From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Mon, 2 Dec 2002 01:56:50 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Mon, 2 Dec 2002 01:56:50 -0500 Received: from holomorphy.com ([66.224.33.161]:50316 "EHLO holomorphy") by vger.kernel.org with ESMTP id ; Mon, 2 Dec 2002 01:56:50 -0500 Date: Sun, 1 Dec 2002 23:04:12 -0800 From: William Lee Irwin III To: Stephen Rothwell Cc: LKML Subject: Re: Maximum Physical Memory on 2.4 and ia32 Message-ID: <20021202070412.GO697@holomorphy.com> Mail-Followup-To: William Lee Irwin III , Stephen Rothwell , LKML References: <20021202120835.4ecb87fd.sfr@canb.auug.org.au> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20021202120835.4ecb87fd.sfr@canb.auug.org.au> User-Agent: Mutt/1.3.25i Organization: The Domain of Holomorphy Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Dec 02, 2002 at 12:08:35PM +1100, Stephen Rothwell wrote: > (http://www.redhat.com/services/techsupport/production/GSS_caveat.html) > What are the "operating system design limits" that restrict the amount of > supported memory to 16GB? I've relayed this privately in less organized private discussions: The design limitation is that a PTE is required to map a PAGE_SIZE -sized region. The kernel allocation unit needs to be enlarged without breaking ABI to reduce the fraction of physical memory reserved for metadata i.e. mem_map[]. This is called page clustering. Hugh Dickins implemented page clustering by creating a distinction between PAGE_SIZE and MMUPAGE_SIZE then introducing an API for dealing with vectors of PTE's. He implemented this for 2.4.6 and 2.4.7. His patches are at ftp://ftp.veritas.com/linux/ In other environments this is an optimization, not a requirement to be able to run, and so it benefits all platforms. There are, of course, other kinds of metadata generated at runtime that may impose limitations on feasibility of workloads on large highmem boxen. Bill