Memory monitoring and management

This article shows you how to check and monitor the memory usage of a Linux® server. You must
consider many different things when managing memory use for a Linux server, but this article focuses
on the following:

• How to view memory usage
• Swap
• Out-of-memory (OOM) killer

How to view memory usage

You can use the free, top, or htop commands to view memory usage.

The free command

Use the free command to display the amount of free and used memory in the system. After you
run it, you should see output similar to the following example:

$ free
              total        used        free      shared  buff/cache   available
Mem:        8009408     1878604      970740      470152     5160064     5341764
Swap:       4194300       92160     4102140

You can add the following flag options to the command to customize the output:

• -h: Makes the output of the command human readable.
• -[b, k, m, g]: Formats the data in the corresponding data type (byte, kilobyte, megabyte, or gigabyte).
• -s: Outputs the data at the specified interval. For example, -s 3 displays data every 3 seconds.

The top and htop commands

Use the top command to display the current processes running on the server. The htop command
displays the same information in a more organized way, however htop is not installed on most
servers by default.

Swap space

Swap space is the amount of space that is reserved whenever the random access memory (RAM) is
used up. You can use the commands in the preceding sections to view the swap space along with the memory. If you
want to learn more about swap space, see
Swap space on Cloud Servers.

OOM Killer

When a server runs low on memory, the system invokes OOM Killer to kill certain processes
in order to free up memory so that the system can keep running. Often when a process is
killed by OOM Killer, you can see an entry in the following log files:

• /var/log/messages (or /var/log/syslog for Ubuntu)
• /var/log/dmesg