Re: [libvirt-users] What's the precision of the domain cpu stats?
On 04/16/2012 02:33 AM, Zhihua Che wrote:
Hi, everyone
I'm using libvirt and resort to virDomainGetCPUStats for cpu
usage by one domain. In my system, Ubuntu-11.10 64bit with 2 cores and
Qemu Hyperviser, this function returns something like this:
virTypedParameter[0]
{
field: "cpu_time"
type: 4
value.ul 51640610899
}
virTypedParameter[1]
{
field: "cpu_time"
type: 4
value.ul 55302820304
}
This 51640610899 and 55302820304 kind of conflict the result
returned by "cat /proc/stat | grep cpu"
cpu 449224 6812 89433 4407261 50327 1 2443 0 13041 0
cpu0 224251 2603 43144 2195672 34682 0 2350 0 6692 0
cpu1 224972 4209 46289 2211589 15644 0 93 0 6349 0
I know the precision of /proc/stat is 1/HZ, so what's the precision
of CPU stats returned by virDomainGetCPUStats?
Per libvirt.h:
/**
* VIR_DOMAIN_CPU_STATS_CPUTIME:
* cpu usage in nanoseconds, as a ullong
*/
#define VIR_DOMAIN_CPU_STATS_CPUTIME "cpu_time"
/**
* VIR_DOMAIN_CPU_STATS_USERTIME:
* cpu time charged to user instructions in nanoseconds, as a ullong
*/
#define VIR_DOMAIN_CPU_STATS_USERTIME "user_time"
Also, it helps to know that virDomainGetCPUStats gets its information
from the cpuacct cgroup; I have no idea how the kernel code relates
cpuacct tracking to /proc/stat tracking, so you'll have to track down
that relationship before you can expect any sanity checking between the
two values.
--
Eric Blake eblake(a)redhat.com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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