Reducing coherence-related overhead in multiprocessor systems

The coherence problem is one of the critical issues designers have to cope with when they apply caching techniques to multiprocessor systems. The copies which most affect consistency are the shared ones, i.e. copies of memory blocks accessed by concurrent processes in a multiprogramming environment; nevertheless, a private data block of a process may become resident in more than one cache-and need to be treated as shared (useless shared copy) with respect to coherence-related operations-as a consequence of the migration of the owner process. These copies reduce the global performance of the system, since they involve a useless (time consuming) transaction on the shared bus on each write operation, to maintain consistency of all remote copies. In the paper, we introduce a hardware solution which can be successfully employed with any snooping protocol to eliminate useless shared copies. Finally, we show how this technique can be applied to a specific coherence protocol, in order to improve global system performance