Re: [Qemu-devel] [Qestion] What status of memory stats feature

[Adam Litke <agl@us.ibm.com>]

On Wed, 2010-12-15 at 15:39 -0200, Luiz Capitulino wrote:
> On Wed, 15 Dec 2010 16:20:05 +0900
> "Ken'ichi Ohmichi" <oomichi@mxs.nes.nec.co.jp> wrote:
> 
> > 
> > Hi,
> > 
> > I tried to get the memory stats by using virDomainMemoryStats() of libvirt,
> > but it could not do it because of the following patch:
> > 
> > [PATCH 03/23] disable guest-provided stats on "info balloon" command
> >   2010/10/01 from Luiz Capitulino
> >   http://www.mail-archive.com/qemu-devel@nongnu.org/msg43024.html 
> > 
> > According to the related thread, the above patch avoids hanging user
> > monitor, and people hope the memory stats feature will be able in the
> > future. So I'd like to know the status of this feature.
> > 
> > Is there the patch for enabling the feature ?
> > If the patch exists, I'd like to try it.
> 
> It doesn't, afaik.

It depends on what you are looking to do.  If you just want to play
around and aren't concerned about lockups, You could undo the above
patch to re-enable the interface (although I cannot guarantee that even
this will work).  

> > What is the essential problem ?
> 
> There are two essential problems here.
> 
> The first one is that QMP lacks true asynchronous command support (it does
> have some code for it, but it's incomplete).

I was never completely clear on the problems with the QMP async support.

> The second problem is that we must not make an existing synchronous command
> asynchronous, because this breaks the ABI.

The memory stats interface has always advertised itself as an async
command so this one shouldn't matter.  Whether those semantics actually
ever worked is another issue.

> > Does some infinite loop happen ?
> 
> No, the guest doesn't respond.
> 
> > Unfortunately, I cannot understand the cause of hanging user monitor.
> 
> The balloon command needs guest cooperation (ie. it talks to the guest),
> if the guest doesn't respond the command will never return.

Isn't that only a problem for the user monitor?  For QMP you might just
never get a response to the stats request.  IIRC, the QMP monitor is
never 'suspended' in the same way that the user monitor is so I wouldn't
expect it to be susceptible to the same lockup issues.

> We don't have a precise ETA for async support, but if it depends only on
> the new error framework work, then we could have it for 0.15. If it depends
> on the full monitor redesign work, then I guess we'll two releases.

I can't provide a definitive answer to this unfortunately since I
haven't looked in awhile (nor am I privy to the details of the full
monitor redesign proposal.

-- 
Thanks,
Adam