Re: [Qemu-devel] [PATCH 6/6] qmp: add balloon-get-memory-stats & event

[Luiz Capitulino <lcapitulino@redhat.com>]

On Fri, 17 Feb 2012 10:55:41 -0600
Anthony Liguori <aliguori@us.ibm.com> wrote:

> On 02/08/2012 02:30 PM, Luiz Capitulino wrote:
> > This commit adds a QMP API for the guest provided memory statistics
> > (long disabled by commit 07b0403dfc2b2ac179ae5b48105096cc2d03375a).
> >
> > The approach taken by the original commit
> > (625a5befc2e3200b396594f002218d235e375da5) was to extend the
> > query-balloon command. It introduced a severe bug though: query-balloon
> > would hang if the guest didn't respond.
> >
> > The approach taken by this commit is asynchronous and thus avoids
> > any QMP hangs.
> >
> > First, a client has to issue the balloon-get-memory-stats command.
> > That command gets the process started by only sending a request to
> > the guest, it doesn't block. When the memory stats are made available
> > by the guest, they are returned to the client as an QMP event.
> >
> > Signed-off-by: Luiz Capitulino<lcapitulino@redhat.com>
> 
> Do we need this to be stable in 1.1?

Well, this is disabled for a long time already and libvirt needs it, so I'd
say asap, but isn't it possible to implement this with current QOM?

> We can do this pretty nicely through QOM.  We can have a polling property in the 
> virtio-balloon driver, that when set, will enable the virtio-balloon device to 
> poll the guest for statistics.
>
> 
> We can also have properties for each of the memory statistics and a timestamp 
> for when the last update was.
> 
> I think this is a friendlier approach for clients, and a cleaner approach from a 
> QEMU perspective.

I agree it's friendlier, but is it a good idea to keep polling the guest for
something that may never be needed by a mngt app (real question)?

We could allow the mngt app to do the polling by adding a query-balloon-stats
command (instead of balloon-get-memory-stats & event). This command could
return the latest available stats if any (with a timestamp) and query the
guest for new stats.

> 
> There's nothing generic about this functionality.  It's extremely specific to 
> virtio-balloon.  We just lacked ways to expose device specific function pre-QOM.
> 
> Regards,
> 
> Anthony Liguori
>