Re: [PATCH v2 2/2] virtio-balloon spec: reintroduce "silent deflation" feature

["Michael S. Tsirkin" <mst@redhat.com>]

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 <pbonzini@redhat.com>

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?

> ---
>  virtio-spec.lyx | 197 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 197 insertions(+)
> 
> diff --git a/virtio-spec.lyx b/virtio-spec.lyx
> index 5c76a87..5da4fde 100644
> --- a/virtio-spec.lyx
> +++ b/virtio-spec.lyx
> @@ -7267,6 +7267,20 @@ VIRTIO_BALLOON_F_STATS_VQ
>  \end_inset
>  
>  (1) A virtqueue for reporting guest memory statistics is present.
> +\change_inserted 1531152142 1347020627
> +
> +\end_layout
> +
> +\begin_layout Description
> +
> +\change_inserted 1531152142 1347020648
> +VIRTIO_BALLOON_F_SILENT_DEFLATE
> +\begin_inset space ~
> +\end_inset
> +
> +(2) Guest need not tell host before pages from the balloon are used.
> +\change_unchanged
> +
>  \end_layout
>  
>  \end_deeper
> @@ -7627,6 +7641,168 @@ VIRTIO_BALLOON_S_MEMFREE The amount of memory not being used for any purpose
>  VIRTIO_BALLOON_S_MEMTOT The total amount of memory available (in bytes).
>  \end_layout
>  
> +\begin_layout Description
> +
> +\change_inserted 1531152142 1368005515
> +Silent
> +\begin_inset space ~
> +\end_inset
> +
> +deflation
> +\end_layout
> +
> +\begin_layout Standard
> +
> +\change_inserted 1531152142 1368005515
> +
> +\series medium
> +Some implementation of the balloon device may not require the guest to deflate
> + the balloon explicitly; instead, the guest may just take a page from its
> + reserve and start using it.
> + This is called 
> +\begin_inset Quotes eld
> +\end_inset
> +
> +silent deflate
> +\begin_inset Quotes erd
> +\end_inset
> +
> +
> +\end_layout
> +
> +\begin_layout Standard
> +
> +\change_inserted 1531152142 1368005515
> +In order to use this feature effectively, both the guest and the host need
> + to know how the other part intends to use the balloon.
> +\end_layout
> +
> +\begin_layout Standard
> +
> +\change_inserted 1531152142 1368005515
> +
> +\series medium
> +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.
> +\end_layout
> +
> +\begin_layout Standard
> +
> +\change_inserted 1531152142 1368005515
> +Knowing that the guest will 
> +\emph on
> +not
> +\emph default
> + deflate silently also benefits the host.
> + For example, if the host is pinning the guest's memory, it may unpin ballooned
> + pages and pin them again upon deflation.
> + This allows cooperative memory overcommit even if the guest's memory is
> + pinned.
> +\end_layout
> +
> +\begin_layout Standard
> +
> +\change_inserted 1531152142 1368005515
> +Thus, there are two possibilities for the host (either it does not support
> + silent deflate, or it does), and three for the guest (it doesn't need silent
> + deflate, it may choose to use it if available, it requires it).
> + Because there are three possibilities for the guest, support for silent
> + deflate is represented by two different feature bits.
> +\end_layout
> +
> +\begin_layout Standard
> +
> +\change_inserted 1531152142 1368005515
> +The feature bits are used as follows:
> +\end_layout
> +
> +\begin_layout Itemize
> +
> +\change_inserted 1531152142 1368005671
> +if the host does not support silent deflate, it must propose the
> +VIRTIO_\SpecialChar \-
> +BALLOON_F_\SpecialChar \-
> +CAN_\SpecialChar \-
> +TELL_\SpecialChar \-
> +HOST feature.
> + If the guest does 
> +\emph on
> +not
> +\emph default
> + negotiate it, the host should assume that the guest will use silent deflate.
> + A host that does not support silent deflate then must do nothing on attempts
> + of the guest to deflate/inflate the balloon, except put the descriptors on
> + the used ring.
> +\end_layout
> +
> +\begin_layout Itemize
> +
> +\change_inserted 1531152142 1368005515
> +a guest that 
> +\emph on
> +must
> +\emph default
> + use silent deflation does not need to negotiate any feature.
> +\end_layout
> +
> +\begin_layout Itemize
> +
> +\change_inserted 1531152142 1368005676
> +a guest that 
> +\emph on
> +will never
> +\emph default
> + use silent deflation only has to propose VIRTIO_\SpecialChar \-
> +BALLOON_F_\SpecialChar \-
> +CAN_\SpecialChar \-
> +TELL_\SpecialChar \-
> +HOST.
> + It need not do anything if the host does not negotiate the feature.
> +\end_layout
> +
> +\begin_layout Itemize
> +
> +\change_inserted 1531152142 1368005686
> +a guest that 
> +\emph on
> +can optionally
> +\emph default
> + use silent deflation should propose both VIRTIO_\SpecialChar \-
> +BALLOON_F_\SpecialChar \-
> +CAN_\SpecialChar \-
> +TELL_\SpecialChar \-
> +HOST
> + and VIRTIO_\SpecialChar \-
> +BALLOON_F_\SpecialChar \-
> +SILENT_\SpecialChar \-
> +DEFLATE.
> + The guest driver can then use silent deflation if and only if the host
> + has negotiated VIRTIO_\SpecialChar \-
> +BALLOON_F_\SpecialChar \-
> +SILENT_\SpecialChar \-
> +DEFLATE too.
> +\end_layout
> +
> +\begin_layout Standard
> +
> +\change_inserted 1531152142 1368005690
> +Hosts are free to propose the VIRTIO_\SpecialChar \-
> +BALLOON_F_\SpecialChar \-
> +CAN_\SpecialChar \-
> +TELL_\SpecialChar \-
> +HOST feature even if they support silent deflation.
> + It will
> + not affect operation of a well-behaving guest; such a guest
> + should never check if the host proposed the feature, and
> + should not assume that lack of this feature implies support
> + for silent deflation. The only bit to check for this purpose is
> + VIRTIO_\SpecialChar \-
> +BALLOON_F_\SpecialChar \-
> +SILENT_\SpecialChar \-
> +DEFLATE.
> +\end_layout
> +
>  \begin_layout Chapter*
>  Appendix H: Rpmsg: Remote Processor Messaging
>  \end_layout
> -- 
> 1.8.2.1