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    SubjectRe: RFC: Ideal Adaptive Spinning Conditions
    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

    > 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

    Darren Hart
    IBM Linux Technology Center
    Real-Time Linux Team

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