From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Wed, 19 Sep 2001 17:46:49 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Wed, 19 Sep 2001 17:46:39 -0400 Received: from ebiederm.dsl.xmission.com ([166.70.28.69]:30550 "EHLO flinx.biederman.org") by vger.kernel.org with ESMTP id ; Wed, 19 Sep 2001 17:46:29 -0400 To: Alan Cox Cc: phillips@bonn-fries.net (Daniel Phillips), rfuller@nsisoftware.com (Rob Fuller), linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: broken VM in 2.4.10-pre9 In-Reply-To: From: ebiederm@xmission.com (Eric W. Biederman) Date: 19 Sep 2001 15:37:26 -0600 In-Reply-To: Message-ID: User-Agent: Gnus/5.0808 (Gnus v5.8.8) Emacs/20.5 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Alan Cox writes: > > On September 17, 2001 06:03 pm, Eric W. Biederman wrote: > > > In linux we have avoided reverse maps (unlike the BSD's) which tends > > > to make the common case fast at the expense of making it more > > > difficult to handle times when the VM system is under extreme load and > > > we are swapping etc. > > > > What do you suppose is the cost of the reverse map? I get the impression you > > > think it's more expensive than it is. > > We can keep the typical page table cost lower than now (including reverse > maps) just by doing some common sense small cleanups to get the page struct > down to 48 bytes on x86 While there is a size cost I suspect you will notice reverse maps a lot more in operations like fork where having them tripples the amount of memory that you need to copy. So you should see a double or more in the time it takes to do a fork. That I think is a significant cost. Eric