From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935804AbYEBPrn (ORCPT ); Fri, 2 May 2008 11:47:43 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S934381AbYEBPrd (ORCPT ); Fri, 2 May 2008 11:47:33 -0400 Received: from smtp-out03.alice-dsl.net ([88.44.63.5]:19587 "EHLO smtp-out03.alice-dsl.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932231AbYEBPrc (ORCPT ); Fri, 2 May 2008 11:47:32 -0400 To: Fabio De Francesco Cc: Arjan van de Ven , linux-kernel@vger.kernel.org, Alan Cox Subject: Re: [x86_32] With 4GB installed, in which cases low mem total is less than 896MB? From: Andi Kleen References: <200804301702.06019.fabiomdf@alice.it> <20080429101852.6719eb59@infradead.org> <200805021638.52540.fabiomdf@alice.it> Date: Fri, 02 May 2008 17:47:17 +0200 In-Reply-To: <200805021638.52540.fabiomdf@alice.it> (Fabio De Francesco's message of "Fri, 2 May 2008 16:38:46 +0200") Message-ID: <87skx0vhbu.fsf@basil.nowhere.org> User-Agent: Gnus/5.1008 (Gnus v5.10.8) Emacs/21.3 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-OriginalArrivalTime: 02 May 2008 15:40:23.0199 (UTC) FILETIME=[D5B52AF0:01C8AC6A] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Fabio De Francesco writes: > > Unfortunetally my poor knowledge of Linux kernel mechanisms (and > code!) prevents me from understanding what could be the causes for > that overhead. Do you mean it is a hardware issue? Or is it a kernel > one? Anyway another machine of mine, that is equipped with 2GB RAM, > doesn't show any proportional reduction of low memory at all. So > this makes me think (oh, dear!). You might want to read http://halobates.de/memorywaste.pdf, in particular 3.5.2. The numbers arjan quoted and in the paper are for 64bit kernels, on 32bit the overhead is ~32bytes/page. This mem_map overhead happens for both highmem and lowmem, but is always allocated in lowmem. -Andi