From owner-freebsd-current@FreeBSD.ORG  Thu Aug  5 22:17:42 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 3C14F16A4CE; Thu,  5 Aug 2004 22:17:42 +0000 (GMT)
Received: from pooker.samsco.org (pooker.samsco.org [168.103.85.57])
	by mx1.FreeBSD.org (Postfix) with ESMTP
	id 9134443D3F; Thu,  5 Aug 2004 22:17:39 +0000 (GMT)
	(envelope-from scottl@samsco.org)
Received: from [192.168.0.201] ([192.168.0.201])
	(authenticated bits=0)
	by pooker.samsco.org (8.12.11/8.12.10) with ESMTP id i75MGpWH008311;
	Thu, 5 Aug 2004 16:16:51 -0600 (MDT)
	(envelope-from scottl@samsco.org)
Message-ID: <4112B184.8010303@samsco.org>
Date: Thu, 05 Aug 2004 16:15:32 -0600
From: Scott Long <scottl@samsco.org>
User-Agent: Mozilla/5.0 (X11; U; FreeBSD i386; en-US; rv:1.7.1) Gecko/20040801
X-Accept-Language: en-us, en
MIME-Version: 1.0
To: John Baldwin <jhb@freebsd.org>
References: <20040805050422.GA41201@cat.robbins.dropbear.id.au>
	<200408051759.53079.jhb@FreeBSD.org>
In-Reply-To: <200408051759.53079.jhb@FreeBSD.org>
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Transfer-Encoding: 7bit
X-Spam-Status: No, hits=0.0 required=3.8 tests=none autolearn=no version=2.63
X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on pooker.samsco.org
cc: freebsd-current@freebsd.org
cc: Tim Robbins <tjr@freebsd.org>
Subject: Re: Atomic operations on i386/amd64
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, 05 Aug 2004 22:17:42 -0000

John Baldwin wrote:
> On Thursday 05 August 2004 01:04 am, Tim Robbins wrote:
> 
>>Is there any particular reason why atomic_load_acq_*() and
>>atomic_store_rel_*() are implemented with CMPXCHG and XCHG instead of
>>MOV on i386/amd64 UP?
> 
> 
> Actually, using mov instead of lock xchg for store_rel reduced performance in 
> some benchmarks Scott ran on an SMP machine, I'm guessing due to the higher 
> latency of locks becoming available to other CPUs.  I'm still waiting for 
> benchmark results on UP to see if the change should be made under #ifndef SMP 
> or some such.
> 
> 
>>Also, could we use MFENCE/LFENCE/SFENCE in combination with MOV on
>>SMP systems instead of LOCK CMPXCHG / (implied LOCK) XCHG?
> 
> 
> MFENCE and LFENCE only exist on the P4.  SFENCE only exists on P3+, so to do 
> so you'd lose the ability to run on PII's and earlier.  Also, if you use more 
> than SFENCE you lose PIII's.  Note that amd64 could probably be changed 
> though since they might all have fences, in which case that might be 
> something to benchmark on both UP and SMP to see what kind of difference it 
> makes.
> 

We always have the ability to define PENTIUM2_CPU, PENTIUM3_CPU, and 
PENTIUM4_CPU cpu types in the kernel and then ifdef the code
appropriately (and ship with the lowest common denominator like we do
for I386/I486/I586/I686.)

Scott