自从几天前我从 15.10 升级到 16.04 以来，我遇到了一个问题：我的内存几乎完全满了！
我的系统上有 8 GB 的 RAM，目前已使用 6.9GB（仅打开 Firefox）。
我运行了 ps_mem 脚本，它显示以下内容：
Private + Shared = RAM used Program
4.0 KiB + 10.0 KiB = 14.0 KiB sh
4.0 KiB + 12.5 KiB = 16.5 KiB vmnet-bridge
4.0 KiB + 20.5 KiB = 24.5 KiB agetty
8.0 KiB + 25.0 KiB = 33.0 KiB vmnet-netifup (2)
4.0 KiB + 31.5 KiB = 35.5 KiB uuidd
4.0 KiB + 35.0 KiB = 39.0 KiB cgmanager
4.0 KiB + 53.0 KiB = 57.0 KiB indicator-bluetooth-service
4.0 KiB + 60.0 KiB = 64.0 KiB gvfsd-metadata
4.0 KiB + 62.0 KiB = 66.0 KiB gvfsd-burn
4.0 KiB + 63.0 KiB = 67.0 KiB gvfsd-fuse
36.0 KiB + 32.0 KiB = 68.0 KiB atd
4.0 KiB + 64.5 KiB = 68.5 KiB indicator-messages-service
4.0 KiB + 68.0 KiB = 72.0 KiB indicator-power-service
4.0 KiB + 69.5 KiB = 73.5 KiB sshd
40.0 KiB + 46.0 KiB = 86.0 KiB rtkit-daemon
4.0 KiB + 86.5 KiB = 90.5 KiB gvfsd-network
68.0 KiB + 25.0 KiB = 93.0 KiB vmware-authdlauncher
80.0 KiB + 14.0 KiB = 94.0 KiB acpid
4.0 KiB + 90.0 KiB = 94.0 KiB (sd-pam)
4.0 KiB + 119.0 KiB = 123.0 KiB timidity
4.0 KiB + 122.0 KiB = 126.0 KiB ModemManager
112.0 KiB + 35.5 KiB = 147.5 KiB cron
8.0 KiB + 157.0 KiB = 165.0 KiB lightdm (2)
168.0 KiB + 27.5 KiB = 195.5 KiB irqbalance
152.0 KiB + 57.5 KiB = 209.5 KiB ibus-dconf
172.0 KiB + 47.5 KiB = 219.5 KiB systemd-timesyncd
4.0 KiB + 215.5 KiB = 219.5 KiB evolution-addressbook-factory
168.0 KiB + 60.5 KiB = 228.5 KiB gvfs-mtp-volume-monitor
156.0 KiB + 76.5 KiB = 232.5 KiB accounts-daemon
140.0 KiB + 97.0 KiB = 237.0 KiB vmnet-dhcpd (2)
200.0 KiB + 58.5 KiB = 258.5 KiB upstart-file-bridge
164.0 KiB + 96.5 KiB = 260.5 KiB upstart-udev-bridge
188.0 KiB + 87.0 KiB = 275.0 KiB gvfsd-trash
188.0 KiB + 96.0 KiB = 284.0 KiB gvfs-afc-volume-monitor
4.0 KiB + 292.5 KiB = 296.5 KiB evolution-source-registry
164.0 KiB + 145.0 KiB = 309.0 KiB gvfsd-dnssd
272.0 KiB + 56.0 KiB = 328.0 KiB gvfs-goa-volume-monitor
4.0 KiB + 326.5 KiB = 330.5 KiB evolution-calendar-factory
296.0 KiB + 52.0 KiB = 348.0 KiB gpg-agent
248.0 KiB + 142.0 KiB = 390.0 KiB gconfd-2
320.0 KiB + 73.0 KiB = 393.0 KiB at-spi-bus-launcher
336.0 KiB + 58.0 KiB = 394.0 KiB vmware-vmblock-
328.0 KiB + 71.0 KiB = 399.0 KiB gvfs-gphoto2-volume-monitor
400.0 KiB + 18.0 KiB = 418.0 KiB vmnet-natd
360.0 KiB + 92.5 KiB = 452.5 KiB thermald
332.0 KiB + 137.5 KiB = 469.5 KiB upstart-dbus-bridge (2)
416.0 KiB + 88.0 KiB = 504.0 KiB at-spi2-registryd
376.0 KiB + 139.0 KiB = 515.0 KiB avahi-daemon (2)
392.0 KiB + 124.0 KiB = 516.0 KiB gvfsd
484.0 KiB + 40.5 KiB = 524.5 KiB systemd-logind
428.0 KiB + 103.0 KiB = 531.0 KiB deja-dup-monitor
488.0 KiB + 47.5 KiB = 535.5 KiB rsyslogd
456.0 KiB + 81.0 KiB = 537.0 KiB polkitd
156.0 KiB + 386.0 KiB = 542.0 KiB dnsmasq (3)
432.0 KiB + 122.0 KiB = 554.0 KiB ibus-engine-simple
544.0 KiB + 68.5 KiB = 612.5 KiB sudo
484.0 KiB + 142.5 KiB = 626.5 KiB upowerd
456.0 KiB + 195.0 KiB = 651.0 KiB cups-browsed
476.0 KiB + 179.0 KiB = 655.0 KiB evolution-addressbook-factory-subprocess
624.0 KiB + 41.5 KiB = 665.5 KiB vmware-usbarbitrator
612.0 KiB + 55.0 KiB = 667.0 KiB systemd-udevd
352.0 KiB + 339.0 KiB = 691.0 KiB nmbd
664.0 KiB + 67.0 KiB = 731.0 KiB dconf-service
508.0 KiB + 257.0 KiB = 765.0 KiB indicator-application-service
700.0 KiB + 81.5 KiB = 781.5 KiB indicator-session-service
568.0 KiB + 294.0 KiB = 862.0 KiB indicator-sound-service
688.0 KiB + 281.0 KiB = 969.0 KiB zeitgeist-datahub
688.0 KiB + 310.5 KiB = 998.5 KiB whoopsie
680.0 KiB + 323.0 KiB = 1.0 MiB libvirtd
456.0 KiB + 580.0 KiB = 1.0 MiB window-stack-bridge
984.0 KiB + 100.0 KiB = 1.1 MiB upstart
796.0 KiB + 316.0 KiB = 1.1 MiB update-notifier
956.0 KiB + 177.0 KiB = 1.1 MiB gvfs-udisks2-volume-monitor
924.0 KiB + 238.0 KiB = 1.1 MiB polkit-gnome-authentication-agent-1
956.0 KiB + 234.0 KiB = 1.2 MiB unity-fallback-mount-helper
1.0 MiB + 191.5 KiB = 1.2 MiB indicator-datetime-service
964.0 KiB + 360.0 KiB = 1.3 MiB evolution-calendar-factory-subprocess (2)
1.0 MiB + 331.5 KiB = 1.3 MiB firewalld
1.1 MiB + 289.5 KiB = 1.3 MiB ibus-x11
940.0 KiB + 446.0 KiB = 1.4 MiB notify-osd
1.2 MiB + 277.0 KiB = 1.4 MiB indicator-printers-service
1.2 MiB + 264.0 KiB = 1.4 MiB gnome-session-binary
1.1 MiB + 331.0 KiB = 1.5 MiB colord
1.4 MiB + 67.5 KiB = 1.5 MiB systemd-journald
1.3 MiB + 284.5 KiB = 1.6 MiB indicator-keyboard-service
1.7 MiB + 142.5 KiB = 1.9 MiB gnome-keyring-daemon
1.4 MiB + 515.0 KiB = 1.9 MiB zeitgeist-daemon
1.8 MiB + 325.0 KiB = 2.1 MiB nm-applet
1.9 MiB + 203.5 KiB = 2.1 MiB ibus-daemon
2.1 MiB + 141.0 KiB = 2.2 MiB systemd (2)
704.0 KiB + 1.7 MiB = 2.4 MiB winbindd (4)
1.1 MiB + 1.5 MiB = 2.6 MiB smbd (3)
2.5 MiB + 204.0 KiB = 2.7 MiB udisksd
2.5 MiB + 532.0 KiB = 3.0 MiB ibus-ui-gtk3
3.2 MiB + 622.0 KiB = 3.8 MiB dbus-daemon (3)
3.5 MiB + 1.3 MiB = 4.8 MiB zeitgeist-fts
4.4 MiB + 643.5 KiB = 5.0 MiB unity-scope-home
4.4 MiB + 704.5 KiB = 5.1 MiB unity-settings-daemon
4.9 MiB + 233.5 KiB = 5.1 MiB NetworkManager
4.8 MiB + 418.5 KiB = 5.2 MiB gnome-software
4.7 MiB + 1.0 MiB = 5.7 MiB unity-files-daemon
5.7 MiB + 280.0 KiB = 6.0 MiB mount.ntfs (2)
6.5 MiB + 1.1 MiB = 7.6 MiB bamfdaemon
6.8 MiB + 1.3 MiB = 8.2 MiB bash (3)
7.4 MiB + 968.0 KiB = 8.3 MiB hud-service
7.7 MiB + 2.4 MiB = 10.1 MiB unity-panel-service
9.3 MiB + 926.0 KiB = 10.2 MiB pulseaudio
10.6 MiB + 177.5 KiB = 10.7 MiB fwupd
10.6 MiB + 1.3 MiB = 11.9 MiB unity-scope-loader
9.7 MiB + 2.6 MiB = 12.3 MiB gnome-terminal-server
12.6 MiB + 2.7 MiB = 15.3 MiB nautilus
19.3 MiB + 2.0 MiB = 21.3 MiB Xorg
20.6 MiB + 4.5 MiB = 25.1 MiB gnome-system-monitor
23.7 MiB + 2.2 MiB = 25.9 MiB oneconf-service
74.9 MiB + 3.5 MiB = 78.3 MiB compiz
465.7 MiB + 6.8 MiB = 472.5 MiB firefox
---------------------------------
823.4 MiB
=================================
但是 cat /proc/meminfo 显示 8,068,136 KB（~ 7,879 MB）中只有 235,172 KB（~ 230 MB）是可用的。
cat /proc/meminfo
MemTotal: 8068136 kB
MemFree: 235172 kB
MemAvailable: 929988 kB
Buffers: 67956 kB
Cached: 609744 kB
SwapCached: 47000 kB
Active: 1053864 kB
Inactive: 378708 kB
Active(anon): 720464 kB
Inactive(anon): 37844 kB
Active(file): 333400 kB
Inactive(file): 340864 kB
Unevictable: 32 kB
Mlocked: 32 kB
SwapTotal: 8279036 kB
SwapFree: 7527772 kB
Dirty: 0 kB
Writeback: 140 kB
AnonPages: 748564 kB
Mapped: 163976 kB
Shmem: 3436 kB
Slab: 112524 kB
SReclaimable: 62840 kB
SUnreclaim: 49684 kB
KernelStack: 10880 kB
PageTables: 42012 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 9241104 kB
Committed_AS: 6502252 kB
VmallocTotal: 34359738367 kB
VmallocUsed: 0 kB
VmallocChunk: 0 kB
HardwareCorrupted: 0 kB
AnonHugePages: 106496 kB
CmaTotal: 0 kB
CmaFree: 0 kB
HugePages_Total: 3000
HugePages_Free: 3000
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 2048 kB
DirectMap4k: 1695128 kB
DirectMap2M: 6586368 kB
编辑 1：输出 free（我自己粗略地从德语翻译为英语）。它与其他部分一致。
total used free shared. buffers/cached available
Memory: 8068136 7470560 209028 36916 388548 475064
Swap: 8279036 207020 8072016
所以，不知何故，我在某个地方缺少了 7,832,964 KB (~ 7,649 MB) 的 RAM，这些 RAM 已被使用但没有列出...有什么想法吗？
我甚至不知道如何用谷歌来解决这个问题。
谢谢。