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

  • To: "Ian Campbell" <Ian.Campbell@xxxxxxxxxx>
  • From: "Andres Lagar-Cavilla" <andres@xxxxxxxxxxxxxxxx>
  • Date: Mon, 27 Feb 2012 06:45:38 -0800
  • Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>, Olaf Hering <olaf@xxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Mon, 27 Feb 2012 14:46:09 +0000
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> 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?



> Ian.

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