Display memory used by processes in human-readable form using ps and awk utilities.
I will use mawk as it is installed by default on Debian based distributions.
$ awk -W version
mawk 1.3.3 Nov 1996, Copyright (C) Michael D. Brennan compiled limits: max NF 32767 sprintf buffer 2040
Use ps command to display the top twenty processes using the most memory. Notice that Resident Set Size (RSS) is defined in kiloBytes (1024 bytes as this is a memory).
$ ps -ax --no-headers --format=pid,rss,command --sort=-rss,pid | head -20
5451 648148 /usr/lib/firefox/firefox -contentproc -childID 67 -isForBrowser -prefsLen 7220 -prefMapSize 169730 -schedulerPrefs 0001,2 -parentBuildID 20181023214826 -greomni /usr/lib/firefox/omni.ja -appomni /usr/lib/firefox/browser/omni.ja -appdir /usr/lib/firefox/browser 3941 true tab 3941 646684 /usr/lib/firefox/firefox -new-window 12932 622172 /usr/lib/firefox/firefox -contentproc -childID 70 -isForBrowser -prefsLen 7220 -prefMapSize 169730 -schedulerPrefs 0001,2 -parentBuildID 20181023214826 -greomni /usr/lib/firefox/omni.ja -appomni /usr/lib/firefox/browser/omni.ja -appdir /usr/lib/firefox/browser 3941 true tab 8580 549512 /usr/lib/firefox/firefox -contentproc -childID 10 -isForBrowser -prefsLen 6427 -prefMapSize 169730 -schedulerPrefs 0001,2 -parentBuildID 20181023214826 -greomni /usr/lib/firefox/omni.ja -appomni /usr/lib/firefox/browser/omni.ja -appdir /usr/lib/firefox/browser 3941 true tab 14041 536904 /usr/lib/virtualbox/VirtualBox --comment Debian --startvm dabea2f0-5c6e-4d44-9f37-8dfc070e1ff8 --no-startvm-errormsgbox 17219 487292 /usr/lib/firefox/firefox -contentproc -childID 29 -isForBrowser -prefsLen 7218 -prefMapSize 169730 -schedulerPrefs 0001,2 -parentBuildID 20181023214826 -greomni /usr/lib/firefox/omni.ja -appomni /usr/lib/firefox/browser/omni.ja -appdir /usr/lib/firefox/browser 3941 true tab 3477 406112 /usr/bin/gnome-shell 4272 268492 /usr/bin/ruby /usr/bin/jekyll server --future -w 467 128224 /lib/systemd/systemd-journald 4088 112420 /usr/lib/firefox/firefox -contentproc -childID 2 -isForBrowser -prefsLen 226 -prefMapSize 169730 -schedulerPrefs 0001,2 -parentBuildID 20181023214826 -greomni /usr/lib/firefox/omni.ja -appomni /usr/lib/firefox/browser/omni.ja -appdir /usr/lib/firefox/browser 3941 true tab 3287 99784 /usr/lib/xorg/Xorg vt2 -displayfd 3 -auth /run/user/1000/gdm/Xauthority -background none -noreset -keeptty -verbose 3 1825 78204 /usr/bin/gnome-shell 309 76452 keepassxc 13972 68024 /usr/lib/virtualbox/VirtualBox 4063 62096 /usr/sbin/netdata -P /run/netdata/netdata.pid -D 4161 58964 /usr/bin/python /usr/libexec/netdata/plugins.d/python.d.plugin 1 3830 56316 /usr/lib/gnome-terminal/gnome-terminal-server 4416 32520 /usr/sbin/libvirtd 4162 27772 /usr/bin/python3 /usr/bin/chrome-gnome-shell /usr/lib/mozilla/native-messaging-hosts/org.gnome.chrome_gnome_shell.json chrome-gnome-shell@gnome.org 18463 27396 /usr/lib/packagekit/packagekitd
The solution to this issue is to use awk to parse and pretty-print Resident Set Size (RSS).
$ ps -ax --no-headers --format=pid,rss,command --sort=-rss,pid | head -20 | awk 'BEGIN{suffixes_len=split("kB MB GB TB",suffixes)} {n_suffix=1; while($2 > 1000 && n_suffix < suffixes_len) {$2 /= 1024; n_suffix++;}; printf "%5s %7.2f %2s %s\n",$1,$2,suffixes[n_suffix],substr($0, index($0,$3))}'
5451 632.96 MB /usr/lib/firefox/firefox -contentproc -childID 67 -isForBrowser -prefsLen 7220 -prefMapSize 169730 -schedulerPrefs 0001,2 -parentBuildID 20181023214826 -greomni /usr/lib/firefox/omni.ja -appomni /usr/lib/firefox/browser/omni.ja -appdir /usr/lib/firefox/browser 3941 true tab 3941 631.53 MB /usr/lib/firefox/firefox -new-window 12932 607.59 MB /usr/lib/firefox/firefox -contentproc -childID 70 -isForBrowser -prefsLen 7220 -prefMapSize 169730 -schedulerPrefs 0001,2 -parentBuildID 20181023214826 -greomni /usr/lib/firefox/omni.ja -appomni /usr/lib/firefox/browser/omni.ja -appdir /usr/lib/firefox/browser 3941 true tab 8580 536.63 MB /usr/lib/firefox/firefox -contentproc -childID 10 -isForBrowser -prefsLen 6427 -prefMapSize 169730 -schedulerPrefs 0001,2 -parentBuildID 20181023214826 -greomni /usr/lib/firefox/omni.ja -appomni /usr/lib/firefox/browser/omni.ja -appdir /usr/lib/firefox/browser 3941 true tab 14041 524.32 MB /usr/lib/virtualbox/VirtualBox --comment Debian --startvm dabea2f0-5c6e-4d44-9f37-8dfc070e1ff8 --no-startvm-errormsgbox 17219 475.87 MB /usr/lib/firefox/firefox -contentproc -childID 29 -isForBrowser -prefsLen 7218 -prefMapSize 169730 -schedulerPrefs 0001,2 -parentBuildID 20181023214826 -greomni /usr/lib/firefox/omni.ja -appomni /usr/lib/firefox/browser/omni.ja -appdir /usr/lib/firefox/browser 3941 true tab 3477 396.59 MB /usr/bin/gnome-shell 4272 262.20 MB /usr/bin/ruby /usr/bin/jekyll server --future -w 467 125.22 MB /lib/systemd/systemd-journald 4088 109.79 MB /usr/lib/firefox/firefox -contentproc -childID 2 -isForBrowser -prefsLen 226 -prefMapSize 169730 -schedulerPrefs 0001,2 -parentBuildID 20181023214826 -greomni /usr/lib/firefox/omni.ja -appomni /usr/lib/firefox/browser/omni.ja -appdir /usr/lib/firefox/browser 3941 true tab 3287 97.45 MB /usr/lib/xorg/Xorg vt2 -displayfd 3 -auth /run/user/1000/gdm/Xauthority -background none -noreset -keeptty -verbose 3 1825 76.37 MB /usr/bin/gnome-shell 309 74.66 MB keepassxc 13972 66.43 MB /usr/lib/virtualbox/VirtualBox 4063 60.64 MB /usr/sbin/netdata -P /run/netdata/netdata.pid -D 4161 57.58 MB /usr/bin/python /usr/libexec/netdata/plugins.d/python.d.plugin 1 3830 55.00 MB /usr/lib/gnome-terminal/gnome-terminal-server 4416 31.76 MB /usr/sbin/libvirtd 4162 27.12 MB /usr/bin/python3 /usr/bin/chrome-gnome-shell /usr/lib/mozilla/native-messaging-hosts/org.gnome.chrome_gnome_shell.json chrome-gnome-shell@gnome.org 18463 26.75 MB /usr/lib/packagekit/packagekitd