Re: NUMA TODO-list for xen-devel

[George Dunlap <george.dunlap@eu.citrix.com>]

On 03/08/12 10:48, Andre Przywara wrote:
>>> I think we could use cpu hot-plug to change the "virtual topology" of
>>> VMs, couldn't we?  We could probably even do that on a running guest
>>> if we really needed to.
>> Hmm, not sure - using hotplug behind the back of the guest might
>> be possible, but you'd first need to hot-unplug the vCPU. That's
>> something that I don't think you can do on HVM guests (and for
>> PV guests, guest visible NUMA support makes even less sense
>> than for HVM ones).
> I don't think that hotplug would really work. I have checked this some
> times ago, at least the Linux NUMA code cannot be really fooled by this.
> The SRAT table is firmware defined and static by nature, so there is no
> code in Linux to change the NUMA topology at runtime. This is especially
> true for the memory layout.
I was more thinking of giving a VM the biggest topology you would want 
at boot, and then asking Linux to online or offline vcpus; for example, 
giving it a 4x2 topology (4 vcores x 2 vnodes).  When running on a 
system with 2 cores per node, you offline 2 vcpus per vnode, giving it 
an effective layout of 2x2.  When running on a system with 4 cores per 
node, you could offline all of the cores on one node, giving it an 
effective topology of 4x1.

Unfortunately, I just realized that you could change the number of vcpus 
in a given node, but you couldn't move the memory around very easily.   
Unless you have memory hotplug? Hmm..... :-)

  -George