[Qemu-devel] Re: [RFD] virtio: Add memory statistics reporting to the balloon driver

[Anthony Liguori]

On 01/07/2010 09:18 AM, Avi Kivity wrote:
> On 01/07/2010 05:12 PM, Anthony Liguori wrote:
> > 3) Make qemu request balloon stats regularly (maybe every 10 seconds) 
> > and display the latest stats with info balloon.  This avoids the 
> > problem in #2 but it means that qemu determines the poll rate instead 
> > of a management tool.
> >
> > 4) Make info-balloon a proper asynchronous command.  We need new 
> > infrastructure to allow a qmp handler to take a callback that can be 
> > used to delay the completion of the command.  This addresses all of 
> > the above problems but it introduces a new one.  Command completion 
> > now depends on the guest.  This potentially could trip up a naive 
> > management tool that doesn't realize that the info-balloon command 
> > may never complete.
> >
> > I'm on the fence between 3 and 4 myself.
>
> Can I tip you over to #4?  #3 means we have no idea when the stats 
> were generated.  With #4, we can't be sure, but it usually be close to 
> when the command returns.
>
> The command should include a timeout so a broken guest won't hang a 
> management tool thread.

Generally, timeouts are evil but if we did something like, wait 10 
seconds and if we don't hear a response from the guest, return the last 
data set, I think I would be okay with that.  It means we may be 
reporting stale data, but at the same time, the data is coming from a 
guest so it can't be considered authoritative anyway.

Regards,

Anthony Liguori