atop

What is atop?

atop is a powerful command-line utility for Linux systems that provides a detailed view of system performance over time. Unlike top, which only displays real-time data, atop logs system activity at regular intervals, allowing you to analyze historical performance trends. This is for identifying recurring issues or pinpointing the root cause of past performance problems. It collects a wide range of metrics, including CPU usage, memory usage, disk I/O, network traffic, and process activity.

Installation

The installation process varies slightly depending on your distribution. For Debian/Ubuntu based systems, use apt:

sudo apt update
sudo apt install atop

For Fedora/CentOS/RHEL, use yum or dnf:

sudo dnf install atop  # or sudo yum install atop

After installation, atop starts logging system activity automatically.

Basic Usage and Output Interpretation

The core functionality of atop revolves around its ability to collect and display historical performance data. To view the current system activity in a similar way to top, use:

atop

This command will present a real-time summary, updating every second. Press q to exit. However, the true power of atop lies in its ability to analyze past performance logs. To view these logs, you need to specify the log file:

atop -r /var/log/atop/atop_20241027

Replace /var/log/atop/atop_20241027 with the actual path to your atop log file. The date in the filename corresponds to the date of the log. You’ll find a detailed breakdown of CPU usage (per core), memory usage, disk I/O, network traffic, and a list of the most resource-intensive processes.

Understanding the output requires some familiarity with system performance metrics. However, the column headers are usually self-explanatory. Key metrics to watch include:

• CPU: Percentage of CPU time used by different processes and system activities.
• MEM: Amount of physical and virtual memory in use.
• DISK: Read and write operations to various disk devices.
• NET: Network traffic in and out.
• PROCESSES: A list of running processes with their respective CPU and memory usage.

Analyzing Specific Time Periods

atop allows granular control over the analysis of historical data. To view data from a specific time period, use the -b (begin) and -e (end) options:

atop -r /var/log/atop/atop_20241027 -b 10:00:00 -e 11:00:00

This command displays the data from 10:00 AM to 11:00 AM on October 27th, 2024.

Focusing on Specific Processes

atop’s ability to filter data is important for troubleshooting. To focus on a specific process (e.g., httpd), use the -p option:

atop -r /var/log/atop/atop_20241027 -p httpd

This will filter the output to show only data related to the httpd process and its resources consumption.

Graphical Output with atoptool

While atop provides a text-based interface, the companion tool atoptool presents data graphically, making it easier to identify trends and anomalies. Installation instructions vary depending on the distribution, usually through the same package manager as atop. Once installed, you can use it like this:

atoptool -r /var/log/atop/atop_20241027

Configuration

The atop daemon runs continuously in the background, logging data to the specified location. You can configure the logging interval, log file size and other settings by modifying the /etc/atop/atop.conf file. Refer to the atop man page (man atop) for detailed information on configuration options.

Advanced Usage

atop offers many other powerful features, such as:

• Multiple Log File Analysis: Combine logs from multiple days for longer-term analysis.
• Saving Output to a File: Redirect the output to a file using >.
• Data Aggregation: Summarize data across different time intervals.

This guide offers a starting point for utilizing atop’s powerful capabilities. Experimentation and reviewing the atop man page are highly recommended for deeper understanding and more advanced usage.