On my production server, which is hosted on digital ocean, if that helps, Ubuntu 12.04, I have RoR 4 and rake 10.1.1.
When I deploy, I run rake assets:precompi
This uses a lot of RAM. To check how much available RAM memory you have free, use the command
free -m
This will show the available RAM in MB
A temporary solution would be to create a swap space.
I was going to add this as a comment to Jason R post above before you go into his steps, just to make sure it is a RAM resource issue.
you could also run
echo {1,2,3} > /proc/sys/vm/drop_caches
to clean up the cache memory, but it probably will not free up enough.