From owner-freebsd-current@FreeBSD.ORG  Thu May  6 11:26:26 2004
Return-Path: <owner-freebsd-current@FreeBSD.ORG>
Delivered-To: freebsd-current@freebsd.org
Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125])
	by hub.freebsd.org (Postfix) with ESMTP
	id D082516A4CE; Thu,  6 May 2004 11:26:26 -0700 (PDT)
Received: from duke.cs.duke.edu (duke.cs.duke.edu [152.3.140.1])
	by mx1.FreeBSD.org (Postfix) with ESMTP
	id 57D5543D45; Thu,  6 May 2004 11:26:26 -0700 (PDT)
	(envelope-from gallatin@cs.duke.edu)
Received: from grasshopper.cs.duke.edu (grasshopper.cs.duke.edu
	[152.3.145.30])
	by duke.cs.duke.edu (8.12.10/8.12.10) with ESMTP id i46IQPxZ018131
	(version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO);
	Thu, 6 May 2004 14:26:25 -0400 (EDT)
Received: (from gallatin@localhost)
	by grasshopper.cs.duke.edu (8.12.9p2/8.12.9/Submit) id i46IQJp0002548;
	Thu, 6 May 2004 14:26:19 -0400 (EDT)
	(envelope-from gallatin)
From: Andrew Gallatin <gallatin@cs.duke.edu>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Message-ID: <16538.33610.967480.120194@grasshopper.cs.duke.edu>
Date: Thu, 6 May 2004 14:26:18 -0400 (EDT)
To: John Baldwin <jhb@FreeBSD.org>
In-Reply-To: <200405061411.27216.jhb@FreeBSD.org>
References: <FE045D4D9F7AED4CBFF1B3B813C85337021AB38C@mail.sandvine.com>
	<20040506184749.R19447@gamplex.bde.org>
	<200405061411.27216.jhb@FreeBSD.org>
X-Mailer: VM 6.75 under 21.1 (patch 12) "Channel Islands" XEmacs Lucid
cc: current@FreeBSD.org
Subject: Re: 4.7 vs 5.2.1 SMP/UP bridging performance
X-BeenThere: freebsd-current@freebsd.org
X-Mailman-Version: 2.1.1
Precedence: list
List-Id: Discussions about the use of FreeBSD-current
	<freebsd-current.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-current>,
	<mailto:freebsd-current-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-current>
List-Post: <mailto:freebsd-current@freebsd.org>
List-Help: <mailto:freebsd-current-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-current>,
	<mailto:freebsd-current-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Thu, 06 May 2004 18:26:26 -0000


John Baldwin writes:

 > and lfence is only on PIV+.  I don't recall when mfence first appeared.. 
 > perhaps PII?  If the lock is really expensive, then perhaps we could make 
 > atomic_cmpset() be actual functions (ugh) rather than inlines that did a 
 > branch to use foofence for PIV rather than the default.  The branches would 
 > suck, but it might be faster than the lock.  Of course, this would greatly 
 > pessimize non-PIV.

According to http://www.sandpile.org/ia32/coherent.htm,
both mfence and lfence require SSE2, so sfence has the broadest
coverage. 

But since only P4 needs it, and since lfence is ~25% cheaper, maybe
there should be a separate config option for it, and it should be a
straight conditional compile option for those of us cursed w/P4s.

Drew