Re: [Xen-devel] [PATCH] RFC: initial libxl support for xenpaging

["Andres Lagar-Cavilla" <andres@xxxxxxxxxxxxxxxx>]

> On Fri, 2012-02-24 at 17:12 +0000, Andres Lagar-Cavilla wrote:
>> Sharing + balloon is mostly a bad idea. It's not forbidden or broken
>> from the hypervisor angle, but it's a lose-lose game. Ballooning a
>> shared page gains you nothing. The ballooner can't know what is shared
>> and what isn't, in order to make an educated decision. And the sharer
>> can't foresee what the balloon will victimize.
>
> Doesn't ballooning generally evict unused pages, because it allocates
> new _free_ pages, even if they were shared is there any benefit to doing
> so?

Certainly the balloon will pick free pages. The sharing daemon should not
have shared those, but it's not unlikely that it will have.

It's a classic semantic gap problem, and we were discussing this in the
context of paging ("the pager should not have paged out page table pages")

Beyond free pages, a prime target for sharing are read-only disk buffers
in the page cache. Those are victim #2 for the balloon.

>
> Or is it more of a second order effect, e.g. ballooning can cause the
> guest to swap out stuff which could have been shared?
>

Absolutely.

Andres

> Ian.
>
>



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