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Re: [libvirt] [RFC][PATCHv1 2/5] libvirt - new API for getting percpu statistics of VM



On Fri, Apr 15, 2011 at 04:04:52PM +0900, KAMEZAWA Hiroyuki wrote:
[...]

The idea and the implementation is sound.  However I have a problem
with this proposed API:

> +typedef struct _virDomainPcpuStat virDomainPcpuStatStruct;
> +
> +struct _virDomainPcpuStat {
> +    unsigned long long cpuTime;
> +};
> +
> +typedef virDomainPcpuStatStruct *virDomainPcpuStatPtr;

[...]

>  /**
> + * virDomainPcpuStats:
> + * @dom: pointer to the domain object
> + * @stats: nr_stats-sized array of stat structures (returned)
> + * @nr_stats: number of cpu statistics requested
> + * @flags: unused, always pass 0
> + *
> + * This function provides per-cpu statistics for the domain. 'cpu' here means
> + * not vcpu.
> + *
> + * Up to 'nr_stats' elements of 'stats' will be populated with cpu statistics
> + * from the domain.  Only statistics supported by the domain, the driver, and
> + * this version of libvirt will be returned.
> + *
> + * Now, only cpuTime per cpu is reported in nanoseconds.
> + *
> + * Returns: The number of stats provided or -1 in case of failure.
> + */
> +int virDomainPcpuStats (virDomainPtr dom, virDomainPcpuStatPtr stats,
> +                          unsigned int nr_stats, unsigned int flags)

Am I correct that the caller sets nr_stats == number of physical
cores, and allocates an array of nr_stats * virDomainPcpuStats
structs?

We could never add more elements to the _virDomainPcpuStat structure,
because that would break the ABI for existing callers.  (Or I guess we
could use the flags field).

I think we should forget about the structure, and just use an array of
unsigned long long's, but I'd like to hear what others think.

Is it likely that we would return other per-physical-CPU stats in
future?

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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