Programming parallel machines is very difficult. First, generating an algorithm requires the programmer to assimilate the interactions of multiple threads of control. Second, synchronization and communication among the threads must be addressed to avoid contention and deadlock. Then, once the program is executing on the parallel system and does not function correctly or performs poorly, the debugging of multiple threads is a complicated problem . Additionally, debugging software is an activity that requires systematic attention to detail. Success is a function of the experienced individual involved and the tools employed. The ability to efficiently debug software requires the wisdom to know what questions to ask, the ability to analyze the answers received, and the knowledge to formulate the best next question. To aid in this interactive process, the programmer needs information about the run-time behavior of the program. © 1991, ACM. All rights reserved.