SIMD machines

Whereas much of the algorithmic work to be described has been implemented on a range of parallel hardware, including both SIMD machines such as... 

Another common programming model is the data parallel model. In this model the parallelism is extracted from the parallel operations that can be performed on arrays of data. Even though this programming model was first introduced on SIMD machines it is a misconception to believe it to be tied to these types of machines. The data parallel model is directly supported by HPF [29]. The model is clearly of great generality and very elegant, but not all parallel algorithms can easily be expressed in HPF and also compiled into efficient code. However, as compilers continue to improve this situation is sure to improve. 

Several types of parallel computing architectures have been used over the years, including pipelined processors, specialized SIMD machines and general MIMD computers. Most parallel computing is done on hybrid supercomputers that combine features from several basic architectures. The best way to define parallel processing in detail is to explain how a program executes on a typical hybrid parallel computer. 

FIGURE 3 A generic block diagram of a distributed-memory SIMD machine. 

The Thinking Machines Corporation (TMC) CM5 MPP system was announced in November 1991. This processor is a distributed-memory MIMD machine that supports both the SIMD and MIMD programming models. The hardware is composed of a fat tree of compute processors that are used and administered from control processors. The compute processing nodes consist... 

While SIMD computers have been shown to be applicable to a much wider range of applications than was first thought possible they have not been commercially successful. Companies which sell these types of machines have had to leave the arena of general purpose computing and can now only be found in a few special areas. In short, SIMD type machines have not been able to cope with the tougher, post cold-war, commercial market and variations of MIMD parallel computers are the only types left on this market. 

The SIMD category comprises both vector computers, such as the CRAY-1, and so-called processor arrays, such as the Thinking Machines Connection Machine and the classic ILLIAC-IV computer. - SIMD computers ate well suited for applications requiring identical operations on uniform data structures, but SIMD computers cannot necessarily be used efficiently for general-purpose calculations. Presently, MIMD is the most common parallel architecture, and the majority of parallel computers used for quantum chemistry applications are of the MIMD type. Examples of MIMD computers are the... 

---