On monitoring Linux servers we may came across the term zombie process many times, Before explaining to you what a Zombie Process is, let me explain to you what the word Zombie means,the dictionary says “a dead body that has been brought back to life by a supernatural force“. And this is what exactly the Zombie Process is all about. When a process finishes execution, it will have an exit status to report to its parent process. Because of this last little bit of information, the process will remain in the operating system’s process table as a zombie process, indicating that it is not to be scheduled for further execution, but that it cannot be completely removed and its process ID cannot be reused until it has been determined that the exit status is no longer needed.
When a child exits, the parent process will receive a SIGCHLD signal to indicate that one of its children has finished executing; the parent process will typically call the wait() system call at this point. That call will provide the parent with the child’s exit status, and will cause the child to be reaped, or removed from the process table.
Zombies can be identified on monitoring linux servers, the output from the Unix ps command by the presence of a “Z” in the “STAT” column. Zombies that exist for more than a short period of time typically indicate a bug in the parent program, or just an uncommon decision to reap children. As with other leaks, the presence of a few zombies is not worrisome in itself, but may indicate a problem that would grow serious under heavier loads. Since there is no memory allocated to zombie processes except for the process table entry itself, the primary concern with many zombies is not running out of memory, but rather running out of process ID numbers.
To remove zombies from a system, the SIGCHLD signal can be sent to the parent manually, using the kill command. If the parent process still refuses to reap the zombie, the next step would be to remove the parent process. When a process loses its parent, init becomes its new parent. Init periodically executes the wait system call to reap any zombies with init as parent.
