Re: [Xen-devel] strange phenomenon on CPU affinity

[Like Zhou <likechou@xxxxxxxxx>]

2013/3/15 Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>

On 15.03.2013 10:08, 周理科 wrote:

Hello,
      My testing machine has 2 quad-core CPU (It supports hyperthreading, but i
disable it in BIOS). I uses Xen 4.0.1 as the hypervisor. When I use 8 VMs to
conduct a test, CPU affinity of the VMs is very strange. Like this:

vm_name  vcpu_num  cpu_affinity
Domain-0    8      any
VM1            4      1,3,5,7
VM2            4      1,3,5,7
VM3            4      1,3,5,7
VM4            4      1,3,5,7
VM5            4      1,3,5,7
VM6            4      0,2,4,6
VM7            4      0,2,4,6
VM8            4      0,2,4,6

I do not set the CPU affinity in the configuration file, and I cannot find when
the hypervisor set the CPU affinity in the source code. In this situation, 4
VCPUs of each VM are binding to 4 PCPUs permanently, and 5 VMs run on a set of
PCPUs, and others run on the other set of PCPUs. It is unfair to these VMs.

I'd suspect NUMA optimization. xend tries to optimize domain placement by
pinning the vcpus of the domains to cores in the same NUMA domain. Normally
the overall performance is better with this optimization. You can disable it
by specifying

numa=off

as an additional xen boot parameter for the hypervisor (not the dom0 kernel).
You can see whether you have NUMA active with

xm info

It will display line(s) like:

node_to_cpu            : node0:0-3

If you see multiple nodes you have NUMA active.


Juergen

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Yes. When I disable NUMA optimization, CPU affinity of each VCPU becomes "any". Thanks.

-- 
Like Zhou
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