Re: RFC: Ideal Adaptive Spinning Conditions

[Rik van Riel]

On 03/31/2010 10:25 PM, Steven Rostedt wrote:
> On Wed, 2010-03-31 at 19:13 -0700, Darren Hart wrote:
> > Steven Rostedt wrote:
> > > On Wed, 2010-03-31 at 16:21 -0700, Darren Hart wrote:
> > >
> > > > o What type of lock hold times do we expect to benefit?
> > >
> > > 0 (that's a zero) :-p
> > >
> > > I haven't seen your patches but you are not doing a heuristic approach,
> > > are you? That is, do not "spin" hoping the lock will suddenly become
> > > free. I was against that for -rt and I would be against that for futex
> > > too.
> >
> > I'm not sure what you're getting at here. Adaptive spinning is indeed
> > hoping the lock will become free while you are spinning and checking
> > it's owner...
>
> I'm talking about the original idea people had of "lets spin for 50us
> and hope it is unlocked before then", which I thought was not a good
> idea.

Maybe not a good idea when running on bare metal, but it
could be a big help when running virtualized.

A lock with a short hold time can, occasionally, have a
very long hold time, when the VCPU holding the lock is
preempted by the host/hypervisor.

An adaptive lock would spin-and-acquire if the lock holder
is running, while turning into a sleep lock when the lock
holder has been preempted.

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