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

[Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>]

On Fri, Dec 03, 2010 at 09:28:25AM -0800, Chris Wright wrote:
> * Srivatsa Vaddagiri (vatsa@linux.vnet.ibm.com) wrote:
> > On Thu, Dec 02, 2010 at 11:14:16AM -0800, 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.
> > 
> > That's not sufficient. Lets we have 3 guests A, B, C that need to be
> > rate limited to 25% on a single cpu system. We create this idle guest
> > D that is 100% cpu hog as per above definition. Now when one of the
> > guest is idle, what ensures that the idle cycles of A is given only
> > to D and not partly to B/C?
> 
> Yeah, I pictured priorties handling this.

All guest are of equal priorty in this case (that's how we are able to divide 
time into 25% chunks), so unless we dynamically boost D's priority based on how
idle other VMs are, its not going to be easy!

- vatsa