Re: kvm & dyntick

Ingo Molnar
2007-01-12T06:20:07Z

* Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org> wrote:

> It occurs to me that kvm could benefit greatly from dyntick:
>
> dyntick-enabled host:
> - generate virtual interrupts at whatever HZ the guest programs its
> timers, be it 100, 250, 1000 or whatever
> - avoid expensive vmexits due to useless timer interrupts
>
> dyntick-enabled guest:
> - reduce the load on the host when the guest is idling
> (currently an idle guest consumes a few percent cpu)

yeah. KVM under -rt already works with dynticks enabled on both the host
and the guest. (but it's more optimal to use a dedicated hypercall to
set the next guest-interrupt)

> What are the current plans wrt dyntick? Is it planned for 2.6.21?

yeah, we hope to have it in v2.6.21.

note that s390 (and more recently Xen too) uses a next_timer_interrupt()
based method to stop the guest tick - which works in terms of reducing
guest load, but it doesnt stop the host-side interrupt. The highest
quality approach is to have dynticks on both the host and the guest, and
this also gives high-resolution timers and a modernized
time/timer-events subsystem for both the host and the guest.

Ingo