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

[Luiz Capitulino]

On Fri, 17 Feb 2012 15:51:33 -0600
Michael Roth <address@hidden> wrote:

> On Fri, Feb 17, 2012 at 03:16:22PM -0200, Luiz Capitulino wrote:
> > On Fri, 17 Feb 2012 10:55:41 -0600
> > Anthony Liguori <address@hidden> 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<address@hidden>
> > > 
> > > 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.

I've talked with Anthony by irc about the implementation details of this and
it will be possible to enable/disable the polling, so this is not an issue
anymore.

> 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.

This is what this series does :) I don't think it's necessary to wait for
async support, we're accepting ad-hoc async mechanisms for other commands and
could use one here too if needed.

> > 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.

Having stale data will be possible with any timer based polling...