htop

htop is an enhanced and more interactive top. It is a terminal user interface that shows processes memory and CPU usage. See peteris.rocks's htop write-up for many details and nuances.

htop is useful to see how busy Rhea (linux server) is before launching a bunch of jobs/threads or otherwise allocating a large cpu pool.

Also see iotop, “load average” in uptime, and, on rhea, http://rhea.wpic.upmc.edu/gangilia

The colors of CPU usage are informative. If you see a lot of gray, disk usage is limiting processing (jobs might finish faster with fewer forks/threads).