Re: [libvirt] [PATCHv2 1/2] Add a new flag VIR_DOMAIN_CPU_STATS_VCPU to virDomainGetCPUStats
On Wed, May 02, 2012 at 01:43:03PM -0600, Eric Blake wrote:
On 04/24/2012 03:20 AM, Hu Tao wrote:
> Currently virDomainGetCPUStats gets total cpu usage, which consists
> of:
>
> 1. vcpu usage: the physical cpu time consumed by virtual cpu(s) of
> domain
> 2. hypervisor: `total cpu usage' - `vcpu usage'
>
> The flag VIR_DOMAIN_CPU_STATS_VCPU is for getting vcpu usages.
> ---
> include/libvirt/libvirt.h.in | 8 ++++++++
> src/libvirt.c | 9 +++++++--
> tools/virsh.c | 16 +++++++++++-----
> 3 files changed, 26 insertions(+), 7 deletions(-)
Question: is it better to make the user call virDomainGetCPUStats twice
(once for total with flags==0, once for vcpu with flags=VCPU), where
there is a race between the two calls? Or should we instead return both
counts in the same call, by having two separate virTypedParameter stat
names and no flag?
Slightly off-topic ...
My opinion on this -- and it applies to many libvirt calls -- is that
APIs where you have to call them twice, once to size the result, and a
second time to get the result, are obtuse and hard to use.
In libguestfs I took the approach that all calls return malloc'd
strings, arrays, etc of the right size. The caller has to free the
data, but that's the lesser of two evils.
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
libguestfs lets you edit virtual machines. Supports shell scripting,
bindings from many languages. http://libguestfs.org