How to monitor resource usage

Method #1: Using the top command

The traditional way of viewing resource usage information on Linux is to use the top program. The top program provides a real-time view that includes system information and a list of running processes. You can also customize which information top displays.

To start top, log in to your account using SSH, and then type the following command:

Copytop

By default, top updates the displayed information every 3 seconds. There are many commands for manipulating the information that top displays. For example, to sort tasks by CPU usage, type P.

To view detailed information about using the top command, type the following command:

Copyman top

Method #2: Using the atop command

Although it has many more functions than the popular top software, the above application is similar to it. Similar to top, above shows an instantaneous summary of the system’s resource utilization. On the other hand, the material displayed above is more in-depth and has more viewing and filtering options. In addition, above maintains system activity log files through a background process. You can view historical resource utilization data by accessing these system activity logs in atop.

To start atop, log in to your account using SSH, and then type the following command:

Copyatop

When you start atop for the first time, the amount of information displayed can be a little overwhelming. Here is a quick summary of the information atop displays:

• The top line displays the server name, as well as the date and time of the last data sample.
• The PRC row displays general statistics, such as time spent in system calls (sys) and user space (user). This line also displays the total number of processes (#proc) and zombie processes (#zombie).
• The CPU and cpu rows display statistics for each CPU.  Each row shows the percentage of time a CPU has spent doing various tasks such as system calls (sys), user space processing (user), servicing hardware interrupts (irq), doing nothing (idle), and waiting for disks (cpuXXX w).
• The CPL row displays the weighted load average for the past 1, 5, and 15 minutes. It also shows the number of context switches (csw). Context switches occur when the processor switches between kernel space and user space.
• The MEM row displays memory usage information.
• The SWP row displays swap memory usage information.
• The PAG row displays paging memory frequency, and should appear rarely on a healthy server.
• The DSK row displays disk usage information.
• The NET rows display usage statistics for each network interface.

To view complete reference information for the atop command, type the following command:

Copyman atop

View server activity history

The atop program maintains activity logs for the past few days. If your web site started experiencing problems yesterday, for example, you could load yesterday’s resource usage data into atop and view what was happening on the system at the time.

To view the server’s activity for today, type the following command:

Copyatop -r

Once the server history has loaded, you can navigate through it by typing t (lowercase t). The time at the top of the screen advances, as you can see. Type T (uppercase T) to navigate through the history backward.

Use the -b option to start above in order to view the server history beginning at a specified time. For instance, enter the following command to examine the server activity starting at 10:15 today:

Copyatop -r -b 10:15

To view resource usage for days in the past, list the files in the /var/log/atop directory to see which days are available. Then start atop using the -r option and the specified log file. For example, the following command instructs atop to load resource usage data for June 16, 2013:

Copyatop -r /var/log/atop/atop_20140316

To quickly and easily view resource usage information for yesterday, type the following command:

Copyatop -r y

Similarly, to view resource usage information for the day before yesterday, type the following command:

Copyatop -r yy