From owner-freebsd-questions@FreeBSD.ORG Sun Mar 27 18:02:00 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 1678D16A4CE for ; Sun, 27 Mar 2005 18:02:00 +0000 (GMT) Received: from smtp9.wanadoo.fr (smtp9.wanadoo.fr [193.252.22.22]) by mx1.FreeBSD.org (Postfix) with ESMTP id 81CC543D54 for ; Sun, 27 Mar 2005 18:01:59 +0000 (GMT) (envelope-from atkielski.anthony@wanadoo.fr) Received: from me-wanadoo.net (localhost [127.0.0.1]) by mwinf0907.wanadoo.fr (SMTP Server) with ESMTP id DF5EB1C00174 for ; Sun, 27 Mar 2005 20:01:57 +0200 (CEST) Received: from pix.atkielski.com (ASt-Lambert-111-2-1-3.w81-50.abo.wanadoo.fr [81.50.80.3]) by mwinf0907.wanadoo.fr (SMTP Server) with ESMTP id B912B1C00145 for ; Sun, 27 Mar 2005 20:01:57 +0200 (CEST) X-ME-UUID: 20050327180157758.B912B1C00145@mwinf0907.wanadoo.fr Date: Sun, 27 Mar 2005 20:01:57 +0200 From: Anthony Atkielski X-Priority: 3 (Normal) Message-ID: <1312606695.20050327200157@wanadoo.fr> To: freebsd-questions@freebsd.org In-Reply-To: <8C701039E12745C-450-3B05F@mblk-d13.sysops.aol.com> References: <1641928994.20050326192811@wanadoo.fr> <8C700529A2DFD74-A44-3A157@mblk-d34.sysops.aol.com> <439876144.20050326220638@wanadoo.fr> <8C7006AE7E80573-FAC-3B652@mblk-r28.sysops.aol.com> <49251524.20050326234521@wanadoo.fr> <8C7007D5D4D30D2-A38-3B313@mblk-r33.sysops.aol.com> <14510304120.20050327123336@wanadoo.fr> <8C701039E12745C-450-3B05F@mblk-d13.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: Sun, 27 Mar 2005 18:02:00 -0000 em1897@aol.com writes: > You know, you spout all of this wonderful theory without considering > the quality of the implementation. Somethings can be derived directly from theory. If you know the design of the hardware, you can predict that two processors will provide x% increment of throughput over a single processor, even if you don't actually measure them. In my case, I cite both theory and my own experience in measuring actual systems. The general principles of behavior of multiprocessor systems are well understood, although specific implementations vary. It is clear, based even on design data alone, that hyperthreading will generally improve throughput and should never diminish it (disregarding OS overhead). It is equally clear that the gain won't be as great as having physically independent processors, but the idea of putting more of the idle processor logic to work is a good one. > And a key point that you consistently overlook is that FreeBSD 5.x is > a particularly poor implementation of SMP. Linux and Dragonfly get 80% > improvement in performance with a 2nd processor, and FreeBSD doesn't. I'd need to see measurements to substantiate this. In general, when it comes to optimization, it's best not to fret too much over how many percentage points of processor power or throughput you gain or lose with specific configuration or implementation choices. If your system is running so close to the wire that five percent makes the difference between 100% busy and less than 100% busy, you need more hardware in any case. > The concept that the kernel is poorly implemented by userland is well > done is just not an assumption that you can make. Actually, it's not something that I spend a lot of time thinking about. Right now, my production system is never more than 0.4% busy. And if it were 99% busy, I'd be looking at faster hardware, no matter what OS or HT/MP options I might have implemented. -- Anthony