On Wed, 22 Feb 2012 09:05:22 -0600 Michael Roth <mdr...@linux.vnet.ibm.com> wrote:
> On Wed, Feb 22, 2012 at 10:48:13AM -0200, Luiz Capitulino wrote: > > On Fri, 17 Feb 2012 15:51:33 -0600 > > Michael Roth <mdr...@linux.vnet.ibm.com> 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 <aligu...@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<lcapitul...@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. > > > > 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. > > I don't mind the ad-hoc implementation details, I just think in this > case it's leaking out into our APIs. With proper async QMP in place we > just re-enable query-balloon and existing synchronous QMP clients and > libvirt interfaces Just Work (albeit with potential for a "timed-out" > response). There's no need for a specialized balloon-get-memory-stats command > at all. It's not possible to re-use query-balloon for this, making a synchronous command asynchronous is an incompatible change. That was exactly the problem we had that forced us to disable this feature (and that's why a new command is required, be it balloon-get-memory-stats or a QOM device property). > And if they want stats asynchronously, proper async QMP does that as well: > they just need to make their QMP client async-aware (basically, don't wait > around for a response, and tag the query-balloon request so you can > match the response to the query). > > So balloon-get-memory-stats is already on the path to being deprecated. Any command implementing its async schema is going to be deprecated when we get proper async support. That's not a problem per se. I mean, a much worse problem is to delay features & functionality because we don't have the perfect means to implement them. But that's a dead discussion I guess, as I already agreed on implementing this as a device property. > We should instead focus on just re-enabling query-balloon, and if that can be > achieved with this approach, then I'm all for it, but I think we all seem to > agree that a timer-based mechanism would be needed instead. > > > > > > > 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... > > > > Sorry, thought you were suggesting we do "lazy" polling where we only > queue up a balloon request after a query-balloon has been issued, in > which case the age of the response is unbounded... well, from a QMP > standpoint, > I guess that *is* what we'd be doing, and we'd just be telling management > tools > to add a wrapper around the interface as a work-around, which seems > suggests it's not the right interface. >