Linux Virtualization - Aug 2007 - Réf. : Re: [PATCH 0/4] Virtual Machine Time Accounting

> Laurent Vivier wrote:
> functionnalities:
>
> > - allow to measure time spent by a CPU in a virtual CPU.
> > - allow to display in /proc/state this value by CPU
> > - allow to display in /proc/<pid>/state this value by process
> > - allow KVM to use these 3 previous functionnalities
> >
>
> So, currently time spent in a kvm guest is accumulated as qemu-kvm
> usertime, right?  Given that qemu knows when its running in qemu vs
No, it is accumulated as kvm system time.
> guest context, couldn't it provide the breakdown between user and guest
> time (ditto lguest)?
by doing this at kernel level, we can:
- measure exactly the guest time,
- move this part of system time to user time (as you think it should be
user time),
- have consistency between system, user and guest time,
- report values in /proc/state and /proc/<pid>/state, at system wide level

I'm not sure we can measure the guest time at the qemu user level.

Perhaps Rusty can say what he thinks about this ?

Regards,
Laurent

> by doing this at kernel level, we can:
> - measure exactly the guest time,
> - move this part of system time to user time (as you think it should be
> user time),
> - have consistency between system, user and guest time,
> - report values in /proc/state and /proc/<pid>/state, at system wide
level
>
> I'm not sure we can measure the guest time at the qemu user level.
>
> Perhaps Rusty can say what he thinks about this ?
>
Even if we cannot _now_, isn't that an easier, and safer change? (and
I don't think we lose anything by design).

Although I don't know KVM to a that deep level, I think it should be
possible to keep the virtual cpus in different process (or threads),
and take the accounting time from there. Perfectly possible to know
the time we spent running (user time), and the time the hypervisor
spent doing things on our behalf (system time).

$0.02.

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