That message is the OOM ('out-of-memory') killer running - it's trying to keep your system running by killing off the least important processes. There's a fairly complex scoring algorithm that decides what processes to sacrifice first - that's where the 'score' in your log messages comes from.

The question is.. what's the spec of your VM, and what's it actually doing? The outputs of 'free', 'top', and 'ps -aux' can be useful in determining what's actually eating all your memory. 20GB should be enough for a fairly decent Devstack install.