Mukherjee, Shubhendu S.Sharma, Shamik D.Hill, Mark D.Larus, James R.Rogers, AnneSaltz, JoelIrregular computation problems underlie many important scientific applications. Although these problems are computationally expensive, and so would seem appropriate for parallel machines, their irregular and unpredictable run-time behavior makes this type of parallel program difficult to write and adversely affects run-time performance. This paper explores three issues---partitioning, mutual exclusion, and data transfer---crucial to the efficient execution of irregular problems on distributed-memory machines. Unlike previous work, we studied the same programs running in three alternative systems on the same hardware base (a Thinking Machines CM-5): the CHAOS irregular application library, Transparent Shared Memory (TSM), and eXtensible Shared Memory (XSM). CHAOS and XSM performed equivalently for all three applications. Both systems were somewhat (13%) to significantly faster (991%) than TSM. (Also cross-referenced as UMIACS-TR-95-46)en-USTechnical Report