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onik
Asked: 2012-04-28 01:21:04 +0800 CST

Too many PHP zombie processes

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I restarted my apache daemon today to reload the config file, but after this i began to see many php zombie processes on the system. The amount varies from 10 to 30 and they all take a little slice of CPU while they die. Where should I begin to debug this problem?

The modification I made was reducing Max Requests Per Child from 0 (massive memory leaks) to 1000. I think that the php processes are from a script that receives data from "dumb" devices, ie. they send a request with GET parameters and don't care about the result.

Some data:

uname -a

# uname -a
Linux <hostname> 2.6.32-71.29.1.el6.x86_64 #1 SMP Mon Jun 27 19:49:27 BST 2011 x86_64 x86_64 x86_64 GNU/Linux

ps -aux | grep php

# ps aux | grep php
user1     5709  1.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5717  1.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5721  1.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5722  2.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5723  2.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5724  2.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5725  2.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5729  2.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5731  2.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5737  2.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5760  2.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5778  1.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5793  2.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5798  1.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5800  1.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5833  2.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5850  2.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5870  3.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5875  2.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5876  2.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5877  2.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5886  0.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5926  0.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5939  0.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5941  0.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5961  0.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5962  0.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5973  0.0  0.0      0     0 ?        Z    12:15   0:00 [php] <defunct>
user1     5977  0.0  0.2 106836  8680 ?        R    12:15   0:00 /usr/bin/php /home/user1/public_html/<script>.php
root      5981  0.0  0.0 103228   836 pts/0    S+   12:15   0:00 grep php

free

# free -m
             total       used       free     shared    buffers     cached
Mem:          3831       3173        658          0        183       2502
-/+ buffers/cache:        487       3344
Swap:         4031          7       4024

uptime

# uptime
 12:18:10 up 105 days, 23:21,  1 user,  load average: 0.23, 0.20, 0.18

Anything else needed to help me debug this?

php apache-2.2 centos6

3 Answers

  1. Best Answer

    It's not a problem. Zombies don't take up CPU, or memory, or anything other than process table slots. The zombies aren't hanging around for very long, you've said, so all that is happening is that the Apache master process is doing things other than waiting around for child processes to exit, so it's sometimes taking a little while before the terminated children get reaped.

    • 4

  2. A zombie prozess is just a process which parent already died/was killed, and was not tidied up yet.

    If this happens during a service restart its perfectly normal. You should read up on linux process states, rough info just here:

    1. Running: This is a state where a process is either in running or ready to run.
    2. Interruptible: This state is a blocked state of a process which awaits for an event or a signal from another process
    3. Uninterruptible: It is also a blocked state. The process is forced to halt for certain condition that a hardware status is waited and a signal could not be handled.
    4. Stopped: Once the process is completed, this state occurs. This process can be restarted
    5. Zombie: In this state, the process will be terminated and the information will still be available in the process table.

    If it happens however during regular usage of your webserver (we are talking about php processes after all), you might fix this temporarily with a cronjob every minute doing an apache graceful restart (service apache2 reload), but this does not solve the underlying problem eating away your available slots.

    In the second case you need to discern which hosting is causing the zombies and the application has to be fixed. 99% of such zombies are caused by badly programmed websites.

    • 0

  3. Please add code pcntl_wait() after forking the process with pcntl_fork()

    $pid = pcntl_wait($status, WNOHANG);
int pcntl_wait ( int &$status [, int $options ] )

    The wait function suspends execution of the current process until a child has exited, or until a signal is delivered whose action is to terminate the current process or to call a signal handling function. If a child has already exited by the time of the call (a so-called "zombie" process), the function returns immediately. Any system resources used by the child are freed

    • 0