Re: domain creation vs querying free memory (xend and xl)

[Olaf Hering <olaf@aepfle.de>]

On Thu, Oct 04, Dan Magenheimer wrote:

> > From: Olaf Hering [mailto:olaf@aepfle.de]
> > Subject: Re: [Xen-devel] domain creation vs querying free memory (xend and xl)
> > 
> > On Mon, Oct 01, Dan Magenheimer wrote:
> > 
> 
> Hi Olaf --
> 
> Thanks for the reply.
> 
> > domain. All of this needs math, not locking.
> >  :
> > As IanJ said, the memory handling code in libxl needs such a feature to
> > do the math right. The proposed handling of
> > sharing/paging/ballooning/PoD/tmem/... in libxl is just a small part of
> > it.
> 
> Unfortunately, as you observe in some of the cases earlier in your reply,
> it is more than a math problem for libxl... it is a crystal ball problem.
> If xl launches a domain D at time T and it takes N seconds before it has
> completed asking the hypervisor for all of the memory M that D will require
> to successfully launch, then xl must determine at time T the maximum memory
> allocated across all running domains for the future time period between
> T and T+N.  In other words, xl must predict the future.

I think xl can predict it, if it takes the target of all domains into
account.  Certainly not down to a handful pages, it would be good enough
to know if the calculated estimate of free memory is good for the new
guest and its specific memory targets.

> Clearly this is impossible especially when page-sharing is not communicating
> its dynamic allocations (e.g. due to page-splitting) to libxl, and tmem
> is not communicating allocations resulting from multiple domains
> simultaenously making tmem hypercalls to libxl, and PoD is not communicating
> its allocations to libxl, and in-guest-kernel selfballooning is not communicating
> allocations to libxl.  Only the hypervisor is aware of every dynamic allocation
> request.

The hypervisor can not predict the future either, and it has even less
info about the individual targets of each domain.

> Does that make sense?

It does, but:
If xl reserves the memory in its own "virtual allocator", or if Xen gets
such functionality, does not really matter, as long as its known how much
exactly needs to be allocated. I think that part is missing.

Olaf