Re: RFC: Ideal Adaptive Spinning Conditions

[Darren Hart]

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.
>
>
> > > > o How much contention is a good match for adaptive spinning?
> > > >    - this is related to the number of threads to run in the test
> > > > o How many spinners should be allowed?
> > > >
> > > > I can share the kernel patches if people are interested, but they are 
> > > > really early, and I'm not sure they are of much value until I better 
> > > > understand the conditions where this is expected to be useful.
> > > Again, I don't know how you implemented your adaptive spinners, but the
> > > trick to it in -rt was that it would only spin while the owner of the
> > > lock was actually running. If it was not running, it would sleep. No
> > > point waiting for a sleeping task to release its lock.
> > It does exactly this.
>
> OK, that's good.
>
> > > Is this what you did? Because, IIRC, this only benefited spinlocks
> > > converted to mutexes. It did not help with semaphores, because
> > > semaphores could be held for a long time. Thus, it was good for short
> > > held locks, but hurt performance on long held locks.
> > Trouble is, I'm still seeing performance penalties even on the shortest 
> > critical section possible (lock();unlock();)
>
> performance penalties compared to what? not having adaptive at all?

Right. See the data in the original mail:


         futex_lock: Result: 635 Kiter/s
futex_lock_adaptive: Result: 542 Kiter/s

So 15% fewer lock/unlock iterations per second with in kernel adaptive 
spinning enabled for a critical section approaching 0 in length. But If 
we agree I'm taking the right approach, then it's time for me to polish 
things up a bit and send them out for review.

--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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