if( hostgroup == NULL ) {
hostgroup = xbt_dynar_new(sizeof(char*), NULL);
xbt_dict_set(parent2hostgroup, parent->name, hostgroup, NULL);
if( hostgroup == NULL ) {
hostgroup = xbt_dynar_new(sizeof(char*), NULL);
xbt_dict_set(parent2hostgroup, parent->name, hostgroup, NULL);