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I changed the value stored in /proc/sys/kernel/pid_max. Do I need to reboot for this new value to take effect?

Shlublu
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Vikas Bansal
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    How did you change it? – Jaroslav Kucera Sep 27 '17 at 14:00
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    @JaroslavKucera echo 999 >/proc/sys/kernel/pid_max. You can increase or decrease as you wish. A change will affect only newly created processes. You can make the change permanent by inserting a kernel.pid_max = 999 into your /etc/sysctl.conf. Short pids are very handy, long pids look very professional (but they are not). The kernel wired-in max is 32-bit pids, so rougly 4billion. The default is 32766. – peterh Sep 27 '17 at 15:19
  • @peterh Thank you, but I wanted to know how Vikas really did it, because modification of /proc/sys/kernel/pid_max isn't permanent. So without modification of sysctl.conf or file in sysctl.d it would be just temporary change. – Jaroslav Kucera Sep 27 '17 at 18:34

1 Answers1

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Should you have increased /proc/sys/kernel/pid_max (by doing cat 100000 > /proc/sys/kernel/pid_max for example), this value is effective right away, there is no need to reboot. I never tried to decrease it though(*).

You can test this the following way (that's somewhat ugly, not to be done on a production machine):

i=0 ; while [ $i -lt 10000] ; do (echo $i > /dev/null); ((i++)); done; ps ax | grep AnythingUnlikelyToBeUsedAlready

(echo $i > /dev/null) is just there to create a new pid at each iteration. 10000 was convenient in my test case but you may adjust it. Here, it loops at run #3 as I previously set my pid_max as described above:

shlublu:~$ i=0 ; while [ $i -lt 10000 ] ; do (echo $i > /dev/null); ((i++)); done; ps ax | grep AnuthingUnlikelyToBeUsedAlready
86880 pts/0    S+     0:00 grep --color=auto AnuthingUnlikelyToBeUsedAlready

shlublu:~$ i=0 ; while [ $i -lt 10000 ] ; do (echo $i > /dev/null); ((i++)); done; ps ax | grep AnuthingUnlikelyToBeUsedAlready
96882 pts/0    S+     0:00 grep --color=auto AnuthingUnlikelyToBeUsedAlready

shlublu:~$ i=0 ; while [ $i -lt 10000 ] ; do (echo $i > /dev/null); ((i++)); done; ps ax | grep AnuthingUnlikelyToBeUsedAlready
 7246 pts/0    S+     0:00 grep --color=auto AnuthingUnlikelyToBeUsedAlready

shlublu:~$ i=0 ; while [ $i -lt 10000 ] ; do (echo $i > /dev/null); ((i++)); done; ps ax | grep AnuthingUnlikelyToBeUsedAlready
17260 pts/0    S+     0:00 grep --color=auto AnuthingUnlikelyToBeUsedAlready

shlublu:~$ i=0 ; while [ $i -lt 10000 ] ; do (echo $i > /dev/null); ((i++)); done; ps ax | grep AnuthingUnlikelyToBeUsedAlready
27262 pts/0    S+     0:00 grep --color=auto AnuthingUnlikelyToBeUsedAlready

But should you restart after having done this you will see that /proc/sys/kernel/pid_max will have been restored to its default value (32768 usually).

To make your setting persistant across restarts you have to edit /etc/sysctl.conf and set kernel.pid_max accordingly.

For example:

kernel.pid_max = 100000

Caveat: pid_max has boundaries that depend on your system. The value you define should be within these limits.

(*) but @peterh did and it works obviously, see comments.

Shlublu
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  • I tried to decrease, works like charm. From now, my PIDs will be reduced to 3 digits :-) Tyvm! – peterh Sep 27 '17 at 15:18
  • @peterh Yolo! :) – Shlublu Sep 27 '17 at 15:20
  • Btw, afaik the 32-bit pid support is an option in the kernels roughly 15 years ago, thus it is probably default in most distro kernels roughly a decade ago. Probably even 64-bit support pids wouldn't require too tremendous patches, but nobody needs them. – peterh Sep 27 '17 at 15:23
  • @peterh This is probably why the default is still 32768. I suppose that extending that value due to an actual need only happens with very very very busy machines. – Shlublu Sep 27 '17 at 15:25
  • @MarkPlotnick That's right, I should have said 'expect' instead. I'll edit that right away. – Shlublu Sep 27 '17 at 15:44