我的 Kubuntu 笔记本电脑
$ uname -a
Linux jiggs 3.16.0-31-generic #43-Ubuntu SMP Tue Mar 10 17:37:36 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
有 6GiB 内存：
# lshw -C memory
*-firmware
description: BIOS
vendor: TOSHIBA
physical id: 0
version: Version 6.50
date: 11/26/2012
size: 128KiB
capacity: 8128KiB
capabilities: isa pci pnp upgrade shadowing vesa cdboot bootselect edd int13floppytoshiba int13floppy720 int5printscreen int9keyboard int14serial int17printer acpi usb biosbootspecification netboot
*-memory
description: System Memory
physical id: 1f
slot: System board or motherboard
size: 6GiB
*-bank:0
description: SODIMM DDR3 Synchronous 1600 MHz (0,6 ns)
product: HMT351S6CFR8C-PB
vendor: Hynix/Hyundai
physical id: 0
serial: 3F720214
slot: DIMM0
size: 4GiB
width: 64 bits
clock: 1600MHz (0.6ns)
*-bank:1
description: SODIMM DDR3 Synchronous 1600 MHz (0,6 ns)
product: HMT325S6CFR8C-PB
vendor: Hynix/Hyundai
physical id: 1
serial: 046584F8
slot: DIMM2
size: 2GiB
width: 64 bits
clock: 1600MHz (0.6ns)
据我推断，我有 6GiB（根据我购买时的规格记忆）。
然而，其他文书却有不同的报告：
$ vmstat -s
4942348 K total memory
3043944 K used memory
1591504 K active memory
1160736 K inactive memory
1898404 K free memory
93484 K buffer memory
810816 K swap cache
4194300 K total swap
242832 K used swap
3951468 K free swap
7151425 non-nice user cpu ticks
22453 nice user cpu ticks
401211 system cpu ticks
94036935 idle cpu ticks
62616 IO-wait cpu ticks
246 IRQ cpu ticks
3030 softirq cpu ticks
0 stolen cpu ticks
1570974 pages paged in
9494712 pages paged out
1695 pages swapped in
18084 pages swapped out
87647428 interrupts
199120848 CPU context switches
1426162570 boot time
41765 forks
或者
$ free -h
total used free shared buffers cached
Mem: 4,7G 2,9G 1,8G 209M 91M 796M
-/+ buffers/cache: 2,0G 2,7G
Swap: 4,0G 237M 3,8G
或者，最后，
$ cat /proc/meminfo
MemTotal: 4942348 kB
MemFree: 1892556 kB
MemAvailable: 2483472 kB
Buffers: 93844 kB
Cached: 815388 kB
SwapCached: 180524 kB
Active: 1595312 kB
Inactive: 1162484 kB
Active(anon): 1128632 kB
Inactive(anon): 934308 kB
Active(file): 466680 kB
Inactive(file): 228176 kB
Unevictable: 68 kB
Mlocked: 68 kB
SwapTotal: 4194300 kB
SwapFree: 3951468 kB
Dirty: 0 kB
Writeback: 0 kB
AnonPages: 1671124 kB
Mapped: 296016 kB
Shmem: 214376 kB
Slab: 167272 kB
SReclaimable: 130004 kB
SUnreclaim: 37268 kB
KernelStack: 9552 kB
PageTables: 56688 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 6665472 kB
Committed_AS: 6178500 kB
VmallocTotal: 34359738367 kB
VmallocUsed: 354304 kB
VmallocChunk: 34359375180 kB
HardwareCorrupted: 0 kB
AnonHugePages: 638976 kB
HugePages_Total: 0
HugePages_Free: 0
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 2048 kB
DirectMap4k: 75328 kB
DirectMap2M: 5027840 kB
最后一个输出可能很重要，因为它确实不是报告任何硬件问题：硬件损坏 = 0 kB。差异从何而来？
编辑：@Sobrique 正确地指出我应该包含 BIOS 信息：
Total Memory Size 6140MB
编辑2：
# dmesg | grep Memory
[ 0.000000] Memory: 5957056K/6166700K available (7384K kernel code, 1145K rwdata, 3408K rodata, 1336K init, 1444K bss, 209644K reserved)
[ 1.201086] [drm] Memory usable by graphics device = 2048M
答案1
奇怪的是，重新启动后，差异不再存在：
$ dmesg | grep Memory
[ 0.000000] Memory: 5957056K/6166700K available (7384K kernel code, 1145K rwdata, 3408K rodata, 1336K init, 1444K bss, 209644K reserved)
[ 1.312970] [drm] Memory usable by graphics device = 2048M
$ free -h
total used free shared buffers cached
Mem: 5.7G 3.8G 1.9G 334M 274M 1.6G
-/+ buffers/cache: 1.9G 3.8G
Swap: 6.0G 0B 6.0G
$ vmstat -s | head -1
5987640 K total memory
$ head -1 /proc/meminfo
MemTotal: 5987640 kB
我当然松了一口气，但也很好奇：我不知道什么可能会导致可用内存暂时减少。
答案2
4G是一个特殊的数字，因为它是2^32。这是一个很好的指标，表明根本原因可能在哪里。遇到 32 位和 31 位内存限制的情况并不罕见。你已经超越了这个 - 你使用的是 4.7G - 所以这对你的 32 位来说不是问题。
经过一番挖掘，我们发现为显卡保留系统 RAM 的情况并不少见。
https://askubuntu.com/questions/82917/ubuntu-detects-only-3-2gb-ram-on-a-64bit-os
要么是这样，要么与 BIOS 相关：
https://askubuntu.com/questions/80721/only-3-2-gb-of-memory-being-used-on-a-64-bit-system http://forums.fedoraforum.org/showthread.php?t=297556