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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Jan 10, 2014 at 04:11:15PM +0000, Ian Campbell wrote:
> 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?

The delta of it: maxmem - memory.
> 
> 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.

The way I read the code it has a split personality:
 - up to 'memory' in the toolstack are allocated.
 - the delta of 'maxmem - memory' are allocated in the hypervisor and
   then freed.

> 
> 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...

This is confusing indeed.
> 
> Ian
> 

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