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[Xen-devel] 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.)


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