Unallocated Memory Space in COMA Multiprocessors

Cache only memory architecture (COMA) for distributed shared memory multiprocessors attempts to provide high utilization of local memory by organizing the local memory as a large cache, called attraction memory (AM), without traditional main memory. To facilitate caching of replicated data, it is desirable to have some of the physical storage space in the AMs left unallocated, i.e. not utilized as a part of the physical address space. Without the unallocated space, excessive relocation and migration of memory blocks between the AMs can happen due to replacement, negating the very purpose of the AM. It is important in a COMA machine that the operating system maintains a certain amount of unallocated memory space to provide good performance. In this paper, we identify an important relation between the amount of unallocated space and the set associativity of the AM, and discuss the trade-off between additional unallocated memory space and higher set associativity.