From owner-freebsd-hackers@FreeBSD.ORG Mon Sep 25 19:57:57 2006 Return-Path: X-Original-To: freebsd-hackers@freebsd.org Delivered-To: freebsd-hackers@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 78B9816A403 for ; Mon, 25 Sep 2006 19:57:57 +0000 (UTC) (envelope-from uspoerlein@gmail.com) Received: from nf-out-0910.google.com (nf-out-0910.google.com [64.233.182.188]) by mx1.FreeBSD.org (Postfix) with ESMTP id B699543D5D for ; Mon, 25 Sep 2006 19:57:56 +0000 (GMT) (envelope-from uspoerlein@gmail.com) Received: by nf-out-0910.google.com with SMTP id n29so24776nfc for ; Mon, 25 Sep 2006 12:57:55 -0700 (PDT) DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=beta; d=gmail.com; h=received:date:from:to:subject:message-id:mail-followup-to:references:mime-version:content-type:content-disposition:in-reply-to; b=B3wVTNMyYQd/27dIJ6F7jzr0Hzp3PLhN5XW7Ll/L6F5n9eC+FMiJeniykYqDUl+8JBHhQ/8xfbqKjV8Z+vMYeDiRGD/i52v8CeYujeiESfEXSHtxZkr4dA1No99wmQSp2nBNhd9Y0Gc3jiGyvm8WIE/lABgZNLII6E6eleSuS6Q= Received: by 10.48.48.15 with SMTP id v15mr97699nfv; Mon, 25 Sep 2006 12:57:55 -0700 (PDT) Received: from roadrunner.q.local ( [85.180.155.61]) by mx.gmail.com with ESMTP id x27sm5748069nfb.2006.09.25.12.57.54; Mon, 25 Sep 2006 12:57:54 -0700 (PDT) Received: from roadrunner.q.local (localhost [127.0.0.1]) by roadrunner.q.local (8.13.8/8.13.6) with ESMTP id k8PJvp5C004496 for ; Mon, 25 Sep 2006 21:57:51 +0200 (CEST) (envelope-from uspoerlein@gmail.com) Received: (from q@localhost) by roadrunner.q.local (8.13.8/8.13.6/Submit) id k8MIiZZR002659 for freebsd-hackers@FreeBSD.ORG; Fri, 22 Sep 2006 20:44:35 +0200 (CEST) (envelope-from uspoerlein@gmail.com) Date: Fri, 22 Sep 2006 20:44:35 +0200 From: Ulrich Spoerlein To: freebsd-hackers@FreeBSD.ORG Message-ID: <20060922184435.GA1250@roadrunner.q.local> Mail-Followup-To: freebsd-hackers@FreeBSD.ORG References: <451140F8.9030500@centtech.com> <200609210831.k8L8VxvK007258@lurza.secnetix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <200609210831.k8L8VxvK007258@lurza.secnetix.de> Cc: Subject: Re: numbers don't lie ... 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: Mon, 25 Sep 2006 19:57:57 -0000 Oliver Fromme wrote: > Eric Anderson wrote: > > Oliver Fromme wrote: > > > Reading /usr/src from a physical disk certainly requires > > > quite some I/O that takes more than zero time. > > > > But, in order to populate the ram disk, you must read /usr/src also from > > something, and that also takes time, which you should include in the > > full scope. > > But when you perform the buildworld several times (as you > should do when you're benchmarking properly), everything > is already in the RAM disk. If you instead rely on caching > but you don't have enough RAM to hold all of src + obj + > toolchain in RAM, then src (or at least parts of it) will > have to be read from the physical disk again upon each > buildworld. .. which makes no difference for the test case presented here. You're missing the point here: they benchmark with '-j8'. If you'd benchmark a -j1 build of md(4) vs. real disks, then you should get drastically different results (provided you start with a cold cache). But on these dual (quad?) CPU machines, with a -j8 build, 6 threads/processes are free to wait for disk I/O a very long time till they are finally scheduled. Thus, specifying high -jN values will mask any disk I/O latency (for reasonable combinations of CPUs and HDDs). Ulrich Spoerlein -- A: Yes. >Q: Are you sure? > >A: Because it reverses the logical flow of conversation. > >>Q: Why is top posting frowned upon?