Messages in this thread | | | Subject | Re: RFC: Ideal Adaptive Spinning Conditions | From | "Peter W. Morreale" <> | Date | Wed, 31 Mar 2010 18:17:46 -0600 |
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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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