Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptivespinning

From: Darren Hart
Date: Tue Apr 06 2010 - 11:33:42 EST

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@xxxxxxxxxxxxx>
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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