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Why Does Top Command on Ubuntu Multicore CPU Show CPU Usage >100%?

Ubuntu 7

Understanding the intricacies of system monitoring can be a daunting task, especially when the information displayed seems counterintuitive. A common question among Ubuntu users is why the top command sometimes shows CPU usage exceeding 100%. In this article, we will explore this phenomenon and provide a clear explanation of why this happens.

Quick Answer

The top command on Ubuntu may show CPU usage exceeding 100% on multicore systems because it displays usage as a percentage of a single CPU core. This is not a bug or error, but an intentional feature designed to provide detailed information about CPU usage for each process and thread.

Understanding the top Command

The top command is a commonly used tool in Unix-like operating systems, including Ubuntu. It provides a dynamic, real-time view of the processes running on a system. One of its features is the display of CPU usage.

When you run the top command, it displays CPU usage as a percentage. However, on multicore systems, this percentage can sometimes exceed 100%. This might seem like an error or a bug, but it’s actually an intentional feature.

CPU Usage on Multicore Systems

On multicore systems, the top command displays CPU usage as a percentage of a single CPU core. This means that if a process is using two cores at 100%, top will display this as 200% CPU usage. This is because top sums up the percentages of CPU usage across all cores.

The reason for this is that top is designed to show the CPU usage of each individual process and thread. If a process is multithreaded and can run on multiple cores simultaneously, top will show the total CPU usage across all cores.

Changing the Display of CPU Usage

If you find the default behavior of top confusing, you can change it to display the overall percentage of available CPUs in use. To do this, press Shift + i while top is running. This will toggle the display mode and show CPU usage as a percentage of total CPU capacity, rather than per core.

Using htop as an Alternative

Another option is to use the htop command, which is a more advanced and user-friendly alternative to top. htop provides a clearer picture of CPU usage on multicore systems, with a visual representation of how the load is distributed across all cores.

To install htop on Ubuntu, use the following command:

sudo apt-get install htop

Then, you can run htop just like top:

htop

Conclusion

In summary, the top command in Ubuntu shows CPU usage greater than 100% on multicore systems because it displays usage as a percentage of a single CPU core. This is not a bug or an error, but an intentional feature designed to provide detailed information about CPU usage for each process and thread.

By understanding how top calculates and displays CPU usage, or by using alternatives like htop, you can get a more accurate picture of your system’s CPU usage. This will help you monitor your system more effectively and troubleshoot performance issues more accurately.

Why does the `top` command on Ubuntu show CPU usage exceeding 100%?

The top command on Ubuntu shows CPU usage exceeding 100% on multicore systems because it displays the usage as a percentage of a single CPU core. If a process is using multiple cores at 100%, top will sum up the percentages across all cores, resulting in a value greater than 100%.

Can the display of CPU usage be changed in the `top` command?

Yes, you can change the display of CPU usage in the top command. While top is running, press Shift + i to toggle the display mode. This will show CPU usage as a percentage of the total CPU capacity, rather than per core.

Is there an alternative to the `top` command on Ubuntu?

Yes, there is an alternative called htop. htop is a more advanced and user-friendly command that provides a clearer picture of CPU usage on multicore systems. It offers a visual representation of how the load is distributed across all cores. To install htop on Ubuntu, use the command: sudo apt-get install htop. Then, you can run htop just like top by typing htop in the terminal.

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