To see if you have a swap files:
sudo swapon -s
No swap file shown? Check how much disk space space you have:
df
To create a swap file:
Step 1: Allocate a file for swap
sudo fallocate -l 2048m /mnt/swap_file.swap
Step 2: Change permission
sudo chmod 600 /mnt/swap_file.swap
Step 3: Format the file for swapping device
sudo mkswap /mnt/swap_file.swap
Step 4: Enable the swap
sudo swapon /mnt/swap_file.swap
Step 5: Make sure the swap is mounted when you Reboot. First, open fstab
sudo nano /etc/fstab
Finally, add entry in fstab (only if it wasn't automatically added)
# /etc/fstab
/mnt/swap_file.swap none swap sw 0 0
Save and exit. You're done adding swap. Now your rake assets:precompile should complete without being killed.
Credits: http://stackoverflow.com/a/22272491