Knowledge Base

How to determine a server's memory usage

This article explains how to determine memory usage statistics on a server by using the free command.

Using the free command

To determine memory usage statistics on a server, follow these steps:

  1. Log in to the server using SSH.
  2. At the command prompt, type the following command:
    free -m

    For easier readability, use the -m option to display memory usage statistics in megabytes. To display statistics in bytes, run the free command without the -m option.

  3. Interpret the free command output. For example, consider the following sample output from a server:

    [email protected]:~# free -m
                 total      used      free    shared   buffers    cached
    Mem:          2003      1338       665         0       149      1015
    -/+ buffers/cache:       172      1830
    Swap:            0         0         0

    You may be tempted to look at the Mem row, scan the used and free columns, and determine that the server is using 1338 MB of RAM, and the amount of free RAM is only 665 MB. However, this is incorrect. In fact, the server is only using 172 MB of RAM, and has 1830 MB of free RAM.

    This is because Linux uses free memory for disk caching to improve performance. This memory, listed in the buffers and cached columns, is available immediately for any application that may need it. Although it is technically being “used” by Linux in the background, for all practical purposes this memory is free and available. So if you add the free memory (665 MB), buffers memory (149 MB), and cached memory (1015 MB) values, you obtain 1829 MB, which is the actual amount of memory available for applications. (The discrepancy between the 1829 MB that we calculated and the 1830 MB shown in the output is due to rounding because we used the -m option. If you run the free command without this option to obtain amounts in bytes, the numbers will add up exactly.)

More Information

For a humorous explanation of memory usage in Linux, please visit http://www.linuxatemyram.com.