How to change memory resources from within the DomU

For a certain job I sometimes need more memory in a DomU (CentOS 5).
The trigger for the start of the job are some conditions that I can only check within that DomU: The DomU is waiting for an upload. Once it is finished it needs to process the received data – best suited would be a ram-disk.

For this scenario I have set up the DomU with a memory setting of 1 GB min and 16 GB max.

Now I am trying to find an elegant way to resize the memory to max/min from within the DomU.

The first solution for this would be using ssh from DomU to all possible Dom0s and then trigger the "xm mem-set" commands there (with sudo).

I've read some things about the xen-store. Triggers were mentioned… So this makes me think that there should be a better way. It turned out that memory balloning would be a better way.

This should work within the DomU:

echo $((4096*1024*1024)) >/proc/xen/balloon

Should resize the memory to 4 GB.

But: cat /proc/xen/ballon says:

Current allocation:  2165536 kB
Requested target:    4192256 kB
Low-mem balloon:    14611680 kB
High-mem balloon:          0 kB
Driver pages:              0 kB
Xen hard limit:      2165536 kB

Now where does that xen hard limit come from?

Result is now that my DomU has 2 GB RAM.
xm list in Dom0 states that the DomU is still at 1 GB RAM…

What's going wrong here?
And yes – since the DomU-version is below 3.0.4 I added "mem=16G" as kernel-boot-parameter to my DomU.

With Dom0 SLES11 SP1 (XEN 4.0.1) and CentOS 5.6 DomU (still XEN 3.0.3?) the
echo 4G >/proc/xen/ballon did not do anything first – but after I did some successful xm mem-sets from the Dom0 (up to 16 GB – which worked), the /proc/xen/ballon did work within the DomU, too.