Messages in this thread | | | Date | Wed, 31 Mar 2010 19:13:20 -0700 | From | Darren Hart <> | Subject | Re: RFC: Ideal Adaptive Spinning Conditions |
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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...
> >> 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.
> 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();)
> 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.
> Then the application would need to determine which to use. An adaptive > spinner for short held locks, and a normal futex for long held locks.
Yes, this was intended to be an optional thing (and certainly not the default).
-- Darren Hart IBM Linux Technology Center Real-Time Linux Team
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