The JVM probably calls fork() and system(), no? Not allowed. Dynamic thread creation? Not allowed. And 50% of your flops go away unless your program uses the BG

The JVM probably calls fork() and system(), no? Not allowed. Dynamic thread creation? Not allowed. And 50% of your flops go away unless your program uses the BG/Q-specific "double hummer" floating point instructions. These are primitive machines, in terms of development environment and typically require significant rewriting to get even "standard" system software working.