Every new process and thread on Linux gets a new PID (short for process ID). PIDs are normally assigned sequentially until they hit some maximum value and roll over. The traditional maximum PID value on Unixes has been some number related to a 16-bit integer, either signed or unsigned, and Linux is no exception; the kernel default is generally still 32768 (which is 2^15 exactly, and so not quite authentic to a signed 16-bit int).

(You can find the current limit in /proc/sys/kernel/pid_max, but it may have been increased through sysctls.)

A few years ago I discovered that a Fedora package had raised this limit on me, which I was able to see because it turned out that my Fedora machines routinely go through a lot of PIDs. I reverted this by removing the package for various reasons, including that I don't really like gigantic process IDs (they bulk up the output of ps, top, and other similar tools). Then recently I updated to Fedora 31, and not too long afterward noticed that I was getting giant process IDs again (as I write this a new shell on one machine gets PID 4,085,915).

This turns out to be a deliberate choice in modern versions of systemd, instead of another stray package deciding it knows best. In Fedora 31 (with systemd 243), /usr/lib/sysctl.d/50-pid-max.conf says:

# Bump the numeric PID range to its maximum of 2^22
# (from the in-kernel default of 2^16), to make PID
# collisions less likely.
kernel.pid_max = 4194304

(Since the PID that new processes get is so close to the maximum, I suspect that I have actually rolled over even this large range a couple of times in the 21 days that this machine has been up since the last time I got around to a kernel update.)

Given that this is a new official systemd thing, I'm going to let it be and live with gigantic PIDs. It's not really worth fighting systemd; it generally doesn't end well for me.

(Hopefully there aren't any programs on the system that assume PIDs are small and always fit into five-character fields in ASCII. Or at least no programs that will fail when this assumption is incorrect, as opposed to producing ugly output.)