In the function
sysdep_DARWIN.c) it uses
task_for_pid to gather the information (e.g. cpu/mem resources) of all processes in the system.
In OSX 10.11 the new "system integrity protection" feature made this call to task_for_pid always failing. In system logs there are logs like this (one line for each pid being checked):
Oct 17 22:03:49 MacBook-Pro kernel: Sandbox: monit(57901) System Policy: deny(1) mach-priv-task-port 1
One possible solution is mentioned in the second reference link.