Max PID in 20.04 increased to 4194304

[Colin Watson]

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.

-- 
Colin Watson                                       [cjwatson at ubuntu.com]