From owner-freebsd-hackers@FreeBSD.ORG  Thu Sep 15 21:21:28 2011
Return-Path: <owner-freebsd-hackers@FreeBSD.ORG>
Delivered-To: freebsd-hackers@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id 640C5106566C;
	Thu, 15 Sep 2011 21:21:28 +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 3ECF08FC15;
	Thu, 15 Sep 2011 21:21:28 +0000 (UTC)
Received: from fledge.watson.org (fledge.watson.org [65.122.17.41])
	by cyrus.watson.org (Postfix) with ESMTPS id B92E646B3B;
	Thu, 15 Sep 2011 17:21:27 -0400 (EDT)
Date: Thu, 15 Sep 2011 22:21:27 +0100 (BST)
From: Robert Watson <rwatson@FreeBSD.org>
X-X-Sender: robert@fledge.watson.org
To: "K. Macy" <kmacy@freebsd.org>
In-Reply-To: <CAHM0Q_P7NRQXay-ho1E--P4QnV5kr0eTo48NT21dbJjpbmAF=Q@mail.gmail.com>
Message-ID: <alpine.BSF.2.00.1109152219380.14815@fledge.watson.org>
References: <CACqU3MXQ6tD804fKymeFeKDnHndSXVvHJwepYztB4DsnNmtMiw@mail.gmail.com>
	<CAHM0Q_P7NRQXay-ho1E--P4QnV5kr0eTo48NT21dbJjpbmAF=Q@mail.gmail.com>
User-Agent: Alpine 2.00 (BSF 1167 2008-08-23)
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed
Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org>,
	Arnaud Lacombe <lacombar@gmail.com>
Subject: Re: buf_ring(9) API precisions
X-BeenThere: freebsd-hackers@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Technical Discussions relating to FreeBSD
	<freebsd-hackers.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>, 
	<mailto:freebsd-hackers-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-hackers>
List-Post: <mailto:freebsd-hackers@freebsd.org>
List-Help: <mailto:freebsd-hackers-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>,
	<mailto:freebsd-hackers-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Thu, 15 Sep 2011 21:21:28 -0000

On Thu, 15 Sep 2011, K. Macy wrote:

>> Why are you making an MD guess, the amount of padding to fit the size of a 
>> cache line, in MI API ? Strangely enough, you did not make this assumption 
>> in, say r205488 (picked randomly).
>
> It has been several years, and I haven't done any work in svn in over a 
> year, I don't remember. I probably meant to refine it in a later iteration.
>
> If you would like to send me a patch addressing this I'd be more than happy 
> to apply it if appropriate. Otherwise, I will deal with it some time after 9 
> settles.
>
> Thanks for pointing this out.

I'm not sure if gcc (and friends) allow __aligned(CACHE_LINE_SIZE) to be used 
on individual elements of a struct (causing appropriate padding to be added), 
but that may be one option here.  Of course, that introduces a further 
alignment requirement on the struct itself, so a moderate amount of care would 
need to be used.

Robert