Re: [virtio-dev] Re: [virtio] New virtio balloon...

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jan 30, 2014 at 11:47:19AM -0500, Luiz Capitulino wrote:
> On Thu, 30 Jan 2014 12:16:29 +0200
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > Also copy virtio-dev since this in clearly implementation ...
> > 
> > On Thu, Jan 30, 2014 at 07:34:30PM +1030, Rusty Russell wrote:
> > > Hi,
> > > 
> > >         I tried to write a new balloon driver; it's completely untested
> > > (as I need to write the device).  The protocol is basically two vqs, one
> > > for the guest to send commands, one for the host to send commands.
> > > 
> > > Some interesting things come out:
> > > 1) We do need to explicitly tell the host where the page is we want.
> > >    This is required for compaction, for example.
> > > 
> > > 2) We need to be able to exceed the balloon target, especially for page
> > >    migration.  Thus there's no mechanism for the device to refuse to
> > >    give us the pages.
> > > 
> > > 3) The device can offer multiple page sizes, but the driver can only
> > >    accept one.  I'm not sure if this is useful, as guests are either
> > >    huge page backed or not, and returning sub-pages isn't useful.
> > > 
> > > Linux demo code follows.
> > > 
> > > Cheers,
> > > Rusty.
> > 
> > More comments:
> > 	- for projects like auto-ballooning that Luiz works on,
> > 	  it's not nice that to swap page 1 for page 2
> > 	  you have to inflate then deflate
> > 	  besides overhead this confuses the host:
> > 	  imagine you tell QEMU to increase target,
> > 	  meanwhile guest inflates temporarily,
> > 	  QEMU thinks okay done, now you suddenly deflate.
> 
> Yes. Just to give more context: one of my auto-ballooning versions broke
> when virtballoon_migratepage() ran. The reason was that my host-side code
> in the balloon device did not expect guest initiated operations. And the
> current spec does imply that all operations are initiated by the host.
> 
> So, first suggestion: if the current spec is still valid, we have to add
> a note there that balloon operations can be initiated by the guest.
> 
> My current code is different, but something it does that could also brake
> due to guest initiated inflate/deflate is that it keeps track of the
> current balloon size. This is done by a counter which is incremented
> on inflate and decremented on deflate. I did that because the device just
> doesn't have this information ('actual' is unreliable, besides it's
> only updated every 256 pages inflated/deflated).
> 
> Second suggestion: I think we need a reliable way to know the current
> balloon size on the host. My counter does work, btw.
> 
> As far as the guest is concerned, my current code just informs the host
> that the guest is facing pressure. This is done through a "message" virtqueue,
> but I think this could just use the guest command virtqueue.
> 
> > A couple of other suggestions:
> > 
> > - how to accomodate memory pressure in guest?
> >   Let's add a field telling host how hard do we
> >   want our memory back
> 
> I agree we have to accommodate pressure in the guest some way, but what
> you proposed is more or less related to auto-ballooning.
> 
> My suggestion would be for the host to tell the guest what to do in
> case of pressure. Like, it could tell the guest to just keep trying like
> it does today or it could ask the guest to stop inflation on pressure
> (which would require an ack from the host, which complicates the
> protocol a bit).

If we need ack anyway, it seems enough to notify host
and then host can ask guest to stop inflating?

> Also, there are two ways to know the guest is under pressure: 1. when
> alloc_page() fails or 2. use in-kernel vmpressure notification like
> auto-balloon does.

2 can detect pressure earlier so seems nicer.

> > - assume you want to over-commit host and start
> >   inflating balloon.
> >   If low on memory it might be better for guest to
> >   wait a bit before inflating.
> >   Also, if host asks for a lot of memory a ton of
> >   allocations will slow guest significantly.
> >   But for guest to do the right thing we need host to tell guest what
> >   are its memory and time contraints.
> >   Let's add a field telling guest how hard do we
> >   want it to give us memory (e.g. time limit)
> 
> I think this is also related to auto-ballooning.
> Maybe we should start
> with a simple device/driver and add all these features on top.


Just shows these are good ideas :).

I tried to propose building blocks in a generic way that will
be useful for multiple features.
autoballoon can build on top but these features
are useful even with manual inflate.

-- 
MST