HowTo: Check Swap Usage and Utilization in Linux

Posted on in Categories , , last updated June 8, 2017

How do I check swap (paging) usage under Linux operating systems using command bash/ksh 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 to show swap usage summary by device
# 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 the htop command:

Linux: Swap Memory Usage Command
Fig.01: Linux: Swap Memory Usage Command

See man pages – swapon(8),free(1),top(1),vmstat(8), for more information.

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.

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11 comment

    1. Disable swap (Take care if the swap memory is in use: information goes from swap to RAM)

      # swapoff -a

      With lvm partition, you can resize it like this:
      Suppose swap partition in /dev/vg0/swap
      # lvresize -L +1G /dev/vg0/swap
      next, (re)setup swap memory :
      # mkswap /dev/vg0/swap
      Now, You can re-enable swap like this:
      # swapon -a

  3. Thank you for sharing. May I please request you to clarify what UNITS are used when printing various numbers in the output of commands you use. I have been searching to get details on the “swapon -s” output column “Size” (the man page omits the units as well).

    Is it reported in bytesm mb,gb?
    Is it reported as multiples of “Memory Page Size (as in getconf PAGESIZE)”?
    Is it reported as multiples of some disk block setting (as in the output of blockdev –getsize64 /dev/swap/swap)
    Some other unit?

    Thank you

    I

  4. I found the answer to my own question, and here it is:

    The units used in “swapon -s” output is “kB (kilo bytes)”.

    OS Details: 

    Red Hat Enterprise Linux Server release 6.3 (Santiago)
Linux XXXXX-002 2.6.32-279.el6.x86_64 #1 SMP Wed Jun 13 18:24:36 EDT 2012 x86_64 x86_64 x86_64 GNU/Linux

    Route to My Solution
    Command:
    #grep SwapTotal /proc/meminfo
    Which gave me the output (with units kB):
    SwapTotal: 104603632 kB
    And swapon -s output is:

    # swapon -s 
Filename                                Type            Size    Used    Priority
/dev/dm-7                               partition       20717560        0      -1
/dev/dm-9                               partition       83886072        0      -2

    Used bc to add up the two “Size” numbers

    #bc
83886072 + 20717560
104603632

    The sum and the “SwapTotal” output from earlier match (104603632).

    Therefore, the units must be kB or kilobytes

  6. Personally, I like to use:
    cat /proc/meminfo | grep Swap

    That will output the total swap stats like so:
    SwapCached: 0 kB
    SwapTotal: 999996 kB
    SwapFree: 999996 kB

    It always displays output, even if there is no swap, which may help if you are using the command in scripts etc.

  8. Why I can’t see swap usage on htop command?, I guess my configuration it’s all messed up, here’s an example of what I get with htop:
    Swp [ 0K/8.00G]

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