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

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Mon, Feb 27, 2012 at 2:45 PM, Andres Lagar-Cavilla
<andres@xxxxxxxxxxxxxxxx> wrote:
>> 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.

Not exactly -- victim #2 would be read-only disk buffers *which have
not been read recently*.  Buffers which are in active use will not be
evicted.  So although evicting these pages from the guests' cache
doesn't buy the system any more memory, it doesn't have a major impact
on the guest either.

In any case, if the guest experiences its own internal memory
pressure, these pages will be the first to go anyway.  After that, it
will go for trying to evict infrequently-used read-write pages --
which, if the pager is active, will already have been paged out to
disk; thus we'll end up with the double-paging problem.  This will
have a much larger impact on performance than uselessly evicting
little-used read-only pages.

So I think that even though sharing+balloon will lead to some
occasional sub-optimal behavior, it's still a lot better than
sharing+paging and no ballooning.  Remember that ballooning was a
technique introduced in the paper by VMWare that talked about page
sharing -- they obviously thought sharing+ballooning was better than
sharing+paging.

 -George

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