On Wed, May 29, 2013 at 10:47:08AM +0200, Paolo Bonzini wrote:
> Il 29/05/2013 09:49, Michael S. Tsirkin ha scritto:
> > On Tue, May 28, 2013 at 07:40:18PM +0200, Paolo Bonzini wrote:
> >> The original idea of the VIRTIO_BALLOON_F_MUST_TELL_HOST feature was to
> >> let drivers skip usage of the deflate queue when leaking the balloon
> >> ("silent deflation"). Guests may benefit from silent deflate by
> >> aggressively inflating the balloon; they know that they will be able to
> >> use ballooned pages without issuing a (blocking) request to the device.
> >>
> >> The previous patch redefined the feature to ensure correctness of the
> >> operation when drivers do not correctly report deflation. This patch
> >> adds back the optimization.
> >>
> >> The new feature bit is for the host to tell the drivers if silent
> >> deflation is actually supported. The meaning of the feature bit is
> >> reversed compared to the original, because the original meaning was
> >> not safe against migration.
> >>
> >> For features to be safe against migration, they have to be defined as
> >> "this is true if the guest _can_ do X". For such a "positive" feature,
> >> migration is possible if the destination supports it, or the source
> >> didn't set it:
> >>
> >> dest support source set ok?
> >> T T T
> >> T F T
> >> F T F
> >> F F T
> >>
> >> Instead, the old feature was defined as "this is true if the guest
> >> _cannot_ do X". For such a "negative" feature, migration is possible
> >> if the destination supports it, or the source sets it:
> >>
> >> dest support source set ok?
> >> T T T
> >> T F F
> >> F T T
> >> F F T
> >>
> >> However, the negotiated features are supposed to be the AND of the
> >> device- and driver-supported features. In the F/T case, the feature
> >> would be negotiated by the source as T, and become F when negotiated on
> >> the destination.
> >>
> >> Signed-off-by: Paolo Bonzini <[email protected]>
> >
> > Do you have any numbers showing how this new feature improves
> > performance?
> > We are able to batch quite a lot of pages in a single deflate
> > request - is the overhead measureable in practice?
>
> It's not only about better times, but also about better algorithms. I
> started writing this after seeing the Google fileballoon driver. For
> that implementation, the deflateq cannot be used at all.
>
> Paolo
It seems weak to claim the algorithm is better without trying it
out in practice.
I asked the fileballoon driver author whether
it can use TELL_HOST or if that will be too much overhead.
He said don't know.
And in particular, I think a feature bit is a bad fit for deciding how
page will be reclaimed. Whether you will want a page back soon can
change over time depending on load etc. So if we can't just tell host
first always, then I think we need to give guest control to do this per
inflate command.
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