Messages in this thread | | | Subject | Re: RFC: Ideal Adaptive Spinning Conditions | From | Steven Rostedt <> | Date | Wed, 31 Mar 2010 22:25:53 -0400 |
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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?
> > > 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. > > It is currently under preempt_disable() just like mutexes. I asked Peter > why it was done that way for mutexes, but didn't really get an answer. > He did point out that since we check need_resched() at every iteration > that we won't run longer than our timeslice anyway, so it shouldn't be a > problem.
Sure it's not a problem ;-)
-- Steve
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