[Xen-devel] Re: [patch 0/6] Guest page hinting version 7.

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

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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