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Full Discussion: zombie
Operating Systems Linux Red Hat zombie Post 302541440 by agama on Sunday 24th of July 2011 03:33:37 PM
Old 07-24-2011
Zombie processes, sometimes called defunct processes, are processes that have completed, but their parent process has not yet received status of their termination. It's not unusual to see a few zombies for a few seconds if the parent process is busy and has not issued a wait() system call to collect the status of a child/children. However, if the zombies persist (their PIDs are always the same), this is an indication that the parent application is either poorly coded, or wedged (looping). Given that your CPU usage is low, I'm guessing the parent(s) aren't wedged.

Zombie processes cannot be killed, as you've likely found out. The good news is that the only system resources they are taking is the slot in the process table; all other real resources (memory, sockets, open files, etc.) were closed/released when the process ended. The zombies only become a problem when their numbers start to "clog" the process table which might have a finite size.

If you have the option to stop and restart the parent, then your zombie processes will be cleaned up with the parent. If the parent is some service that you must keep running in order to prevent down-time, then you're stuck.

If you want to see which process(es) own the zombies, capture the output of a ps -elf (or ps -ajx on FreeBSD) and look for the zombies. For each zombie the parent process id (PPID) should be listed (column 3 usually) and that can be used to find it's parent (look for the process with the PPID you found listed as the PID -- usually column 2).

This illustrates the output from the ps command showing the process a.out having a defunct/zombie child process:

Code:
0 S scooter  31578  4537  0  84   4 -   406 -      15:29 pts/3    00:00:00 a.out
1 Z scooter  31579 31578  0  84   4 -     0 exit   15:29 pts/3    00:00:00 [a.out] <defunct>

 

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preap(1)							   User Commands							  preap(1)

NAME
preap - force a defunct process to be reaped by its parent SYNOPSIS
preap [-F] pid... DESCRIPTION
A defunct (or zombie) process is one whose exit status has yet to be reaped by its parent. The exit status is reaped via the wait(3C), waitid(2), or waitpid(3C) system call. In the normal course of system operation, zombies may occur, but are typically short-lived. This may happen if a parent exits without having reaped the exit status of some or all of its children. In that case, those children are reparented to PID 1. See init(1M), which periodically reaps such processes. An irresponsible parent process may not exit for a very long time and thus leave zombies on the system. Since the operating system destroys nearly all components of a process before it becomes defunct, such defunct processes do not normally impact system operation. However, they do consume a small amount of system memory. preap forces the parent of the process specified by pid to waitid(3C) for pid, if pid represents a defunct process. preap will attempt to prevent the administrator from unwisely reaping a child process which might soon be reaped by the parent, if: o The process is a child of init(1M). o The parent process is stopped and might wait on the child when it is again allowed to run. o The process has been defunct for less than one minute. OPTIONS
The following option is supported: -F Forces the parent to reap the child, overriding safety checks. OPERANDS
The following operand is supported: pid Process ID list. EXIT STATUS
The following exit values are returned by preap, which prints the exit status of each target process reaped: 0 Successfully operation. non-zero Failure, such as no such process, permission denied, or invalid option. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Availability |SUNWesu (32-bit) | +-----------------------------+-----------------------------+ | |SUNWesxu (64-bit) | +-----------------------------+-----------------------------+ SEE ALSO
proc(1), init(1M), waitid(2), wait(3C), waitpid(3C), proc(4), attributes(5) WARNINGS
preap should be applied sparingly and only in situations in which the administrator or developer has confirmed that defunct processes will not be reaped by the parent process. Otherwise, applying preap may damage the parent process in unpredictable ways. SunOS 5.10 26 Mar 2001 preap(1)
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