[libvirt-users] cpu time totals
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Hi!
I'm comparing the cpu usage of a VM from virDomainGetInfo and the
usages returned per-vcpu from virDomainGetVcpus and the totals do
not match up.
I expect that the cpu usages reported per-vcpu, when summed, should
equal the total cpu usage of the domain, but there is quite a large
gap between them. Is there something I'm missing?
See this example code using the python bindings:
>> import libvirt
>> c = libvirt.open('qemu:///system')
>> d = c.lookupByID(51)
>> d.info()[4]
111390000000L
>> d.vcpus()[0]
[(0, 1, 12720000000L, 1), (1, 1,
2980000000L, 0), (2, 1, 3150000000L, 1), (3, 1, 2600000000L, 0)]
>> sum([vcpu[2] for vcpu in d.vcpus()[0]])
21520000000L
>> d.info()[4] - sum([vcpu[2] for vcpu in d.vcpus()[0]])
91760000000L
Why is this additional 91 seconds not reflected in time spent by the
vcpus? What exactly is the meaning of the cpuTime parameter of the
virVcpuInfo struct?
Thanks,
--Igor
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On 09/13/2011 03:46 PM, Igor Serebryany wrote:
Hi!
I'm comparing the cpu usage of a VM from virDomainGetInfo and the
usages returned per-vcpu from virDomainGetVcpus and the totals do
not match up.
I expect that the cpu usages reported per-vcpu, when summed, should
equal the total cpu usage of the domain, but there is quite a large
gap between them. Is there something I'm missing?
Yes - emulation overhead. That is, there is some work done by the
hypervisor process itself but not directly attributable to any of the
work done by the vcpus visible in the guest. For example, in qemu, the
time spent in opening qcow2 files counts against the overall domain
time, but not towards any of the vcpu times.
Why is this additional 91 seconds not reflected in time spent by
the
vcpus? What exactly is the meaning of the cpuTime parameter of the
virVcpuInfo struct?
How much total time has been occupied by the overall hypervisor instance
running the domain, which is probably larger than the amount of vcpu
time racked up from the child's point of view. The smaller the
overhead, the more efficient the virtualization.
--
Eric Blake eblake(a)redhat.com +1-801-349-2682
Libvirt virtualization library http://libvirt.org
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Igor Serebryany