我的 Ubuntu 20.04.1 笔记本是从以前的版本（18.10 -> 19.04 -> 19.10 -> 20.04）升级而来的最新 Ubuntu 桌面安装。最近，我注意到gdm系统中添加了一个名为的“系统用户”：
# id gdm
uid=123(gdm) gid=128(gdm) groups=128(gdm)
# grep gdm /etc/passwd /etc/shadow
/etc/passwd:gdm:x:123:128:Gnome Display Manager:/var/lib/gdm3:/bin/false
/etc/shadow:gdm:*:17821:0:99999:7:::
当我以桌面用户身份登录后，此用户的系统进程与我的普通桌面用户的系统进程类似。但是，gdm进程似乎在启动后立即启动，甚至在我以普通用户身份登录之前：
# ps -fu gdm
UID PID PPID C STIME TTY TIME CMD
gdm 1223 1 0 13:28 ? 00:00:00 /lib/systemd/systemd --user
gdm 1224 1223 0 13:28 ? 00:00:00 (sd-pam)
gdm 1241 1223 0 13:28 ? 00:00:00 /usr/bin/pulseaudio --daemonize=no --log-target=journal
gdm 1244 1223 0 13:28 ? 00:00:00 /usr/libexec/tracker-miner-fs
gdm 1246 1218 0 13:28 tty1 00:00:00 /usr/lib/gdm3/gdm-x-session gnome-session --autostart /usr/share/gdm/greeter/autostart
gdm 1249 1246 0 13:28 tty1 00:00:01 /usr/lib/xorg/Xorg vt1 -displayfd 3 -auth /run/user/123/gdm/Xauthority -background none -noreset -keeptty -verbose 3
gdm 1252 1223 0 13:28 ? 00:00:00 /usr/bin/dbus-daemon --session --address=systemd: --nofork --nopidfile --systemd-activation --syslog-only
gdm 1262 1223 0 13:28 ? 00:00:00 /usr/libexec/gvfsd
gdm 1277 1223 0 13:28 ? 00:00:00 /usr/libexec/gvfsd-fuse /run/user/123/gvfs -f -o big_writes
gdm 1284 1223 0 13:28 ? 00:00:00 /usr/libexec/gvfs-udisks2-volume-monitor
gdm 1291 1223 0 13:28 ? 00:00:00 /usr/libexec/gvfs-gphoto2-volume-monitor
gdm 1295 1223 0 13:28 ? 00:00:00 /usr/libexec/gvfs-goa-volume-monitor
gdm 1299 1223 0 13:28 ? 00:00:00 /usr/libexec/goa-daemon
gdm 1335 1223 0 13:28 ? 00:00:00 /usr/libexec/goa-identity-service
gdm 1341 1223 0 13:28 ? 00:00:00 /usr/libexec/gvfs-mtp-volume-monitor
gdm 1346 1223 0 13:28 ? 00:00:00 /usr/libexec/gvfs-afc-volume-monitor
gdm 1764 1246 0 13:28 tty1 00:00:00 /usr/libexec/gnome-session-binary --systemd --autostart /usr/share/gdm/greeter/autostart
gdm 1766 1223 0 13:28 ? 00:00:00 /usr/libexec/at-spi-bus-launcher
gdm 1771 1766 0 13:28 ? 00:00:00 /usr/bin/dbus-daemon --config-file=/usr/share/defaults/at-spi2/accessibility.conf --nofork --print-address 3
gdm 1779 1223 0 13:28 ? 00:00:00 /usr/libexec/gnome-session-ctl --monitor
gdm 1783 1223 0 13:28 ? 00:00:00 /usr/bin/gnome-keyring-daemon --start --components ssh
gdm 1792 1223 0 13:28 ? 00:00:00 /usr/libexec/gnome-session-binary --systemd-service --session=gnome-login
gdm 1799 1223 0 13:28 ? 00:00:03 /usr/bin/gnome-shell
gdm 1839 1799 0 13:28 ? 00:00:00 ibus-daemon --panel disable --xim
gdm 1842 1839 0 13:28 ? 00:00:00 /usr/libexec/ibus-memconf
gdm 1845 1223 0 13:28 ? 00:00:00 /usr/libexec/ibus-x11 --kill-daemon
gdm 1847 1223 0 13:28 ? 00:00:00 /usr/libexec/ibus-portal
gdm 1857 1223 0 13:28 ? 00:00:00 /usr/libexec/at-spi2-registryd --use-gnome-session
gdm 1861 1223 0 13:28 ? 00:00:00 /usr/libexec/xdg-permission-store
gdm 1876 1223 0 13:28 ? 00:00:00 /usr/bin/gjs /usr/share/gnome-shell/org.gnome.Shell.Notifications
gdm 1896 1223 0 13:28 ? 00:00:00 /usr/libexec/gsd-a11y-settings
gdm 1897 1223 0 13:28 ? 00:00:00 /usr/libexec/gsd-color
gdm 1898 1223 0 13:28 ? 00:00:00 /usr/libexec/gsd-keyboard
gdm 1899 1223 0 13:28 ? 00:00:00 /usr/libexec/gsd-media-keys
gdm 1901 1223 0 13:28 ? 00:00:00 /usr/libexec/gsd-power
gdm 1903 1223 0 13:28 ? 00:00:00 /usr/libexec/gsd-print-notifications
gdm 1906 1223 0 13:28 ? 00:00:00 /usr/libexec/gsd-rfkill
gdm 1910 1223 0 13:28 ? 00:00:00 /usr/libexec/gsd-smartcard
gdm 1911 1223 0 13:28 ? 00:00:00 /usr/libexec/gsd-sound
gdm 1912 1223 0 13:28 ? 00:00:00 /usr/libexec/gsd-usb-protection
gdm 1914 1223 0 13:28 ? 00:00:00 /usr/libexec/gsd-wacom
gdm 1915 1223 0 13:28 ? 00:00:00 /usr/libexec/gsd-wwan
gdm 1917 1223 0 13:28 ? 00:00:00 /usr/libexec/gsd-xsettings
gdm 1946 1223 0 13:28 ? 00:00:00 /usr/libexec/gsd-printer
gdm 1967 1839 0 13:28 ? 00:00:00 /usr/libexec/ibus-engine-simple
为什么这个gdm用户似乎有一些进程，就像在启动期间从桌面登录一样？这是正常的吗？这些进程消耗内存的原因是什么？