Re: Preemptable Ticket Spinlock

[Gleb Natapov <gleb@redhat.com>]

On Mon, Apr 22, 2013 at 07:08:06PM -0400, Rik van Riel wrote:
> 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.
> 
FWIW RHEL6 uses unfair spinlock when it runs as a guest. We never got
reports about problems due to this on any scale.

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

--
			Gleb.