Find out how much swap a process is using on Linux

The key files you are interested in for finding out which processes are using swap are:

/proc/${PID}/smaps
/proc/${PID}/status
/proc/${PID}/stat

To see swap space used by a particular process, first find it’s PID:

pidof systemd

or

pgrep systemd

then grep the status file:

grep --color VmSwap /proc/12345/status