Wednesday, October 12, 2011

What is a Zombie Process ?

It’s a pretty much common question asked in most of the interviews related to Linux, and most of the time people got it confused with Orphan Process. But these two are totally different from each other. A Zombie Process is nearly the same thing which we see in lot of horror movies. Like the dead people which don’t have any life force left, Zombie processes are the dead processes sitting in the process table and doing nothing.
To explain this in much better way, “Zombie process or defunct process is a process that have completed the execution, have released all the resources (CPU, memory) but still had an entry in the process table.”

Reasons of Zombie Process:

Most of the time, the reason for existence of Zombie process is bad coding. Normally, when a child (subprocess) finishes it’s task and exits, then it’s parent is suppose to use the “wait” system call and get the status of the process. So, until the parent process don’t check for the child’s exit status, the process is a Zombie process, but it usually is very small duration. But if due to any reason (bad programming or a bug), the parent process didn’t check the status of the child and don’t call “wait”, the child process stays in the state of Zombie waiting for parent to check it’s status.

Finding Zombie processes:

To check whether you have any Zombie processes in your system, simply run linux command line utility “top”. It shows the number of zombie processes at the upper-right side of its output.
At the same time, you can use one more command to check that
# ps aux
All the processes which are having “z” in their Stat column are Zombie processes.

Killing a Zombie Process:

Well, before taking any decision of killing the Zombie process, you should wait, as it is possible that the parent process is intentionally leaving the process in a zombie state to ensure that future children that it may create will not receive the same pid. Or perhaps the parent is occupied, and will reap the child process momentarily.
If that didn’t happen then you can send a SIGCHLD signal to the parent process of zombie which will instruct parents to reap their zombie children.
# kill -s SIGCHLD 
Even if this don’t work, then the last option you will have is to kill the parent process. You can easily find out the parent’s process ID with this command:
# ps aux -eo ppid | grep 
# kill -9 
So when a Zombie process loses it’s parent process, it becomes orphan and adopted by “init”. Init periodically executes the wait system call to reap any zombies with init as parent.


QWhy I can’t kill a Zombie process with “kill” command ?
A. Zombie process is already dead, so killing them with “kill -9″ won’t help at all.
QIs it bad to have Zombie processes on your system ?
A. Well, as Zombie processes are not taking any resources of your system, leaving a small entry in process table, it’s not at all harmful to have Zombie processes in your system, but it may hurt you sometime under heavy load. So, it’s always better not to have them.
QI am seeing a lots of Zombie processes around in my system, what could be the reason ?
A. If this is the case then you are using some code/software in your system which is having a lot of bugs or not properly written. Look for something like that and fix it.
QIs Zombie process different from an Orphan process ?
A. Yes, Zombie is something which is already dead, but Orphan processes are those whose parents are dead.