RE: [Xen-users] allocating memory to dom0

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> john maclean
> Sent: 19 October 2006 22:44
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-users] allocating memory to dom0
> 
> Hey chaps, having a minor dispute with a friend. According the README
> in the Xen sources....
> "
>    The Xen command line takes a number of optional arguments described
>    in the manual. The most common is 'dom0_mem=xxxM' which sets the
>    amount of memory to allocate for use by your initial virtual
>    machine (known as domain 0). Note that Xen itself reserves about
>    32MB memory for internal use, which is not available for allocation
>    to virtual machines.
> .."
> 
> I've taken this to mean that one allocates say, half of one's RAM to
> dom0 with the command `dom0_mem=xxxM`. The domUs will then use up to
> the other half of the box's RAM. Is this correct?
> 
> My buddy says that this is unnecessary as the dom0 only needs 32M of
> ram. It will give the RAM away to the domU chaps until it has no more
> RAM to spare.

Neither of you are directly wrong... 

You can let Dom0 sacrifice it's own memory for the DomU - I personally
prefer to know where I stand - if I lock Dom0 to (say) 256MB, then I
know how much memory is available to DomU's. You may be able to run a
Dom0 on 32MB, but I'd say that you need a bit more than that if you're
using a standard installation, 100MB+ would be more realistic. 

One reason to use a fixed Dom0 memory amount is that some Kernel
allocations are based on percentages of the memory, and once they have
been made, the can't be trivially changed, which means that there may be
(say) 5% of the 16GB of RAM used for some buffer-space or such (5% of
16GB is 820 MB). If you set the dom0_mem to 256MB, the kernel will use
5% of 256MB for that buffer, and you'll have those 800MB available later
on. 

There is a setting in the file /etc/xend-config.sxp that sets how much
minimum Dom0 memory is. Not sure what the default is - it can be changed
up or down. 

--
Mats
> 
> -- 
> John Maclean  - 07739 171 531
> MSc (DIC)
> 
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