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Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support

From: Bharata B Rao
Subject: Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support
Date: Fri, 29 Apr 2016 12:29:40 +0530
User-agent: Mutt/1.5.23 (2014-03-12)

On Fri, Apr 29, 2016 at 08:45:37AM +0200, Thomas Huth wrote:
> On 29.04.2016 05:24, David Gibson wrote:
> > On Tue, Apr 26, 2016 at 04:03:37PM -0500, Michael Roth wrote:
> ...
> >> In the case of pseries, the DIMM abstraction isn't really exposed to
> >> the guest, but rather the memory blocks we use to make the backing
> >> memdev memory available to the guest. During unplug, the guest
> >> completely releases these blocks back to QEMU, and if it can only
> >> release a subset of what's requested it does not attempt to recover.
> >> We can potentially change that behavior on the guest side, since
> >> partially-freed DIMMs aren't currently useful on the host-side...
> >>
> >> But, in the case of pseries, I wonder if it makes sense to maybe go
> >> ahead and MADV_DONTNEED the ranges backing these released blocks so the
> >> host can at least partially reclaim the memory from a partially
> >> unplugged DIMM?
> > 
> > Urgh.. I can see the benefit, but I'm a bit uneasy about making the
> > DIMM semantics different in this way on Power.
> > 
> > I'm shoehorning the PAPR DR memory mechanism into the qemu DIMM model
> > was a good idea after all.
> Ignorant question (sorry, I really don't have much experience yet here):
> Could we maybe align the size of the LMBs with the size of the DIMMs?
> E.g. make the LMBs bigger or the DIMMs smaller, so that they match?

Should work, but the question is what should be the right size so that
we have good granularity of hotplug but also not run out of mem slots
thereby limiting us on the maxmem. I remember you changed the memslots
to 512 in KVM, but we are yet to move up from 32 in QEMU for sPAPR though.


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