Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757862Ab0DFXly (ORCPT ); Tue, 6 Apr 2010 19:41:54 -0400 Received: from terminus.zytor.com ([198.137.202.10]:48838 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757789Ab0DFXln (ORCPT ); Tue, 6 Apr 2010 19:41:43 -0400 Message-ID: <4BBBC470.4050002@zytor.com> Date: Tue, 06 Apr 2010 16:32:00 -0700 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.9) Gecko/20100330 Fedora/3.0.4-1.fc12 Thunderbird/3.0.4 MIME-Version: 1.0 To: Youngwhan Song CC: Venkatram Tummala , Joel Fernandes , Frank Hu , hayfeng Lee , "linux-kernel@vger.kernel.org" , "linux-kernel@zh-kernel.org" , "kernelnewbies@nl.linux.org" Subject: Re: why choose 896MB to the start point of ZONE_HIGHMEM References: <4BBB7AC9.5060008@zytor.com> <4BBB8F07.60401@zytor.com> <3715922601579231267@unknownmsgid> In-Reply-To: <3715922601579231267@unknownmsgid> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1031 Lines: 26 On 04/06/2010 04:27 PM, Youngwhan Song wrote: > Nice explanation, Venkatram, > > Just one question pop up mind. > > What if actual physical memory is only 256MB? How does kernel divide > virtual memory? Do we need to specify the region to kernel? Or will > kernel itself decide it automatically? > If there is less than 896 MB of physical memory, the vmalloc region is automatically extended (in your case, it will be 768 MB in size.) There will be no HIGHMEM in such a case, and if you are compiling your own kernel you will gain considerable speed by disabling HIGHMEM support completely. This, of course, was the norm back when Linux was first created, and a typical amount of memory was 8 MB or so. That we'd have gigabytes of memory seemed very distant at the time. -hpa -- 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/