please read my previous post here carefully. your yml file is incorrect.
this should work for a non-busy forum. do you have a swap file at all? run free -h at the command line and post the output here. it should look something like this (but smaller numbers):
~# free -h
total used free shared buff/cache available
Mem: 3.7Gi 2.5Gi 324Mi 116Mi 1.2Gi 1.2Gi
Swap: 2.0Gi 366Mi 1.6Gi