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Re: [Xen-devel] NUMA TODO-list for xen-devel



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

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