throttle -- Throttles by the number of child processes.
throttle waits that the number of the child processes is less than a limit. Firstly, throttle waits for 0.1 second, and the n-th retry (n < 10) waits for n * 0.1 seconds. The n-th retry (n >= 10) waits for 1 second.
void sub::throttle(int limit)