Available Memory on 4GB system

Available Memory on 4GB system

[Peter]

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.

(BTW we are now offering Xen based VPS hosting - having been running UML 
based servers up to now.  We find the 2.0.6 setup performs well, and the 
VPSs have been very stable.  Other than the minor gripe about the 
memory, we have been very happy with the software.  Thanks for 
everyone's work on this.)

Regards, Peter
http://rimuhosting.com

Re: Available Memory on 4GB system

[Keir Fraser]

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