mjhjmtu

Today, I woke my PC up from suspend and it was very slow. I suspected a high RAM usage, then free gave me:

renan@vector:~> free -tm
 total used free shared buff/cache available
Mem: 7785 5957 264 209 1563 1359
Swap: 8395 27 8368
Total: 16181 5984 8633

Then I closed everything I was running (Firefox with a bunch of tabs, basically) and the RAM usage was still high:

renan@vector:~> free -tm
 total used free shared buff/cache available
Mem: 7785 5109 1180 129 1494 2287
Swap: 8395 27 8368
Total: 16181 5136 9549

And even after logging off and shutting my display manager down the RAM usage
is still high:

 total used free shared buff/cache available
Mem: 7785 4488 1821 47 1475 2989
Swap: 8395 6 8389
Total: 16181 4494 10210

I tried the "ps_mem" utility and all the processes are only using ~300 MB RAM, so why is 'free' saying I'm using nearly 4.5 GB? Something doesn't add up.

 Private + Shared = RAM used Program

160.0 KiB + 33.5 KiB = 193.5 KiB agetty
188.0 KiB + 34.5 KiB = 222.5 KiB vnstatd
324.0 KiB + 42.5 KiB = 366.5 KiB auditd
308.0 KiB + 75.5 KiB = 383.5 KiB rtkit-daemon
352.0 KiB + 56.5 KiB = 408.5 KiB irqbalance
492.0 KiB + 53.5 KiB = 545.5 KiB lvmetad
504.0 KiB + 161.5 KiB = 665.5 KiB mcelog
616.0 KiB + 168.5 KiB = 784.5 KiB agent
648.0 KiB + 160.5 KiB = 808.5 KiB xfconfd
736.0 KiB + 113.5 KiB = 849.5 KiB avahi-daemon
712.0 KiB + 177.5 KiB = 889.5 KiB gconfd-2
652.0 KiB + 249.5 KiB = 901.5 KiB master
884.0 KiB + 55.5 KiB = 939.5 KiB cron
812.0 KiB + 164.5 KiB = 976.5 KiB gvfs-goa-volume-monitor
812.0 KiB + 170.5 KiB = 982.5 KiB gvfs-mtp-volume-monitor
780.0 KiB + 221.5 KiB = 1.0 MiB sshd
684.0 KiB + 319.5 KiB = 1.0 MiB login
 1.0 MiB + 66.5 KiB = 1.0 MiB nscd
808.0 KiB + 301.5 KiB = 1.1 MiB at-spi-bus-launcher
856.0 KiB + 254.5 KiB = 1.1 MiB at-spi2-registryd
944.0 KiB + 170.5 KiB = 1.1 MiB gvfsd-metadata
 1.0 MiB + 123.5 KiB = 1.1 MiB dconf-service
 1.0 MiB + 80.5 KiB = 1.1 MiB mount.ntfs
176.0 KiB + 964.5 KiB = 1.1 MiB smbd-notifyd
184.0 KiB + 1.0 MiB = 1.2 MiB cleanupd
 1.0 MiB + 242.5 KiB = 1.3 MiB obexd
952.0 KiB + 386.5 KiB = 1.3 MiB systemd-logind
944.0 KiB + 480.5 KiB = 1.4 MiB qmgr
 1.2 MiB + 181.5 KiB = 1.4 MiB gvfs-gphoto2-volume-monitor
 1.1 MiB + 290.5 KiB = 1.4 MiB gvfs-afc-volume-monitor
 1.4 MiB + 144.5 KiB = 1.6 MiB smartd
 1.5 MiB + 114.5 KiB = 1.6 MiB bluetoothd
 1.3 MiB + 424.5 KiB = 1.7 MiB gvfsd-trash
524.0 KiB + 1.2 MiB = 1.7 MiB lpqd
 1.3 MiB + 554.5 KiB = 1.8 MiB pickup
 1.8 MiB + 191.5 KiB = 1.9 MiB ModemManager
 2.0 MiB + 23.5 KiB = 2.1 MiB haveged
 1.8 MiB + 319.5 KiB = 2.2 MiB upowerd
 2.1 MiB + 280.5 KiB = 2.4 MiB wpa_supplicant
 1.9 MiB + 526.5 KiB = 2.5 MiB cupsd
 1.9 MiB + 727.5 KiB = 2.6 MiB sudo
 2.4 MiB + 1.2 MiB = 3.5 MiB dhclient (2)
 3.2 MiB + 424.5 KiB = 3.6 MiB udisksd
 3.4 MiB + 343.5 KiB = 3.8 MiB systemd-udevd
 1.4 MiB + 2.4 MiB = 3.8 MiB smbd
 2.4 MiB + 1.4 MiB = 3.8 MiB goa-identity-service
 2.6 MiB + 1.3 MiB = 3.9 MiB nmbd
 3.8 MiB + 77.5 KiB = 3.9 MiB rsyslogd
 3.7 MiB + 238.5 KiB = 3.9 MiB accounts-daemon
 3.6 MiB + 823.0 KiB = 4.4 MiB (sd-pam) (2)
 3.3 MiB + 1.1 MiB = 4.5 MiB lightdm (3)
 4.0 MiB + 538.5 KiB = 4.5 MiB gvfs-udisks2-volume-monitor
 4.1 MiB + 651.0 KiB = 4.7 MiB gvfsd (2)
 4.2 MiB + 766.0 KiB = 4.9 MiB dbus-daemon (4)
 6.0 MiB + 43.5 KiB = 6.0 MiB bash
 6.2 MiB + 498.5 KiB = 6.7 MiB NetworkManager
 4.8 MiB + 2.5 MiB = 7.2 MiB systemd (3)
 7.8 MiB + 339.5 KiB = 8.2 MiB systemd-journald
 7.8 MiB + 473.0 KiB = 8.3 MiB gvfsd-fuse (2)
 5.8 MiB + 3.8 MiB = 9.6 MiB goa-daemon
 8.2 MiB + 3.8 MiB = 12.0 MiB deja-dup-monitor
 20.8 MiB + 398.5 KiB = 21.2 MiB polkitd
 20.9 MiB + 2.0 MiB = 22.9 MiB applet.py
 31.7 MiB + 8.4 MiB = 40.1 MiB Xorg
 44.5 MiB + 9.0 MiB = 53.5 MiB lightdm-gtk-greeter
---------------------------------
 298.3 MiB
=================================

I am using openSUSE Tumbleweed with kernel 4.18.15 and my desktop is MATE. Why is free reporting so high a RAM usage even with nearly nothing running?