Parallel computer models
(b) A hierarchical cluster model
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- The COMA Model
- Figure 1.5 The COMA model of a multiprocessor
- Distributed-Memory Multicomputers
(b) A hierarchical cluster model Figure 1.4 The shared memory is physically distributed to all processors, called local memories. The collection of all local memories forms a global address space accessible by all processors. It is faster to access a local memory with a local processor. The access of remote memory attached to other processors takes longer due to the added delay through the interconnection network. The BBN TC-2000 Butterfly multiprocessor assumes the configuration. Besides distributed memories, globally shared memory can be added to a multiprocessor system. In this case, there are three memory-access patterns: The fastest is local memory access. The next is global memory access. The slowest is access of remote memory. As a matter of fact, the models can be easily modified to allow a mixture of shared memory and private memory with pre specified access rights. A hierarchically structured multiprocessor is modeled. The processors are divided into several clusters. Each cluster is itself an UMA or a NUMA multiprocessor. The clusters are connected to global shared-memory modules. The entire system is considered a NUMA multiprocessor. All processors belonging to the same cluster are allowed to uniformly access the cluster shared-memory modules. All clusters have equal access to the global memory. However, the access time to the cluster memory is shorter than that to the global memory. One can specify the access right among intercluster memories in various ways. The Cedar multiprocessor, built at the University of Illinois, assumes such a structure in which each cluster is an Alliant FX/80 multiprocessor. The COMA Model: A multiprocessor using cache-only memory assumes the COMA model. The COMA model (Figure 1.5) is a special case of a NUMA machine, in which the distributed main memories are converted to caches. There is no memory hierarchy at each processor node. All the caches form a global address space. Remote cache access is assisted by the distributed cache directories. Depending on the interconnection network used, sometimes hierarchical directories may be used to help locate copies of cache blocks. Initial data placement is not critical because data will eventually migrate to where it will be used. 
Figure 1.5 The COMA model of a multiprocessor Besides the UMA, NUMA, and COMA models specified above, other variations exist for mutliprocessors. For example, a cache-coherent non-uniform memory access (CCNUMA) model can be specified with distributed shared memory and cache directories. One can also insist on a cache-coherent COMA machine in which all cache copies must be kept consistent. Distributed-Memory Multicomputers A distributed-memory multicomputer system is modeled in Figure 1.6. The system consists of multiple computers, often called nodes, interconnected by a message-passing network. Each node is an autonomous computer consisting of a processor, local memory, and sometimes attached disks or I/O peripherals. 
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