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

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Message-ID: <i2ta36005b51004101653ka0411bc6z5841f35cbd1e41d1@mail.gmail.com>
Date:	Sat, 10 Apr 2010 16:53:33 -0700
From:	Ulrich Drepper <drepper@...il.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Darren Hart <dvhltc@...ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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 Sat, Apr 10, 2010 at 16:35, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
> You only need one page per 4096 threads

Very expensive.  Each cache line would be fought over by 64 threads.
Constant RFOs make context switches significantly slower.

At most 4096/64 = 64 threads should share one page.  One page would
still cover almost all processes.
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