Max PID in 20.04 increased to 4194304

[Walt Mankowski]

On Sun, Apr 26, 2020 at 11:22:04AM +0100, Colin Watson wrote:
> On Sat, Apr 25, 2020 at 09:29:45PM -0400, Walt Mankowski wrote:
> > I upgraded from 19.10 to 20.04 earlier today. Later on I ran ps and
> > was surprised when I saw some very large process IDs. It looks like
> > the default maximum PID (set in /proc/sys/kernel/pid_max) has
> > increased from 32767 to 4194304.
> > 
> > I don't see this new behavior mentioned in the release notes at
> > https://wiki.ubuntu.com/FocalFossa/ReleaseNotes. Was it intentional?
> 
> Yes - it's in the NEWS file for systemd 243 (there are far too many
> changes in a two-year LTS release cycle to be able to reproduce all of
> them in the overall release notes):
> 
>         * On 64 bit systems, the "kernel.pid_max" sysctl is now bumped to
>           4194304 by default, i.e. the full 22bit range the kernel allows, up
>           from the old 16bit range. This should improve security and
>           robustness, as PID collisions are made less likely (though certainly
>           still possible). There are rumours this might create compatibility
>           problems, though at this moment no practical ones are known to
>           us. Downstream distributions are hence advised to undo this change in
>           their builds if they are concerned about maximum compatibility, but
>           for everybody else we recommend leaving the value bumped. Besides
>           improving security and robustness this should also simplify things as
>           the maximum number of allowed concurrent tasks was previously bounded
>           by both "kernel.pid_max" and "kernel.threads-max" and now effectively
>           only a single knob is left ("kernel.threads-max"). There have been
>           concerns that usability is affected by this change because larger PID
>           numbers are harder to type, but we believe the change from 5 digits
>           to 7 digits doesn't hamper usability.
> 
> Seems like a basically good idea to me.

Ah, so it was systemd! Totally agree it seems like a good idea.

Thanks.

Walt