From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S264067AbUEXGYk (ORCPT ); Mon, 24 May 2004 02:24:40 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S264066AbUEXGYg (ORCPT ); Mon, 24 May 2004 02:24:36 -0400 Received: from mx2.elte.hu ([157.181.151.9]:42892 "EHLO mx2.elte.hu") by vger.kernel.org with ESMTP id S264057AbUEXGY3 (ORCPT ); Mon, 24 May 2004 02:24:29 -0400 Date: Mon, 24 May 2004 10:25:22 +0200 From: Ingo Molnar To: Linus Torvalds Cc: Phy Prabab , linux-kernel@vger.kernel.org Subject: Re: 4g/4g for 2.6.6 Message-ID: <20040524082522.GA23725@elte.hu> References: <20040523194302.81454.qmail@web90007.mail.scd.yahoo.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.4.1i X-ELTE-SpamVersion: MailScanner 4.26.8-itk2 (ELTE 1.1) SpamAssassin 2.63 ClamAV 0.65 X-ELTE-VirusStatus: clean X-ELTE-SpamCheck: no X-ELTE-SpamCheck-Details: score=-4.9, required 5.9, autolearn=not spam, BAYES_00 -4.90 X-ELTE-SpamLevel: X-ELTE-SpamScore: -4 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org * Linus Torvalds wrote: > Quite frankly, a number of us are hoping that we can make them > unnecessary. The cost of the 4g/4g split is absolutely _huge_ on some > things, including basic stuff like kernel compiles. > > The only valid reason for the 4g split is that the VM doesn't always > behave well with huge amounts of highmem. The anonvma stuff in > 2.6.7-pre1 is hoped to make that much less of an issue. > > Personally, if we never need to merge 4g for real, I'll be really > really happy. I see it as a huge ugly hack. i agree with the hack part - but the performance aspect has been blown out of proportion. 4:4 has the same cost on kernel compiles as highpte. There are also real workloads where it actually helps performance. also, the 4:4 overhead is really a hardware problem - and there are x86-compatible CPUs (amd64) where the TLB flush problem has already been solved: on amd64 the 4:4 feature has no noticeable overhead. So as long as people opt for a 32-bit OS (even on a 64-bit CPU, for whatever weird compatibility reason), 4:4 can be useful. For the other workloads i as much hope as everyone else that people switch to a 64-bit OS on x86-64 ASAP! also, while a quick transition to x86-64 will most likely happen, the large installed base of big x86 boxes is a matter of fact too - and they wont vanish into thin air. Also, there will always be specific user workloads where lowmem grows to large values. Not to mention the fact that 4:4 is a nice debugging/security tool as you cannot dereference user pointers ;) - it has caught countless bugs already. Plus there are specific workloads that want 4GB of userspace (and no, 3.5 GB wont do). So 4:4 will have its niches where it will live on. We could argue on and on how quickly 'x86 with more than 4GB of RAM' and 'x86 with 4GB of a userspace' will become a niche, hopefully it happens fast but we've got to keep our options open. Ingo