From owner-freebsd-net@FreeBSD.ORG Fri Jan 11 20:42:03 2013 Return-Path: Delivered-To: freebsd-net@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) by hub.freebsd.org (Postfix) with ESMTP id 624BC93E for ; Fri, 11 Jan 2013 20:42:03 +0000 (UTC) (envelope-from jhb@freebsd.org) Received: from bigwig.baldwin.cx (bigknife-pt.tunnel.tserv9.chi1.ipv6.he.net [IPv6:2001:470:1f10:75::2]) by mx1.freebsd.org (Postfix) with ESMTP id 3A6DCF71 for ; Fri, 11 Jan 2013 20:42:03 +0000 (UTC) Received: from ralph.baldwin.cx (c-68-39-198-164.hsd1.de.comcast.net [68.39.198.164]) by bigwig.baldwin.cx (Postfix) with ESMTPSA id A6763B963; Fri, 11 Jan 2013 15:42:02 -0500 (EST) From: John Baldwin To: freebsd-net@freebsd.org Subject: Re: To SMP or not to SMP Date: Fri, 11 Jan 2013 10:39:17 -0500 User-Agent: KMail/1.13.7 (FreeBSD/9.1-PRERELEASE; KDE/4.8.4; amd64; ; ) References: <1357611958.66651.YahooMailClassic@web121603.mail.ne1.yahoo.com> <20130110193659.GA27156@server.rulingia.com> In-Reply-To: <20130110193659.GA27156@server.rulingia.com> MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-15" Content-Transfer-Encoding: 7bit Message-Id: <201301111039.17673.jhb@freebsd.org> X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.2.7 (bigwig.baldwin.cx); Fri, 11 Jan 2013 15:42:02 -0500 (EST) Cc: Barney Cordoba , Peter Jeremy X-BeenThere: freebsd-net@freebsd.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: Networking and TCP/IP with FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 11 Jan 2013 20:42:03 -0000 On Thursday, January 10, 2013 02:36:59 PM Peter Jeremy wrote: > On 2013-Jan-07 18:25:58 -0800, Barney Cordoba wrote: > >I have a situation where I have to run 9.1 on an old single core > >box. Does anyone have a handle on whether it's better to build a non > >SMP kernel or to just use a standard SMP build with just the one > >core? > > Another input for this decision is kern/173322. Currently on x86, > atomic operations within kernel modules are implemented using calls > to code in the kernel, which do or don't use lock prefixes depending > on whethur the kernel was built as SMP. My proposed change changes > kernel modules to inline atomic operations but always include lock > prefixes (effectively reverting r49999). I'm appreciate anyone who > feels like testing the impact of this change. Presumably a locked atomic op is cheaper than a function call then? The current setup assumes the opposite. I think we should actually do this for atomics in modules on x86: 1) If a module is built standalone, it should do whichever is cheaper: a function call or always use "LOCK". 2) If a module is built as part of the kernel build, it should use inlined atomics that match what the kernel does. Thus, modules built with a non-SMP kernel would use inlined atomic ops that do not use LOCK. We have a way to detect this now (some HAVE_FOO #define added in the past few years) that we didn't back when this bit of atomic.h was written. -- John Baldwin