From owner-freebsd-current@FreeBSD.ORG Mon Aug 25 10:14:30 2003 Return-Path: Delivered-To: freebsd-current@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 2FAB216A4BF for ; Mon, 25 Aug 2003 10:14:30 -0700 (PDT) Received: from mail.tcoip.com.br (erato.tco.net.br [200.220.254.10]) by mx1.FreeBSD.org (Postfix) with ESMTP id 4623D43F75 for ; Mon, 25 Aug 2003 10:14:27 -0700 (PDT) (envelope-from dcs@tcoip.com.br) Received: from tcoip.com.br ([10.0.2.6]) by mail.tcoip.com.br (8.11.6/8.11.6) with ESMTP id h7PHEJj03547 for ; Mon, 25 Aug 2003 14:14:19 -0300 Message-ID: <3F4A43EA.9090500@tcoip.com.br> Date: Mon, 25 Aug 2003 14:14:18 -0300 From: "Daniel C. Sobral" User-Agent: Mozilla/5.0 (X11; U; FreeBSD i386; en-US; rv:1.4) Gecko/20030702 X-Accept-Language: en-us, en, pt-br, ja MIME-Version: 1.0 To: current@freebsd.org References: <3F4A1CE2.6080806@freebsd.org> <20030825164907.GA17503@dragon.nuxi.com> In-Reply-To: <20030825164907.GA17503@dragon.nuxi.com> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: HTT on current X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.1 Precedence: list List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 25 Aug 2003 17:14:30 -0000 David O'Brien wrote: > On Mon, Aug 25, 2003 at 08:27:46AM -0600, Scott Long wrote: > >>Since HTT can lead to performance degradation in some (many?) cases, >>the second logical CPU's are halted by default. They are enabled, >>however, in order for interrupt routing to work right. Work is ongoing >>to make an HTT-aware scheduler, and make the enabling of the logical >>cores optional. > > > I've heard this several times and don't doubt it, but it would be nice to > know more about the issue. What type of cases? What benchmarks have > been run showing this? Well, I haven't actually seen any case where there was a performance gain instead of degradation. There are two problems with HTT. First, L1/L2 cache issues. Second, the virtual CPUs are not independent, and there are many cases where instructions in one virtual CPU stall the other. So take, for example, the case of a userland application on CPU0 stalling the kernel on CPU1. The case where HTT presents gains are for applications compiled in a way to maximize HTT benefits and minimize HTT stalling. It would be perhaps be best to restrict HTT usage to threads of a same application, and avoid them at all when in kernel. Intel disagrees, of course, and I haven't been a low level person for many, many years, so YMMV. :-) -- Daniel C. Sobral (8-DCS) Gerencia de Operacoes Divisao de Comunicacao de Dados Coordenacao de Seguranca VIVO Centro Oeste Norte Fones: 55-61-313-7654/Cel: 55-61-9618-0904 E-mail: Daniel.Capo@tco.net.br Daniel.Sobral@tcoip.com.br dcs@tcoip.com.br Outros: dcs@newsguy.com dcs@freebsd.org capo@notorious.bsdconspiracy.net A raccoon tangled with a 23,000 volt line today. The results blacked out 1400 homes and, of course, one raccoon. -- Steel City News