From owner-freebsd-arch@FreeBSD.ORG Sun Jul 5 19:25:39 2009 Return-Path: Delivered-To: freebsd-arch@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 7EAA61065688 for ; Sun, 5 Jul 2009 19:25:39 +0000 (UTC) (envelope-from adrian.chadd@gmail.com) Received: from rv-out-0506.google.com (rv-out-0506.google.com [209.85.198.226]) by mx1.freebsd.org (Postfix) with ESMTP id 4DF998FC0A for ; Sun, 5 Jul 2009 19:25:39 +0000 (UTC) (envelope-from adrian.chadd@gmail.com) Received: by rv-out-0506.google.com with SMTP id f9so1149759rvb.43 for ; Sun, 05 Jul 2009 12:25:39 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=gamma; h=domainkey-signature:mime-version:sender:received:in-reply-to :references:date:x-google-sender-auth:message-id:subject:from:to:cc :content-type:content-transfer-encoding; bh=MwWBUiUmA3rfFwSy+RSR4TPhZqHzxWKp5S5AkmbxZDg=; b=BfQATZi29aHj22LDAfJ/VxiQKZgNGja3B9AjtwngB5JE/XoyJbj4YaJnhSCkcuu++P w9vD69K3RIDF6XGOwPAvVpH+LB8/w8PIaU4xRIyloqXVceJdxFxYmUSeIv3p6MMftaRM lv3991+TAP4WBgRtAfQlLG26j7Hv5mRSS3TbI= DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:sender:in-reply-to:references:date :x-google-sender-auth:message-id:subject:from:to:cc:content-type :content-transfer-encoding; b=m7mpvxBUIXffRnknVe/YAvkcd/qt7Wig8azV67UlGfYI+z5Kri/EO0uJbxjdVJN8ye 08c+n6zRuCfM2w5u9nYarHehMlqu1d9VCqk2RnoCc7cZEw7f8ypcp4AZhsKWO8Fanooo yAgbkze7xtOElGRPfhEnEbLxI+eOwIA3bB+sY= MIME-Version: 1.0 Sender: adrian.chadd@gmail.com Received: by 10.140.248.15 with SMTP id v15mr317122rvh.246.1246820316605; Sun, 05 Jul 2009 11:58:36 -0700 (PDT) In-Reply-To: <4A50F619.4020101@FreeBSD.org> References: <4A4FAA2D.3020409@FreeBSD.org> <20090705100044.4053e2f9@ernst.jennejohn.org> <4A50667F.7080608@FreeBSD.org> <20090705223126.I42918@delplex.bde.org> <4A50BA9A.9080005@FreeBSD.org> <20090706005851.L1439@besplex.bde.org> <4A50DEE8.6080406@FreeBSD.org> <20090706034250.C2240@besplex.bde.org> <4A50F619.4020101@FreeBSD.org> Date: Mon, 6 Jul 2009 02:58:36 +0800 X-Google-Sender-Auth: bf2a0323a2968e38 Message-ID: From: Adrian Chadd To: Alexander Motin Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: freebsd-arch@freebsd.org Subject: Re: DFLTPHYS vs MAXPHYS X-BeenThere: freebsd-arch@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Discussion related to FreeBSD architecture List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sun, 05 Jul 2009 19:25:39 -0000 2009/7/6 Alexander Motin : > In this tests you've got almost only negative side of effect, as you have > said, due to cache misses. Do you really have CPU with so small L2 cache? > Some kind of P3 or old Celeron? But with 64K MAXPHYS you just didn't get any > benefit from using bigger block size. All the world isn't your current desktop box with only SATA devices :) There have been and will be plenty of little embedded CPUs with tiny amounts of cache for quite some time to come. You're also doing simple stream IO tests. Please re-think the thought experiment with a whole lot of parallel IO going on rather than just straight single stream IO. Also, please realise that part of having your cache thrashed is what it does to the performance of -other- code. dd may be fast, but if you're constantly purging your caches by copying around all of that data, subsequent code has to go and freshen the cache again. On older and anaemic embedded/low power boxes the cost of a cache miss vs cache hit can still be quite expensive. 2c, Adrian