Re: Need info on guest swapping and other things in KVM

[Avi Kivity <avi@qumranet.com>]

Please don't top-post.

Arjun wrote:
> Avi,
> Thanks for your response.
>
> Regarding the query on "Guest Swapping", I'm referring to the 
> mechanism that the KVM host can use to swap out a guest's pages. Since 
> a guest OS will have its own swapping mechanism, then how will the 
> host ensure that if it choses to swap out a guest's page, it will not 
> conflict with the guest's own swapping mechanism.
> Put another way: I understand (from docs, mailing list etc.) that KVM 
> allows the host to overbook/overcommit physical memory via the guest 
> swapping mechanism. i.e. the guest thinks it has x number of physical 
> pages but actually the host has swapped out some of the guests pages 
> and given the freed up physical memory to another guest VM. So the 
> guest actually has less than x pages in physical memory but doesn't 
> know it.
>

The guest and host swapping algorithms are completely independent.  It 
is quite possible for the host to select an incorrect page (from a 
performance point of view) for swapping.

There is a proposal (search for "CMM2") to allow cooperation between 
guest and host in this area.

-- 
Any sufficiently difficult bug is indistinguishable from a feature.


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