both managing their pages with an LRU-like algorithm 
you'll get some nasty interactions, which page hinting tries to alleviate.

> Automatic ballooning requires that something on the host figures
> out how much memory each guest needs and sizes the guests
> appropriately.  All the proposed policies for that which I have
> seen have some nasty corner cases or are simply very limited
> in scope.

Well, you could apply something equivalent to a global LRU: ask for more 
pages from guests who have the most unused pages.  (I'm not saying that 
its necessarily a useful policy.)

    J
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