From owner-freebsd-net@FreeBSD.ORG Mon Jan 14 03:01:30 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 B1CB5E74; Mon, 14 Jan 2013 03:01:30 +0000 (UTC) (envelope-from bryanv@daemoninthecloset.org) Received: from torment.daemoninthecloset.org (ip-static-94-242-209-234.as5577.net [94.242.209.234]) by mx1.freebsd.org (Postfix) with ESMTP id 79C76E78; Mon, 14 Jan 2013 03:01:30 +0000 (UTC) Received: from sage.daemoninthecloset.org (unknown [70.114.209.60]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client CN "sage.daemoninthecloset.org", Issuer "daemoninthecloset.org" (verified OK)) by torment.localdomain (Postfix) with ESMTPS id EA99542C0867; Sun, 13 Jan 2013 07:17:16 +0100 (CET) X-Virus-Scanned: amavisd-new at daemoninthecloset.org Received: from sage.daemoninthecloset.org (sage.daemoninthecloset.org [127.0.1.1]) by sage.daemoninthecloset.org (Postfix) with ESMTP id AAFC07AA57; Sun, 13 Jan 2013 00:15:15 -0600 (CST) Date: Sun, 13 Jan 2013 00:15:13 -0600 (CST) From: Bryan Venteicher To: John Baldwin Message-ID: <330287752.17.1358057713463.JavaMail.root@daemoninthecloset.org> In-Reply-To: <201301111039.17673.jhb@freebsd.org> Subject: Re: To SMP or not to SMP MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Originating-IP: [10.51.1.14] X-Mailer: Zimbra 7.2.0_GA_2669 (ZimbraWebClient - GC23 (Mac)/7.2.0_GA_2669) Cc: freebsd-net@freebsd.org, 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: Mon, 14 Jan 2013 03:01:30 -0000 ----- Original Message ----- > From: "John Baldwin" > To: freebsd-net@freebsd.org > Cc: "Barney Cordoba" , "Peter Jeremy" > Sent: Friday, January 11, 2013 9:39:17 AM > Subject: Re: To SMP or not to SMP > > 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. > It would be nice to have the LOCK variants available even on UP kernels in non-hackish way. For VirtIO, we need to handle an guest UP kernel running on an SMP host. Whether this is an #define that forces the SMP atomics to be inlined, or if they're exposed with an _smp suffix. VirtIO currently uses mb() to enforce ordering. I have a patch to change to use atomic(9), but can only do so when VirtIO is included in the an SMP kernel (among other constraints - must have 16-bit atomic operations too). (FreeBSD's VirtIO is x86 only for now - but that will be changing soon; I haven't looked if other arch's atomic(9) behave differently for UP/SMP.) > -- > John Baldwin > _______________________________________________ > freebsd-net@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-net > To unsubscribe, send any mail to > "freebsd-net-unsubscribe@freebsd.org" >