sub_1 can run immediately
sub_2 can run immediately
sub_3 can run only after sub_1 finishes
sub_4 can run only after sub_1 finishes
sub_5 can run only after sub_2 finishes
sub_6 can run only after sub_2 finishes
sub_6 can run only after both sub_1 and sub_2 finish
sub_7 can run only after both sub_1 and sub_2 finish