From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S261156AbVADLKk (ORCPT ); Tue, 4 Jan 2005 06:10:40 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S261160AbVADLKk (ORCPT ); Tue, 4 Jan 2005 06:10:40 -0500 Received: from colin2.muc.de ([193.149.48.15]:16398 "HELO colin2.muc.de") by vger.kernel.org with SMTP id S261156AbVADLJm (ORCPT ); Tue, 4 Jan 2005 06:09:42 -0500 Date: 4 Jan 2005 12:09:40 +0100 Date: Tue, 4 Jan 2005 12:09:40 +0100 From: Andi Kleen To: William Lee Irwin III Cc: colin@coesta.com, linux-kernel@vger.kernel.org Subject: Re: Max CPUs on x86_64 under 2.6.x Message-ID: <20050104110940.GA32022@muc.de> References: <44438.202.154.120.74.1104760841.squirrel@www.coesta.com> <20050104022034.GB2708@holomorphy.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20050104022034.GB2708@holomorphy.com> User-Agent: Mutt/1.4.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 03, 2005 at 06:20:34PM -0800, William Lee Irwin III wrote: > "Colin Coe" writes: > >> Why is the number of CPUs on the x86_64 architecture only 8 but under i386 > >> it is 255? > >> I've searched the list archives and Google but can't find an answer. > > On Tue, Jan 04, 2005 at 01:34:50AM +0100, Andi Kleen wrote: > > Post 2.6.10 x86-64 will support more CPUs. 2.6.10 actually does too, > > but the Kconfig hadn't been changed then. Previously there was an > > 8 CPU APIC driver limit, however it turned out later that it doesn't > > apply to some Opteron machines. > > Barring cpus with a different onboard interrupt controller from the > xAPIC or the use of external interrupt controllers to assist with cpu > addressing, 255 serves as an architectural limit for Opteron as well. Yes, 255 is the limit, but not 8. Opteron can enable a special flat mode that allows flat APIC addressing upto 255. I assume the BIOS will set that bit on machines with that many CPUs. I recently audited the flat APIC code and I think it should work without changes to 255 CPUs, but I wasn't able to test it so far. However 2.6.10 supports clustered mode now anyways, so you could probably use more CPUs given the right x86-64 IBM machine. I don't know if that has been tested so far. The change to extend NR_CPUs and the max number of nodes just went into Linus' tree, you would need a recent bk snapshot. http://linux.bkbits.net:8080/linux-2.6/cset@41da1ff62QYI89HDgrcKwnBAz6wgQg?nav=index.html|ChangeSet@-1d -Andi