PGPool memory requirements
It's not documented in the official docs (as of now - Feb 2018), but PgPool's memory footprint is quite heavy.

In my recent testing PgPool II version 3.6 needs as much as 140 MB RAM per child process. (the number of child processes is defined in num_init_children). This is private process memory - not shared. 

This means approximately 8GB for each 50 clients.

Compared to PostgreSQL, this is 5-10 times more (PostgreSQL can easily handle 250 sessions on 8 GB RAM). Plus, PostgreSQL uses shared buffer cache which is much more cost effective.