Re: [PATCH] kvm-vmx: add module parameter to avoid trapping HLT instructions (v2)

Srivatsa Vaddagiri
2010-12-03T11:22:04Z

On Fri, Dec 03, 2010 at 11:40:27AM +0200, Avi Kivity wrote:
> On 12/02/2010 09:14 PM, Chris Wright wrote:
> >Perhaps it should be a VM level option. And then invert the notion.
> >Create one idle domain w/out hlt trap. Give that VM a vcpu per pcpu
> >(pin in place probably). And have that VM do nothing other than hlt.
> >Then it's always runnable according to scheduler, and can "consume" the
> >extra work that CFS wants to give away.
>
> What's the difference between this and the Linux idle threads?

If we have 3 VMs and want to give them 25% each of a CPU, then having just idle
thread would end up giving them 33%. One way of achieving 25% rate limit is to
create a "dummy" or "filler" VM, and let it compete for resource, thus
rate-limiting everyone to 25% in this case. Essentially we are tackling
rate-limit problem by creating additional "filler" VMs/threads that will compete
for resource, thus keeping in check how much cpu resource is consumed by "real"
VMs. Admittedly not as neat as having a in-kernel support for rate-limit.

- vatsa