Messages in this thread | | | Date | Wed, 31 Mar 2010 22:15:48 -0700 | From | Darren Hart <> | Subject | Re: RFC: Ideal Adaptive Spinning Conditions |
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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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