How to deal with zombie processes?

What is a zombie process?

We all know how processes work. We start a program, start our task, and then when the task is completed, we stop the process. When a process stops, it is removed from the process table.

You can view the current process through System-Monitor.

However, sometimes some programs still remain in the process table even after they are executed.

So, these processes that have completed their life cycle but still remain in the process table are called "zombie processes".

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How are they produced?

When you run a program, it creates a parent process and many child processes. All these child processes consume the memory and CPU resources allocated to them by the kernel.

After these child processes complete execution, they will send an Exit signal and then die. This Exit signal needs to be read by the parent process. The parent process needs to subsequently call the wait command to read the exit status of the child process and remove the child process from the process table.

If the parent process correctly reads the Exit signal of the child process, the child process will be deleted from the process table.

But if the parent process fails to read the Exit signal of the child process, although the child process has completed execution and is in a dead state, it will not be deleted from the process table.

Are zombie processes harmful to the system?

Won't. Since zombie processes do not do anything, do not use any resources and do not affect other processes, there is no harm in the existence of zombie processes. However, because the exit status and other process information in the process table are also stored in memory, the presence of too many zombie processes can sometimes be a problem.

You can imagine it like this:

"You are the owner of a construction company. You pay workers every day based on their workload. There is a worker who comes to the construction site every day and just sits there. You don't have to pay him, and he doesn't do any work. He Just come every day and sit there, that’s all!”

This worker is a living example of a zombie process. But, if you have a lot of zombie workers, your construction site will be very congested, making it difficult for normal workers to work.

So how to find the zombie process?

Open the terminal and enter the following command:
ps aux | grep Z

The details of all zombie processes in the process table will be listed.

How to kill zombie processes?

Under normal circumstances we can use SIGKILL
signal to kill the process, but the zombie process is already dead, and you cannot kill something that is already dead. So the command you need to enter should be
kill -s SIGCHLD pid

Replace the pid here with the process id of the parent process, so that the parent process will delete all completed and dead child processes.

You can think of it as:

"You find a body in the middle of the road, so you contact the family of the deceased, who will then take the body away from the road."

However, many programs are not so well written and cannot delete these child zombies (otherwise you will not see these zombies in the first place). Therefore the only way to ensure that child zombies are deleted is to kill their parent process.

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