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    SubjectRe: RFC: Ideal Adaptive Spinning Conditions
    Avi Kivity wrote:
    > On 04/01/2010 06:54 PM, Darren Hart wrote:
    >>> A lock(); unlock(); loop spends most of its time with the lock held
    >>> or contended. Can you something like this:
    >>>
    >>>
    >>> lock();
    >>> for (i = 0; i < 1000; ++i)
    >>> asm volatile ("" : : : "memory");
    >>> unlock();
    >>> for (i = 0; i < 10000; ++i)
    >>> asm volatile ("" : : : "memory");
    >>
    >>
    >>
    >> Great idea. I'll be doing a more rigorous investigation on this of
    >> course, but I thought I'd share the results of just dumping this into
    >> the testcase:
    >>
    >> # ./futex_lock -i10000000
    >> futex_lock: Measure FUTEX_LOCK operations per second
    >> Arguments: iterations=10000000 threads=256 adaptive=0
    >> Result: 420 Kiter/s
    >> lock calls: 9999872
    >> lock syscalls: 665824 (6.66%)
    >> unlock calls: 9999872
    >> unlock syscalls: 861240 (8.61%)
    >>
    >> # ./futex_lock -a -i10000000
    >> futex_lock: Measure FUTEX_LOCK operations per second
    >> Arguments: iterations=10000000 threads=256 adaptive=1
    >> Result: 426 Kiter/s
    >> lock calls: 9999872
    >> lock syscalls: 558787 (5.59%)
    >> unlock calls: 9999872
    >> unlock syscalls: 603412 (6.03%)
    >>
    >> This is the first time I've seen adaptive locking have an advantage!
    >> The second set of runs showed a slightly greater advantage. Note that
    >> this was still with spinners being limited to one.
    >
    > Question - do all threads finish at the same time, or wildly different
    > times?

    I'm not sure, I can add some fairness metrics to the test that will help
    characterize how that's working. My suspicion is that there will be
    several threads that don't make any progress until the very end - since
    adaptive spinning is an "unfair" locking technique.

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


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