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Re: [Xen-devel] [PATCH 2/4] [HVM] introduce CPU affinity for allocate_ph

To: "Ryan Harper" <ryanh@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH 2/4] [HVM] introduce CPU affinity for allocate_physmap call
From: "Andre Przywara" <andre.przywara@xxxxxxx>
Date: Wed, 15 Aug 2007 12:12:56 +0200
Cc: Christoph Egger <christoph.egger@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Keir Fraser <keir@xxxxxxxxxxxxx>
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Ryan Harper wrote:
One concern has been the static nature of the ACPI SRAT data versus the
dynamic ability of the vcpu to cpu mapping.  If the scheduler is
migrating the guest vcpu to various cpus, then the SRAT information is
likely to be incorrect.
I think this is a problem even for the native OSes when you think of CPU- and/or memory-hotplugging. Although Linux can do CPU hotplugging, AFAIK NUMA isn't currently considered in this process. I think the most feasible approach would be to rebuild all affected structures when the hotplug event occurs. This will probably considered quite rare and thus could be potentially more costly, so I this is not something you want to do every time Xen decides to reschedule a VCPU. So IMHO pinning VCPUs to a certain node (actually all cores within this node) is OK for now.

That said, if one creates a vnode, and it sufficiently restricts the
vcpu affinity, then accurate SRAT information can be exported for the
guest to utilize.
My patch does this automatically. CPU affinity information from the config file is ignored and each VCPUs affinity is set to match the NUMA topology.


Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
Tel: +49 351 277-84917
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