s4u::Host::on_state_change_cb(host_state_change);
s4u::Exec::on_start_cb(add_active_exec);
s4u::Activity::on_completion_cb(remove_active_exec);
s4u::Host::on_state_change_cb(host_state_change);
s4u::Exec::on_start_cb(add_active_exec);
s4u::Activity::on_completion_cb(remove_active_exec);