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SubjectRe: RFC: Ideal Adaptive Spinning Conditions
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On Wed, 2010-03-31 at 19:38 -0400, 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.
>
> > 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.

Right. This was *critical* for the adaptive rtmutex. Note in the RT
patch, everybody spins as long as the current owner is on CPU.

FWIW, IIRC, Solaris has a heuristic approach where incoming tasks spin
for a period of time before going to sleep. (Cray UINCOS did the same)

>
> 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.
>

nod. The entire premise was based on the fact that we were converting
spinlocks, which by definition were short held locks. What I found
during early development was that the sleep/wakeup cycle was more
intrusive for RT than spinning.

IIRC, I measured something like 380k context switches/second prior to
the adaptive patches for a dbench test and we cut this down to somewhere
around 50k, with a corresponding increase in throughput. (I can't
remember specific numbers any more, it was a while ago... ;-)

When applied to semaphores, the benefit was in the noise range as I
recall..

(dbench was chosen due to the heavy contention on the dcache spinlock)


Best,
-PWM


> 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.
>
> 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.
>
> -- Steve
>
>




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