Re: RFC: Ideal Adaptive Spinning Conditions

[Steven Rostedt]

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?

> 
> > If userspace is going to do this, I guess the blocked task would need to
> > go into kernel, and spin there (with preempt enabled) if the task is
> > still active and holding the lock.
> 
> It is currently under preempt_disable() just like mutexes. I asked Peter 
> why it was done that way for mutexes, but didn't really get an answer. 
> He did point out that since we check need_resched() at every iteration 
> that we won't run longer than our timeslice anyway, so it shouldn't be a 
>   problem.

Sure it's not a problem ;-)

-- Steve


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