Re: kvm & dyntick

[Avi Kivity <avi@qumranet.com>]

Ingo Molnar wrote:
> * Ingo Molnar <mingo@elte.hu> 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.

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