Linux vmstat Command Tutorial for Beginners (5 Examples)

If you are looking for a command line utility that you can use to access information about processes, CPU activity, memory, and more, you'll be glad to know that vmstat does this for you.

In this article, we will discuss basics of this tool using some easy to understand examples. But before we do that, it's worth mentioning that all examples in this article have been tested on an Ubuntu 18.04 LTS machine.

Linux vmstat command

The vmstat command in Linux reports virtual memory statistics. Following is its syntax:

vmstat [options] [delay [count]]

And here's how the tool's man page explains it:

       vmstat  reports  information about processes, memory, paging, block IO,
       traps, disks and cpu activity.

       The first report produced gives averages since the last reboot.   Addi?
       tional  reports  give information on a sampling period of length delay.
       The process and memory reports are instantaneous in either case.

Following are some Q&A-styled examples that should give you a better idea on how the vmstat command works:

Q1. How to use vmstat?

For basic usage, just execute 'vmstat' sans any option.

vmstat

For example, in my case, the above command produced the following output:

procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 2  0 805624 770924  41792 838568    2   10    35    57  392  225 10  2 86  1  0

As the first row indicates, the output is divided into six sections. The tool's man page contains detailed information about these values. Here's the excerpt:

Procs
       r: The number of runnable processes (running or waiting for run time).
       b: The number of processes in uninterruptible sleep.

   Memory
       swpd: the amount of virtual memory used.
       free: the amount of idle memory.
       buff: the amount of memory used as buffers.
       cache: the amount of memory used as cache.
       inact: the amount of inactive memory.  (-a option)
       active: the amount of active memory.  (-a option)

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

   IO
       bi: Blocks received from a block device (blocks/s).
       bo: Blocks sent to a block device (blocks/s).

   System
       in: The number of interrupts per second, including the clock.
       cs: The number of context switches per second.

   CPU
       These are percentages of total CPU time.
       us: Time spent running non-kernel code.  (user time, including nice time)
       sy: Time spent running kernel code.  (system time)
       id: Time spent idle.  Prior to Linux 2.5.41, this includes IO-wait time.
       wa: Time spent waiting for IO.  Prior to Linux 2.5.41, included in idle.
       st: Time stolen from a virtual machine.  Prior to Linux 2.6.11, unknown.

Q2. How to make vmstat auto update its output?

By default, vmstat produces output once. However, if you want vmstat to automatically update its output, then you can do that by specifying a numeric value as input to the command.

This numeric value acts as delay (in seconds) after which the output is updated. For example:

vmstat 3

This way, vmstat output will get refreshed every 3 seconds.

procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 1  0 824320 1067960  20500 507612    2   10    36    56  412  296 11  2 86  1  0
 0  0 824320 1068116  20532 507176    0    0     0    39 2648 6093  4  1 93  1  0
 0  0 824320 1062420  20548 507160    0    0     0    48 2603 5895  5  1 93  1  0
 0  0 824320 1060180  20556 507152    0    0     0   289 2815 6092  4  1 94  0  0
 1  0 824320 1059940  20580 509216    0    0     0    31 2578 5937  5  1 93  1  0
...
...
...

Q3. How to make vmstat display slabinfo?

For slabinfo, you need to use the -m command line option. Note that your Linux distribution's kernel needs to support slabinfo for this option to work. If support is there, then you need to run the command with sudo permissions.

sudo vmstat -m

For example, in my case, here's an excerpt of the output produced:

Cache                       Num  Total   Size  Pages
ext4_groupinfo_4k          1568   1568    144     28
drm_i915_gem_request        112    112    576     28
i915_vma                    934   1012    704     23
drm_i915_gem_object         896   1050    768     21
RAWv6                       140    140   1152     28
UDPv6                       104    104   1216     26
tw_sock_TCPv6                17     17    240     17
request_sock_TCPv6            0      0    304     26
TCPv6                        60     60   2176     15
kcopyd_job                    0      0   3312      9
dm_uevent                     0      0   2632     12
cfq_io_cq                   442    442    120     34
cfq_queue                   408    408    240     17
bsg_cmd                       0      0    216     18
mqueue_inode_cache           68     68    960     17
fuse_request                 40     40    400     20
fuse_inode                   19     19    832     19
ecryptfs_inode_cache          0      0   1024     16
...
...
...

Q4. How to make vmstat report disk activity summary stats?

Use the -D command line option for it.

vmstat -D

Here's the output this command produced on my system:

           21 disks 
            8 partitions
       158637 total reads
        31589 merged reads
      7376734 read sectors
      2099724 milli reading
       198763 writes
       553378 merged writes
     11743120 written sectors
      6396664 milli writing
            0 inprogress IO
         1248 milli spent IO

Q5. How to make vmstat append timestamp to each output line?

To make sure the vmstat command appends timestamp to each output line, use the -t command line:

vmstat -t
$ vmstat 2 -t
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu----- -----timestamp-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st                 IST
 1  0 822528 855556  36820 689220    1    9    34    54   62   68 11  2 86  1  0 2018-09-27 16:29:40
 0  0 822528 849612  36828 692792    0    0     0    30 2894 7141  7  2 91  1  0 2018-09-27 16:29:42
 1  0 822528 851820  36856 697288    0    0     0   276 3081 7072 17  2 80  1  0 2018-09-27 16:29:44
 0  0 822528 850968  36856 697420    0    0     0     0 3025 8067  7  2 91  0  0 2018-09-27 16:29:46
 0  0 822528 853480  36856 693360    0    0     0     0 2762 6285  5  1 93  0  0 2018-09-27 16:29:48
 0  0 822528 853128  36872 692796    0    0     0   140 2881 6534  6  1 91  2  0 2018-09-27 16:29:50
 0  0 822528 846924  36872 697108    0    0     0     0 2843 6637  6  2 93  0  0 2018-09-27 16:29:52

The highlighted entries show the timestamp included in each line.

Conclusion

Depending on the kind of work you do on the Linux command line, the vmstat command can prove to be really helpful. Apart from the options discussed here, there are a lot of other command line options that this tool offers. Head here to learn more.

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