From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752897AbXD3HR7 (ORCPT ); Mon, 30 Apr 2007 03:17:59 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754529AbXD3HR7 (ORCPT ); Mon, 30 Apr 2007 03:17:59 -0400 Received: from smtp1.linux-foundation.org ([65.172.181.25]:60462 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752897AbXD3HR6 (ORCPT ); Mon, 30 Apr 2007 03:17:58 -0400 Date: Mon, 30 Apr 2007 00:17:55 -0700 From: Andrew Morton To: "Bradley Chapman" Cc: linux-kernel@vger.kernel.org Subject: Re: CONFIG_HIGHMEM4G and 1GB RAM Message-Id: <20070430001755.e9f203c9.akpm@linux-foundation.org> In-Reply-To: References: X-Mailer: Sylpheed version 2.2.7 (GTK+ 2.8.17; x86_64-unknown-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Sat, 28 Apr 2007 21:26:42 +0100 "Bradley Chapman" wrote: > After installing a second 512MB SODIMM into my laptop, I noticed that > my 2.6.18.3 kernel only detected and used 896MB of the resulting 1GB, > and informed me that I needed to enable high memory in order to access > the remaining 127MB. > > While doing so, I noticed that the help text stated that enabling high > memory (in my case, CONFIG_HIGHMEM4G) should only be done if the > system the kernel is running on has more than 1GB, but less than 4GB - > it said that systems with 1GB should leave it off, which resulted in > an eighth of the installed memory going missing. A bit more research > found a three year old thread on this list where it was stated that > enabling CONFIG_HIGHMEM4G sometimes slows the system down due to > sometimes undesirable changes to the way the kernel sets up the memory > map. > > Basically, all I want to know is whether or not enabling > CONFIG_HIGHMEM4G for a laptop that has exactly 1GB of RAM will result > in any performance degradation. > I would expect the advantages of the additional 128MB to considerably outweigh the cost of turning on CONFIG_HIGHMEM4G. That cost will be a little extra CPU consumption inside the kernel, but the great majority of CPU consumption usually happens in userspace anyway.