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Re: [Xen-devel] Proposed new "memory capacity claim" hypercall/feature

> From: Dario Faggioli [mailto:raistlin@xxxxxxxx]
> Subject: Re: Proposed new "memory capacity claim" hypercall/feature
> On Mon, 2012-10-29 at 22:35 +0000, Tim Deegan wrote:
> > At 10:06 -0700 on 29 Oct (1351505175), Dan Magenheimer wrote:
> > > In the case of a dying domain, a XENMEM_release operation
> > > is implied and must be executed by the hypervisor.
> > >
> > > Ideally, the quantity of unclaimed memory for each domain and
> > > for the system should be query-able.  This may require additional
> > > memory_op hypercalls.
> > >
> > > I'd very much appreciate feedback on this proposed design!
> >
> > As I said, I'm not opposed to this, though even after reading through
> > the other thread I'm not convinced that it's necessary (except in cases
> > where guest-controlled operations are allowed to consume unbounded
> > memory, which frankly gives me the heebie-jeebies).
> >
> Let me also ask something.
> Playing with NUMA systems I've been in the situation where it would be
> nice to know not only how much free memory we have in general, but how
> much free memory there is in a specific (set of) node(s), and that in
> many places, from the hypervisor, to libxc, to top level toolstack.
> Right now I ask this to Xen, but that is indeed prone to races and
> TOCTOU issues if we allow for domain creation and ballooning

TOCTOU... hadn't seen that term before, but I agree it describes
the problem succinctly.  Thanks, I will begin using that now!

> (tmem/paging/...) to happen concurrently between themselves and between
> each other (as noted in the long thread that preceded this one).
> Question is, the "claim" mechanism you're proposing is by no means NUMA
> node-aware, right?

I hadn't thought about NUMA, but I think the claim mechanism
could be augmented to attempt to stake a claim on a specified
node, or on any node that has sufficient memory.  AFAICT
this might complicate the arithmetic a bit but should work.
Let me prototype the NUMA-ignorant mechanism first though...


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