Re: [PATCH RFC 2/2] kvm: Be courteous to other VMs in overcommittedscenario in PLE handler

[Peter Zijlstra]

On Mon, 2012-09-24 at 17:43 +0200, Avi Kivity wrote:
> Wouldn't this correspond to the scheduler interrupt firing and causing a
> reschedule?  I thought the timer was programmed for exactly the point in
> time that CFS considers the right time for a switch.  But I'm basing
> this on my mental model of CFS, not CFS itself. 

No, we tried this for hrtimer kernels for a while, but programming
hrtimers the whole time (every actual task-switch) turns out to be far
too expensive. So we're back to HZ ticks and 'polling' the preemption
state.

Even if we remove all the hrtimer infrastructure overhead (can do with a
few hacks) setting the hardware requires going out to the LAPIC, which
is stupid slow.

Some hardware actually has fast/reliable/usable timers, sadly none of it
is popular.
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