Open Stack

Hi,

Lately, on my production openstack Newton setup, I've ran into a 
situation that defies my assumptions regarding memory management on 
Openstack compute nodes and I've been looking for explanations. 
Basically, we had a VM with a flavor that limited it to 96 GB of ram, 
which, to be quite honest, we never thought we could ever reach. This is 
a very important VM where we wanted to avoid running out of memory at 
all cost. The VM itself generally uses about 12 GB of ram.

We were surprised when we noticed yesterday that this VM, which has been 
running for several months, was using all its 96 GB on the compute host. 
Despite that, in the guest, the OS was indicating a memory usage of 
about 12 GB. The only explanation I see to this is that at some point in 
time, the host had to allocate all the 96GB of ram to the VM process and 
it never took back the allocated ram. This prevented the creation of 
more guests on the node as it was showing it didn't have enough memory left.

Now, I was under the assumption that memory ballooning was integrated 
into nova and that the amount of allocated memory to a specific guest 
would deflate once that guest did not need the memory. After 
verification, I've found blueprints for it, but I see no trace of any 
implementation anywhere.

I also notice that on most of our compute nodes, the amount of ram used 
is much lower than the amount of ram allocated to VMs, which I do 
believe is normal.

So basically, my question is, how does openstack actually manage ram 
allocation? Will it ever take back the unused ram of a guest process? 
Can I force it to take back that ram?

-- 
Jean-Philippe Méthot
Openstack system administrator
PlanetHoster inc.
www.planethoster.net