Re: [Xen-devel] Claim mode and HVM PoD interact badly

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2014-01-10 at 11:05 -0500, Konrad Rzeszutek Wilk wrote:
> On Fri, Jan 10, 2014 at 03:56:13PM +0000, Ian Campbell wrote:
> > On Fri, 2014-01-10 at 10:28 -0500, Konrad Rzeszutek Wilk wrote:
> > > On Fri, Jan 10, 2014 at 03:16:25PM +0000, Ian Campbell wrote:
> > > > On Fri, 2014-01-10 at 09:58 -0500, Konrad Rzeszutek Wilk wrote:
> > > > > Which implies to me that we _need_ the 'maxmem' amount of memory at 
> > > > > boot time.
> > > > > And then it is the responsibility of the balloon driver to give the 
> > > > > memory
> > > > > back (and this is where the 'static-max' et al come in play to tell 
> > > > > the
> > > > > balloon driver to balloon out).
> > > > 
> > > > PoD exists purely so that we don't need the 'maxmem' amount of memory at
> > > > boot time. It is basically there in order to let the guest get booted
> > > > far enough to load the balloon driver to give the memory back.
> > > > 
> > > > It's basically a boot time zero-page sharing mechanism AIUI.
> > > 
> > > But it does look to gulp up hypervisor memory and return it during
> > > allocation of memory for the guest.
> > 
> > It should be less than the maxmem-memory amount though. Perhaps because
> > Wei is using relatively small sizes the pod cache ends up being a
> > similar size to the saving?
> > 
> > Or maybe I just don't understand PoD, since the code you quote does seem
> > to contradict that.
> > 
> > Or maybe libxl's calculation of pod_target is wrong?
> > 
> > > From reading the code the patch seems correct - we will _need_ that
> > > extra 128MB 'claim' to allocate/free the 128MB extra pages. They
> > > are temporary as we do free them.
> > 
> > It does makes sense that the PoD cache should be included in the claim,
> > I just don't get why the cache is so big...
> 
> I think it expands and shrinks to make sure that the memory is present
> in the hypervisor. If there is not enough memory it would -ENOMEM and
> the toolstack would know immediately.
> 
> But that seems silly - as that memory might be in the future used
> by other guests and then you won't be able to use said cache. But since
> it is a "cache" I guess that is OK.

Wait, isn't the "cache" here just the target memory size?

PoD uses up to that size to populate guest pages, and will try to
reclaim zeroed pages from the guest so that it never grows bigger than
the target size.

I think the confusion here is that Wei had target=128M and maxmem=256M
so the difference was 128M which served as a nice red-herring...

Ian


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