The vmstat Command

You can also determine memory and swap usage with the vmstat command, which provides a summary of what top produces. In the example, memory is still 15% free (158,196MB used from a total of 130,780) and only 80MB of the swap partition is being used.

     [root@bigboy tmp]# vmstat
     procs ------memory----- --swap- ---io-- -system- --cpu--
      r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us
     sy id wa
      0  0     80 158196 276272 357236    0    0     1     0    0    4  4
     1 95  0
     [root@bigboy tmp]#

As your memory fills up, your system temporarily stores programs and data on your hard disk's swap partition. Excess swapping of programs and data between disk and memory can cause your system to slow down significantly and memory usage should be monitored to allow you to plan ways to either increase RAM or tune the way your system operates.