From owner-freebsd-hackers@FreeBSD.ORG Fri Dec 15 00:12:34 2006 Return-Path: X-Original-To: freebsd-hackers@freebsd.org Delivered-To: freebsd-hackers@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id 02F9616A407 for ; Fri, 15 Dec 2006 00:12:34 +0000 (UTC) (envelope-from soralx@cydem.org) Received: from cydem.org (S0106000103ce4c9c.vc.shawcable.net [24.87.27.3]) by mx1.FreeBSD.org (Postfix) with ESMTP id AD6DD43CAB for ; Fri, 15 Dec 2006 00:03:51 +0000 (GMT) (envelope-from soralx@cydem.org) Received: from freen0de (unknown [192.168.0.251]) by cydem.org (Postfix/FreeBSD) with ESMTP id 5D1FF90A9F; Thu, 14 Dec 2006 16:05:26 -0800 (PST) Date: Thu, 14 Dec 2006 16:05:20 -0800 From: To: Message-ID: <20061214160520.2d28ff3a@freen0de> In-Reply-To: <200612141437.28724.mldodson@houston.rr.com> References: <200612141437.28724.mldodson@houston.rr.com> X-Mailer: Sylpheed-Claws 2.5.2 (GTK+ 2.10.6; i386-portbld-freebsd6.2) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: mldodson@houston.rr.com Subject: Re: "Syncing cpus" on a multi-cpu, dual core system X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 15 Dec 2006 00:12:34 -0000 > On a computational chemistry list I subscribe to there is a > current thread about multi-cpu systems needing to have the cpu > frequencies synced (this is in a Linux context). This is > evidently not just having the cpus running at nominally the same > frequency but something else in addition. A posting in the thread > said variations less than 0.1% were not problematic. However, the > poster said it was an issue in a dual cpu, dual core system he had > set up. > > My questions are: > 1. Is this real or an urban legend? If CPUs use the same FSB (as is the case with dual-core chip), they are already in sync. Right? For system that use multiple FSB clocks [like dual-(dualcore-CPU) systems], it might be possible to vary the clocks (as much as the manufacturer allows without hw modifications: e.g., SpeedStep, or something similar). Why someone would want to have CPUs running at precisely the same frequency is beyond my imagination. > Bud Dodson [SorAlx] ridin' VN1500-B2