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

[Michael Roth <mdroth@linux.vnet.ibm.com>]

On Fri, Feb 17, 2012 at 03:16:22PM -0200, Luiz Capitulino wrote:
> 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)?

Probably not, but then again you'd only need like 1-second granularity.
Also, I think we can do away with the polling once async QMP is in
place, so we wouldn't be stuck with it necessarilly.

> 
> 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.

The downside there is you could read some really stale data that way,
to the point where any app that really cared would likely throw out the first
result.

> 
> > 
> > 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
> > 
> 
>