Re: kvm & dyntick

[Ingo Molnar <mingo@elte.hu>]

* Avi Kivity <avi@qumranet.com> 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