Re: Preemptable Ticket Spinlock

[Rik van Riel]

On 04/22/2013 04:55 PM, Peter Zijlstra wrote:
> On Mon, 2013-04-22 at 16:46 -0400, Jiannan Ouyang wrote:

> > - pv-preemptable-lock has much less performance variance compare to
> > pv_lock, because it adapts to preemption within  VM,
> >    other than using rescheduling that increase VM interference
>
> I would say it has a _much_ worse worst case (and thus worse variance)
> than the paravirt ticket implementation from Jeremy. While full
> paravirt ticket lock results in vcpu scheduling it does maintain
> fairness.
>
> If you drop strict fairness you can end up in unbounded starvation
> cases and those are very ugly indeed.

If needed, Jiannan's scheme could easily be bounded to prevent
infinite starvation. For example, we could allow only the first
8 CPUs in line to jump the queue.

However, given the way that virtual CPUs get scheduled in and
out all the time, I suspect starvation is not a worry, and we
will not need the additional complexity to deal with it.

You may want to play around with virtualization a bit, to get
a feel for how things work in virt land.

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