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

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Message-ID: <1270568249.20295.37.camel@laptop>
Date:	Tue, 06 Apr 2010 17:37:29 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Darren Hart <dvhltc@...ibm.com>
Cc:	Ulrich Drepper <drepper@...il.com>, Avi Kivity <avi@...hat.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 08:33 -0700, Darren Hart wrote:
> 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@...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.
> 
> 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.

Sure, but I'm still not sure why FUTEX_LOCK ends up making more syscalls
than FUTEX_WAIT based locking. Both should only do the syscall when the
lock is contended, both should only ever do 1 syscall per acquire,
right?



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