From owner-freebsd-hackers@FreeBSD.ORG Sat Dec 16 18:30:15 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 2D9AE16A412 for ; Sat, 16 Dec 2006 18:30:15 +0000 (UTC) (envelope-from imp@bsdimp.com) Received: from harmony.bsdimp.com (vc4-2-0-87.dsl.netrack.net [199.45.160.85]) by mx1.FreeBSD.org (Postfix) with ESMTP id 970DC43CA6 for ; Sat, 16 Dec 2006 18:30:14 +0000 (GMT) (envelope-from imp@bsdimp.com) Received: from localhost (localhost [127.0.0.1]) by harmony.bsdimp.com (8.13.4/8.13.4) with ESMTP id kBGITHZP095896; Sat, 16 Dec 2006 11:29:17 -0700 (MST) (envelope-from imp@bsdimp.com) Date: Sat, 16 Dec 2006 11:30:17 -0700 (MST) Message-Id: <20061216.113017.-345495563.imp@bsdimp.com> To: mldodson@houston.rr.com From: "M. Warner Losh" In-Reply-To: <200612141437.28724.mldodson@houston.rr.com> References: <200612141437.28724.mldodson@houston.rr.com> X-Mailer: Mew version 4.2 on Emacs 21.3 / Mule 5.0 (SAKAKI) Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-2.0 (harmony.bsdimp.com [127.0.0.1]); Sat, 16 Dec 2006 11:29:17 -0700 (MST) Cc: freebsd-hackers@freebsd.org 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: Sat, 16 Dec 2006 18:30:15 -0000 In message: <200612141437.28724.mldodson@houston.rr.com> "M. L. Dodson" writes: : 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? : 2. If real, is this a Linuxism or is FreeBSD affected as well? : 3. How do you "sync" the cpus, if it is needed? : 4. anything else some one wants to expound on along this line. Linux keeps the cpu's frequencies 'synchronized' so that it can use the fast time keeping hardware (TSC). FreeBSD uses different mechanisms for its timekeeping, so doesn't need to keep them in sync at all, and doesn't even try at this point. Maybe this is what they are talking about... Warner