Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934464AbXKPAlT (ORCPT ); Thu, 15 Nov 2007 19:41:19 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753550AbXKPAlF (ORCPT ); Thu, 15 Nov 2007 19:41:05 -0500 Received: from holomorphy.com ([66.93.40.71]:40103 "EHLO holomorphy.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1762592AbXKPAlE (ORCPT ); Thu, 15 Nov 2007 19:41:04 -0500 Date: Thu, 15 Nov 2007 16:40:12 -0800 From: William Lee Irwin III To: "H. Peter Anvin" Cc: Linus Torvalds , Jeremy Fitzhardinge , Andi Kleen , Ingo Molnar , Thomas Gleixner , Nick Piggin , Linux Kernel Mailing List Subject: Re: Why preallocate pmd in x86 32-bit PAE? Message-ID: <20071116004012.GE1806@holomorphy.com> References: <473CC0AC.3020500@goop.org> <473CCB66.2080601@zytor.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <473CCB66.2080601@zytor.com> Organization: The Domain of Holomorphy User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1238 Lines: 28 Linus Torvalds wrote: >> IIRC, the present bit is ignored in the magic 4-entry PGD. All entries >> have to be present. On Thu, Nov 15, 2007 at 02:42:46PM -0800, H. Peter Anvin wrote: > This is true, although you could point a PGD to an all-zero page if you > really wanted to. You have to re-load CR3 after modifying the top-level > entries. There may be bigger fish to fry in terms of per-process overhead, if you're trying to cut that down. The trouble with trying to address some of those is that there is mutual antagonism between compactness and expansibility in the process address space layout, so you'll end up instantiating a lot more than you want barring some sort of provision for a compact address space layout. Pagetable sharing is a far more powerful resource scalability method, though it also needs cooperation in user address space layout to reap its gains. There are other overheads, of course, though they're more typically per-something besides processes. -- wli - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/