sub main_loop {
my $parser_thread;
  while(
    schedule_count()
    and $hit_count < $hit_limit       #could be commented out
    and time() < $expiration
    and ! $QUIT_NOW
    and scalar threads->list(threads::running) <= 25
  ) {
    #yield();
    $parser_thread = async { process_url( next_scheduled_url() );};
  }
  $parser_thread->join;
  return;
}