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

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Message-ID: <1270543721.1597.748.camel@laptop>
Date:	Tue, 06 Apr 2010 10:48:41 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	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 00:15 +0300, Avi Kivity wrote:
> 
> An interesting (but perhaps difficult to achieve) optimization would be 
> to spin in userspace. 

System call overhead isn't that large, but if you want to amortize that
you can do a very small spin in userspace and then take the syscall:

  try
  spin
  try
  syscall

like.

Complicating anything beyond that is just not worth it.

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