Re: kvm & dyntick

Avi Kivity
2007-01-12T17:04:31Z

Ingo Molnar wrote:
> * Ingo Molnar <mingo-X9Un+BFzKDI@public.gmane.org> wrote:
>
>
>>> 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)
>>
>
> using the dynticks code from the -rt kernel makes the overhead of an
> idle guest go down by a factor of 10-15:
>
> PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
> 2556 mingo 15 0 598m 159m 157m R 1.5 8.0 0:26.20 qemu
>
>

As usual, great news.

> ( for this to work on my system i have added a 'hyper' clocksource
> hypercall API for KVM guests to use - this is needed instead of the
> running-to-slowly TSC. )
>

What's the problem with the TSC? The only issue I'm aware of is that
the tsc might go backwards if the vcpu is migrated to another host cpu
(easily fixed).

A pv clocksource makes sense in any case.

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.