Messages in this thread | | | Date | Tue, 06 Apr 2010 08:33:07 -0700 | From | Darren Hart <> | Subject | Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive spinning |
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Peter Zijlstra wrote: > On Tue, 2010-04-06 at 07:47 -0700, Ulrich Drepper wrote: >> On Tue, Apr 6, 2010 at 01:48, Peter Zijlstra <peterz@infradead.org> >> wrote: >>> try >>> spin >>> try >>> syscall >> This is available for a long time in the mutex implementation >> (PTHREAD_MUTEX_ADAPTIVE_NP mutex type). It hasn't show much >> improvement if any. There were some people demanding this support for >> as far as I know they are not using it now. This is adaptive >> spinning, learning from previous calls how long to wait. But it's >> still unguided. There is no way to get information like "the owner >> has been descheduled". > > That's where the FUTEX_LOCK thing comes in, it does all those, the above > was a single spin loop to amortize the syscall overhead. > > I wouldn't make it any more complex than a single pause ins, syscalls > are terribly cheap these days.
And yet they still seem to have a real impact on the futex_lock benchmark. Perhaps I am just still looking at pathological cases, but there is a strong correlation between high syscall counts and really low iterations per second. Granted this also correlates with lock contention. However, when using the same period and duty-cycle I find that a locking mechanism that makes significantly fewer syscalls also significantly outperforms one that makes more. Kind of handwavy stilly, I'll have more numbers this afternoon.
-- Darren Hart IBM Linux Technology Center Real-Time Linux Team
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