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

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Date:	Tue, 06 Apr 2010 16:51:18 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ulrich Drepper <drepper@...il.com>
Cc:	Avi Kivity <avi@...hat.com>, Darren Hart <dvhltc@...ibm.com>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Eric Dumazet <eric.dumazet@...il.com>,
	"Peter W. Morreale" <pmorreale@...ell.com>,
	Rik van Riel <riel@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Sven-Thorsten Dietrich <sdietrich@...ell.com>,
	Chris Mason <chris.mason@...cle.com>,
	John Cooper <john.cooper@...rd-harmonic.com>,
	Chris Wright <chrisw@...s-sol.org>
Subject: Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive 
 spinning

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

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