sub _default_thread {
   my $thr_id = shift;
   if (defined $s_coderef[$thr_id] && ref $s_coderef[$thr_id] eq 'CODE') {
      $s_coderef[$thr_id]->(@{ $s_param[$thr_id] });
      $s_coderef[$thr_id] = undef;
   }
   else { sleep(1) }
}