This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
Home Products Support Community News


Re: [Xen-devel] Available Memory on 4GB system

To: Peter <p.spamcatcher@xxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Available Memory on 4GB system
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Fri, 29 Jul 2005 11:15:08 +0100
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
Delivery-date: Fri, 29 Jul 2005 10:09:16 +0000
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
In-reply-to: <42E98C94.7070101@xxxxxxxxxxxxxxx>
List-help: <mailto:xen-devel-request@lists.xensource.com?subject=help>
List-id: Xen developer discussion <xen-devel.lists.xensource.com>
List-post: <mailto:xen-devel@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
References: <42E98C94.7070101@xxxxxxxxxxxxxxx>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx

On 29 Jul 2005, at 02:55, Peter wrote:

We have a dual Xeon server with 4x1GB memory sticks. With a non-Xen kernel it boots up with ~4GB of memory. With the Xen kernel it boots up with ~3.3GB of memory.

I know this has been discussed before, but I'm still not clear why the xen kernel boots up with less memory available than a regular Linux kernel. Would someone mind patiently explaining it once more?

I want to make sure there is no kernel .config option we're setting which is making memory unavailable to us. And from what I read the pae changes in 3.0 should allow us to see more (all?) of the memory.

Memory-mapped I/o spaces (e.g., the video frame buffer) are always mapped below 4GB and consume many megabytes of space. If all 4GB of your RAM were mapped contiguously from address 0, some of this RAM would inevitably overlap with the I/O space and would hence be inaccessible (and so wasted).

The BIOS avoids this by mapping some of your RAM (anything up to a gigabyte, typically) above the 4GB boundary. However, this makes it inaccessible without PAE. So, you cannot get at that memory until you switch to pae Xen (which is probably not advisable just yet).

 -- Keir

Xen-devel mailing list

<Prev in Thread] Current Thread [Next in Thread>