Function start 

pub fn start() -> bool

Starts the background thread, which ends a decay epoch every 5 seconds in all arenas, so that pages free for 5 to 10 seconds go back to the OS even while the process allocates nothing. Returns whether this call started it; it runs until the process exits.

A child process after fork has no such thread and may call this again.

Panics

When the OS cannot create a thread.