COMA Machines Can Be Easily Built

this paper, however the Simple COMA design removes a significant amount of complexity from the hardware protocol handler compared to CC-NUMA and COMA machines, regardless of the choices made. For both COMA and CC-NUMA, the hardware protocol handler is responsible for the replacement of a cache-line from the attraction memory or second level cache when contention arises. In the case of CC-NUMA, the presence of a "home" memory location on a particular node guarantees a place where copies of a datum can be retrieved from, and more importantly written back to, when there is no longer space in one of the caches in the machine. For COMA, the situation is more complex; there is no designated home for each particular datum. In this case, although like the CC-NUMA shared copies of data can simply be discarded, careful coordination is required to prevent all the copies of a datum being disposed of at the same instant. Equally imporantly, if a node has the only copy of a datum, care must be taken to safely move that datum to another node in the event of a replacement. Simple COMA does not suffer these problems. If a page is mapped, space is reserved for each coherence unit within it in the attraction memory - even if the coherence unit is invalid. Thus in the event of an eviction from another node being necessary, space is already available for the data. Moreover, in Simple COMA replacements are instigated by software, which means eviction and the replacement of data can occur in a more orderly fashion - simplifying the coherence protocol design. 4 PERFORMANCE CONSIDERATION