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

From: Peter Zijlstra
Date: Tue Apr 06 2010 - 10:51:32 EST

On Tue, 2010-04-06 at 07:47 -0700, Ulrich Drepper wrote:
> On Tue, Apr 6, 2010 at 01:48, Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> 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.

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