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    SubjectRe: RFC: Ideal Adaptive Spinning Conditions
    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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