ps output - Difference between VSZ vs RSS memory usage

ps command is a great tool to get a relevant information for each process in the Linux system. The ps command is yet another tool among many which reads and interprets data located in the virtual directory /proc/. If you need to read raw process stats data head over /proc directory and locate directory named after process ID ( PID ) of the process you are looking for. Below as a sample output of the ps command:

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root      1354  1.7  0.4 221272 37184 tty1     Ss+  07:18   5:55 /usr/bin/Xorg 
lubos   8803  0.0  0.0 116536  3260 pts/3    Ss+  09:54   0:00 /bin/bash
lubos   8885  0.0  0.0 116668  3480 pts/4    Ss+  09:59   0:00 /bin/bash
lubos   9294  0.0  0.0 116536  3320 pts/5    Ss   10:01   0:00 /bin/bash
root     11633  0.0  0.0 200656  3616 pts/5    S    11:18   0:00 su
root     11642  0.0  0.0 116644  3452 pts/5    S+   11:18   0:00 bash

The most obvious values here are USER, PID, etc. However, there are two additional columns which need more explanation:

VSZ - Virtual Set Size

The Virtual Set Size is a memory size assigned to a process ( program ) during the initial execution. The Virtual Set Size memory is simply a number of how much memory a process has available for its execution.

RSS - Resident Set Size

As oppose to VSZ ( Virtual Set Size ), RSS is a memory currently used by a process. This is a actual number in kilobytes of how much RAM the current process is using.