HowTo: Check Swap Usage in Linux

How do I check swap (paging) usage under Linux operating systems using command line options?

Swap space (also known as paging) is nothing but computer memory management involving swapping regions of memory to and from storage.

You can see swap usage summary by device using any one of the following commands. You may have to login as root user to use the following commands. The maximum useful size of a swap area depends on the architecture and the kernel version. For Linuux kernels after v2.3.3+ there is no such limitation on swap size.

Option #1: /proc/swaps file

Type the following command to see total and used swap size:
# cat /proc/swaps
Sample outputs:

Filename    Type  Size Used Priority
/dev/sda3                               partition 6291448 65680 0

Option #2: swapon command

Type the following command:
# swapon -s
Sample outputs:

Filename    Type  Size Used Priority
/dev/sda3                               partition 6291448 65680 0

Option #3: free command

Use the free command as follows:
# free -g
# free -k
# free -m
Sample outputs:

             total       used       free     shared    buffers     cached
Mem:         11909      11645        264          0        324       8980
-/+ buffers/cache:       2341       9568
Swap:         6143         64       6079

Option #4: vmstat command

Type the following vmstat command:
# vmstat
# vmstat 1 5
Sample outputs:

procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
 1  9 1209512 101352   1504 127980    0    3    11    20   60   55  3  1 95  1
 2 11 1209640 101292   1508 134132  844  424  5608   964 23280 15012  2  8 20 70
 0 10 1210052 108132   1532 125764  648  660 10548   916 22237 18103  3 10 11 77
 1 13 1209892 106484   1500 128052  796  240 10484   980 24024 12692  2  8 24 67
 1  9 1209332 113412   1500 124028 1608  168  2472   620 28854 13761  2  8 20 70

Note down the following output from swap field:

1. si: Amount of memory swapped in from disk (/s).
2. so: Amount of memory swapped to disk (/s).

Option #5: top/atop/htop command

Type the following commands:
# atop
# htop
# top
Sample outputs (from top command):

top - 02:54:24 up 15:24,  4 users,  load average: 0.45, 4.84, 6.75
Tasks: 266 total,   1 running, 264 sleeping,   0 stopped,   1 zombie
Cpu(s):  3.2%us,  1.4%sy,  0.0%ni, 94.4%id,  1.0%wa,  0.0%hi,  0.1%si,  0.0%st
Mem:   8120568k total,  7673584k used,   446984k free,     4516k buffers
Swap: 15859708k total,  1167408k used, 14692300k free,  1151972k cached
  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
13491 vivek     20   0 1137m 279m 6692 S   10  3.5  19:17.47 firefox
 5663 vivek     10 -10 1564m 1.1g  59m S    8 14.5   5:10.94 vmware-vmx
 2661 root      20   0  352m 185m 8604 S    6  2.3  65:40.17 Xorg
 3752 vivek     20   0 3566m 2.6g  12m S    6 33.6  63:44.35 compiz
 4798 vivek     20   0  900m  50m 4992 S    2  0.6   0:11.04 chrome
 5539 vivek     20   0 1388m 838m 780m S    2 10.6   1:45.78 VirtualBox
 6297 root      20   0     0    0    0 S    2  0.0   0:00.15 kworker/2:0
 6646 root      20   0 19252 1404  936 R    2  0.0   0:00.01 top
    1 root      20   0  8404  644  608 S    0  0.0   0:03.32 init
    2 root      20   0     0    0    0 S    0  0.0   0:00.03 kthreadd
    3 root      20   0     0    0    0 S    0  0.0   0:02.30 ksoftirqd/0
    6 root      RT   0     0    0    0 S    0  0.0   0:00.00 migration/0
    7 root      RT   0     0    0    0 S    0  0.0   0:00.24 watchdog/0
   37 root       0 -20     0    0    0 S    0  0.0   0:00.00 cpuset
   38 root       0 -20     0    0    0 S    0  0.0   0:00.00 khelper
   39 root      20   0     0    0    0 S    0  0.0   0:00.00 kdevtmpfs
   40 root       0 -20     0    0    0 S    0  0.0   0:00.00 netns

Sample outputs from htop command:

Fig.01: Linux: Swap Memory Usage Command