forkbench will fork off many child processes (settable on the command
line with -c, default is 1000).

Each child will write a single character into a pipe, and the parent
will read that single character out of the pipe.  Then the child will
block and wait for SIGTERM.  The parent will take the time for creating
the child and receiving the character.

Ideally, this time should be more or less constant.
