From owner-freebsd-questions@FreeBSD.ORG Sat Mar 26 21:17:31 2005 Return-Path: Delivered-To: freebsd-questions@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 74DAD16A4CF for ; Sat, 26 Mar 2005 21:17:31 +0000 (GMT) Received: from smtp11.wanadoo.fr (smtp11.wanadoo.fr [193.252.22.31]) by mx1.FreeBSD.org (Postfix) with ESMTP id E939443D5A for ; Sat, 26 Mar 2005 21:17:30 +0000 (GMT) (envelope-from atkielski.anthony@wanadoo.fr) Received: from me-wanadoo.net (unknown [127.0.0.1]) by mwinf1107.wanadoo.fr (SMTP Server) with ESMTP id 284941C000A7 for ; Sat, 26 Mar 2005 22:17:30 +0100 (CET) Received: from pix.atkielski.com (ASt-Lambert-111-2-1-3.w81-50.abo.wanadoo.fr [81.50.80.3]) by mwinf1107.wanadoo.fr (SMTP Server) with ESMTP id 06A941C000A3 for ; Sat, 26 Mar 2005 22:17:29 +0100 (CET) X-ME-UUID: 20050326211730274.06A941C000A3@mwinf1107.wanadoo.fr Date: Sat, 26 Mar 2005 22:17:29 +0100 From: Anthony Atkielski X-Priority: 3 (Normal) Message-ID: <1923568236.20050326221729@wanadoo.fr> To: freebsd-questions@freebsd.org In-Reply-To: <8C70054F30E8C8E-A44-3A1E4@mblk-d34.sysops.aol.com> References: <1641928994.20050326192811@wanadoo.fr> <8C700529A2DFD74-A44-3A157@mblk-d34.sysops.aol.com> <4245BCE1.6000201@makeworld.com> <8C70054F30E8C8E-A44-3A1E4@mblk-d34.sysops.aol.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Subject: Re: hyper threading. X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.1 Precedence: list Reply-To: freebsd-questions@freebsd.org List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 26 Mar 2005 21:17:31 -0000 em1897@aol.com writes: > I am offerring the correct information. Turning on SMP on > an HT machine will kill the systems performance much > more than hyperthreading will gain. Why? I've explained why hyperthreading can provide a modest gain in performance. Now explain to me why it would not. > I told him to test. > The degradation is easily measurable. If you can say with certainty that a degradation occurs, then you've already tested, in which case you can show your work. If you haven't tested, then you can't say anything with certainty, in which case your opinions are pure conjecture. A quick look at actual research done by various parties on the Web reveals that HT does provide the modest improvements to which I've alluded. It's not as impressive as two processors, but then again, nobody claimed it would be. It just makes better use of one processor and allows you to get more for your money from that processor. One advantage that I had not previous mentioned is that the availability of a logical processor for dispatch can improve response time in certain scenarios, even if the overall processor power doesn't increase that much. When compute-bound processes monopolize a single processor, the response time of the entire system can suffer; but if you have a second processor waiting for dispatch (even a logical HT processor), you can immediately attend to other tasks even as the compute-bound process runs, as long as it isn't launching multiple threads (which most such processes won't do). -- Anthony