[libvirt] [PATCH 2/5] virCaps: expose huge page info

[Daniel P. Berrange]

On Thu, Jun 12, 2014 at 07:21:47PM +0200, Martin Kletzander wrote:
> On Thu, Jun 12, 2014 at 02:30:50PM +0100, Daniel P. Berrange wrote:
> >On Tue, Jun 10, 2014 at 07:21:12PM +0200, Michal Privoznik wrote:
> >>There are two places where you'll find info on huge pages. The first
> >>one is under <cpu/> element, where all supported huge page sizes are
> >>listed. Then the second one is under each <cell/> element which refers
> >>to concrete NUMA node. At this place, the size of huge page's pool is
> >>reported. So the capabilities XML looks something like this:
> >>
> >><capabilities>
> >>
> >>  <host>
> >>    <uuid>01281cda-f352-cb11-a9db-e905fe22010c</uuid>
> >>    <cpu>
> >>      <arch>x86_64</arch>
> >>      <model>Westmere</model>
> >>      <vendor>Intel</vendor>
> >>      <topology sockets='1' cores='1' threads='1'/>
> >>      ...
> >>      <pages unit='KiB' size='1048576'/>
> >>      <pages unit='KiB' size='2048'/>
> >
> >Should have normal sized pages (ie 4k on x86) too, to avoid
> >apps having to special case small pages.
> >
> 
> Since we have to special-case small pages and kernel (at least to my
> knowledge) doesn't expose that information by classic means, I think
> reporting only hugepages is actually what we want here.  For normal
> memory there are existing APIs already.
> 
> Hugepages are different mainly because of one thing.  The fact that
> there are some hugepages allocated is known by the user of the machine
> (be it mgmt app or an admin) and these hugepages were allocated for
> some purpose.  It is fairly OK to presume that the number of hugepages
> (free or total) will change only when and if the user wants to
> (e.g. running a machine with specified size and hugepages).  That
> cannot be said about small pages, though, and I think it is fair
> reason to special-case normal pages like this.

That difference is something that's only relevant to the person who
is provisioning the machine though. For applications consuming the
libvirt APIs it is not relevant. For OpenStack we really want to have
normal size pages dealt with the in the same way as huge pages since
it will simplify our schedular/placement logic. So I really want these
APIs to do this in libvirt so that OpenStack doesn't have to reverse
engineer this itself.

Regards,
Daniel
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