1. A grouping method for threads, each thread comprising basic modules and data, the thread being assigned to processors in a program for a multiprocessor system, the program comprising the basic modules and a parallel statement describing relationships between parallel threads for the basic modules, the grouping method executed in the multiprocessor system comprising: displaying a dataflow graph visually showing a parallel relationship between the data and the basic modules based on the parallel statement when the program is executed; selecting a basic module group for grouping a basic module indicated by a node of the dataflow graph and a priority for grouping in accordance with a user input; changing graph data structure generation information for displaying the dataflow graph in accordance with a selection of the basic module group; and making the basic module group to be reflected in the parallel statement in accordance with the changed graph data structure generation information, wherein the dataflow graph comprises data entries, nodes in the basic modules, and edges configured to connect the data entries and the nodes; wherein at least said displaying is implemented by computer hardware.