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On Thu, Feb 05, 2015 at 11:21:05AM -0600, Chris Friesen wrote:
> On 02/05/2015 10:32 AM, Daniel P. Berrange wrote:
> >On Thu, Feb 05, 2015 at 10:28:56AM -0600, Chris Friesen wrote:
> 
> >>For what it's worth, I was able to make hugepages work with an older qemu by
> >>commenting out two lines in
> >>virt.libvirt.config.LibvirtConfigGuestMemoryBacking.format_dom()
> >>
> >>     def format_dom(self):
> >>         root = super(LibvirtConfigGuestMemoryBacking, self).format_dom()
> >>
> >>         if self.hugepages:
> >>             hugepages = etree.Element("hugepages")
> >>             #for item in self.hugepages:
> >>             #    hugepages.append(item.format_dom())
> >>             root.append(hugepages)
> >>
> >>
> >>This results in XML that looks like:
> >>
> >>   <memoryBacking>
> >>     <hugepages/>
> >>   </memoryBacking>
> >>
> >>
> >>And a qemu commandline that looks like
> >>
> >>-mem-prealloc -mem-path /mnt/huge-2048kB/libvirt/qemu
> >
> >With that there is no guarantee that the huge pages are being allocated
> >from the NUMA node on which the guest is actually placed by Nova, hence
> >we did not intend to support that.
> 
> It's possible that the end-user didn't indicate a preference for NUMA.  If
> they just asked for hugepages and we have the ability to give it to them I
> think we should do so.
> 
> In the likely common case of an instance with a single NUMA node, I think
> this will likely give the desired behaviour since the default kernel
> behaviour is to prefer allocating from the numa node that requested the
> memory.  As long as qemu affinity is set before it allocates memory we
> should be okay.
> 
> The only case that isn't covered is if the flavor specifies multiple numa
> nodes.  In that case maybe the scheduler filters should be aware of that and
> refuse to assign an instance with multiple numa nodes to a compute node with
> an older qemu.

Having the scheduler need to care about versions of software installed on
nodes is a whole heap of extra complexity for no credible gain. It is
perfectly reasonable to just mandate the newer QEMU for this IMHO and
avoid that complexity in Nova.

Regards,
Daniel
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