Re: [Xen-devel] [RFC] Transcendent Memory ("tmem"): a new approach to physical memory management

[Alan Cox <alan@xxxxxxxxxxxxxxxxxxx>]

> Could you point me to the two one liners?  If they are
> simply informing the hypervisor, that is certainly a step
> in the right direction.  IBM's CMM2 is of course much more

Yes - they hook arch_free_page and arch_alloc_page so that free pages are
known to the hypervisor layer.

> the Linux kernel actively participates in the "admission
> policy" so this information need not be inferred outside
> of the kernel by the hypervisor.

Yes - the patches are very interesting and you take it a stage further
than the S/390 hooks by exposing a lot more to the hypervisor.

> I'm not trying to implement distributed shared memory.
> There is no "across the network" except what the cluster
> fs handles already.  The clusterfs must ensure that the

That was what confused me about the shared pools. I had assumed that
shared pools would imply DSM simply because two guests could use a shared
pool and one of them get live migrated without the other.

> SSD interface or might help for hot-swap memory.

Not something I'd thought about. The problem with hot swap is generally
one of managing to get stuff removed from a given physical page of RAM.
Having more flexible allocators probably helps there simply because you
can make space to relocate pages underneath the guest.

> I also think it might be used like compcache, but with
> the advantage that clean page cache pages can also be
> compressed.

Would it be useful to distinguish between pages the OS definitely doesn't
care about (freed) and pages that can vanish, at least in terms of
reclaiming them between guests. It seems that truely free pages are the
first target and there may even be a proper heirarchy.


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