From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755753AbYEHMei (ORCPT ); Thu, 8 May 2008 08:34:38 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752051AbYEHMe3 (ORCPT ); Thu, 8 May 2008 08:34:29 -0400 Received: from smtpq2.tilbu1.nb.home.nl ([213.51.146.201]:44231 "EHLO smtpq2.tilbu1.nb.home.nl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751876AbYEHMe1 (ORCPT ); Thu, 8 May 2008 08:34:27 -0400 Message-ID: <4822F39A.90608@keyaccess.nl> Date: Thu, 08 May 2008 14:35:38 +0200 From: Rene Herman User-Agent: Thunderbird 2.0.0.14 (X11/20080421) MIME-Version: 1.0 To: Fabio De Francesco CC: linux-kernel@vger.kernel.org Subject: Re: [x86_32] With 4GB installed, in which cases low mem total is less than 896MB? References: <200804301702.06019.fabiomdf@alice.it> <481DCA14.9030302@keyaccess.nl> <200805080948.19914.fabiomdf@alice.it> In-Reply-To: <200805080948.19914.fabiomdf@alice.it> Content-Type: text/plain; charset=ISO-8859-15; format=flowed Content-Transfer-Encoding: 7bit X-Spam-Score: -1.0 (-) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08-05-08 09:48, Fabio De Francesco wrote: >>> I've got a Linux box, Intel Core 2 T7700 with 4GB RAM installed, with >> Others have already answered as well so this just as by the way and just in >> case... why are you running a 32-bit kernel on it? > It's only because when I got the box I had to make it run as soon as > possible, sooner than I'd had time to investigate which possible > benefits I would have had from running a 64bit distribution. > > Further, since I had to install a Gentoo GNU/Linux distribution by > "emerging" from a chroot(-ed) environment inside an Ubuntu 32bit one > (it takes too long to explain why), I supposed I couldn't do that for > installing a 64bit Gentoo. Still today I don't know whether or not I > assumed that wrongly. > > Today I would switch to a 64bit Gentoo but, first I don't know how to > do that in a secure and fast way, second I still don't know if it is > worth the job... > > Could you please summarize which benefits I would gain from that? Not directly as it's quite dependent on your use of the box but generally I'd quite definitely use 64-bit on that. Running a 32-bit OS on a 64-bit capable 4G machine today is sort of like running MS-DOS on a 64 MB PC a decade ago (or like putting a Fiat 500 engine in an Alfa Romeo if you prefer car analogies...). 32-bit architectures have a 32-bit address space but this does not translate directly into being able to cope with 4G of memory, and certainly not well. You already remarked on the 896 MB lowmem; note that it might've been called 'sanemem' just as well really. If you want to address more than that 896M you have to resort to highmem or adjust the traditional UNIX 3/1 address space split down. Worse still in your situation -- not just more than 896M but 4G -- the system can not even address all of your memory _at all_ without hardware hacks such as PAE due to it also needing a bit of the physical address space for things such as hardware mappings. In any case, not any single application can make use of all of it on 32-bit. Basically, 32-bit stopped being a good idea once physical memory below 1G (or 768M) stopped being a good idea. If you'd experience much practical downside depends on use. Highmem, lots of it, is slower but you might not care if you don't use all that memory intensively, nor about the application addressing limit, and with 4G you'd not yet be running into situations that the amount of lowmem available gets to be too little to support the amount of highmem ... ... but running a 32-bit on that machine while it's capable of 64-bit is still Just Wrong(TM). It works, but it's stapling kludges onto kludges. Rene.