From owner-svn-src-head@FreeBSD.ORG  Fri Jun 18 12:08:56 2010
Return-Path: <owner-svn-src-head@FreeBSD.ORG>
Delivered-To: svn-src-head@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id D5ED81065672;
	Fri, 18 Jun 2010 12:08:56 +0000 (UTC)
	(envelope-from rwatson@FreeBSD.org)
Received: from cyrus.watson.org (cyrus.watson.org [65.122.17.42])
	by mx1.freebsd.org (Postfix) with ESMTP id A8D8D8FC0C;
	Fri, 18 Jun 2010 12:08:56 +0000 (UTC)
Received: from fledge.watson.org (fledge.watson.org [65.122.17.41])
	by cyrus.watson.org (Postfix) with ESMTPS id 2D36746C5B;
	Fri, 18 Jun 2010 08:08:56 -0400 (EDT)
Date: Fri, 18 Jun 2010 13:08:56 +0100 (BST)
From: Robert Watson <rwatson@FreeBSD.org>
X-X-Sender: robert@fledge.watson.org
To: Lawrence Stewart <lstewart@freebsd.org>
In-Reply-To: <4C1AD292.5070508@freebsd.org>
Message-ID: <alpine.BSF.2.00.1006181307360.90129@fledge.watson.org>
References: <201006130239.o5D2du3m086332@svn.freebsd.org>
	<20100613101025.GD1320@garage.freebsd.pl>
	<4C158B71.205@freebsd.org>
	<20100614085205.GD13238@deviant.kiev.zoral.com.ua>
	<4C1605A7.2000202@freebsd.org>
	<20100614104349.GF13238@deviant.kiev.zoral.com.ua>
	<4C198A90.3060905@freebsd.org>
	<20100617071300.GX13238@deviant.kiev.zoral.com.ua>
	<4C1AD292.5070508@freebsd.org>
User-Agent: Alpine 2.00 (BSF 1167 2008-08-23)
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed
Cc: Matthew Fleming <mdf@freebsd.org>, src-committers@freebsd.org,
	Pawel Jakub Dawidek <pjd@freebsd.org>,
	John Baldwin <jhb@freebsd.org>, svn-src-all@freebsd.org,
	svn-src-head@freebsd.org, brde@optusnet.com.au,
	Kostik Belousov <kostikbel@gmail.com>
Subject: Re: svn commit: r209119 - head/sys/sys
X-BeenThere: svn-src-head@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: SVN commit messages for the src tree for head/-current
	<svn-src-head.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/svn-src-head>,
	<mailto:svn-src-head-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/svn-src-head>
List-Post: <mailto:svn-src-head@freebsd.org>
List-Help: <mailto:svn-src-head-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/svn-src-head>,
	<mailto:svn-src-head-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Fri, 18 Jun 2010 12:08:56 -0000


On Fri, 18 Jun 2010, Lawrence Stewart wrote:

> True but I figured on large SMP systems where the potential to process more 
> is likely, 32bit counters per cpu may be enough to avoid overflow but the 
> aggregate number of events may exceed a 32bit variable. I suspect you're 
> right though and that if there's a likely chance the aggregate could 
> overflow, then the DPCPU var should simply be made 64bit to also remove any 
> possibility of individual PCPU counters overflowing.
>
> I'll commit the above version of the macro this evening (GMT+10) unless I 
> hear any objections. Thanks to all of you for your input.

The only reservation I have, really, is that 64-bit writes are non-atomic on 
i386 and other 32-bit architectures (or, at least, I think they are).  This 
means DPCPU_SUM may encounter non-atomicity rather than just staleness in the 
values it reads as it iterates.  That said, we should probably use 64-bit 
anyway, because 32-bit counters are gauche. :-)

Robert