4.4 Software Design Implications
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4.4 Software Design Implications

The values of the ratios in Table are critically important, as they limit the efficiency of a given parallel application. To see this, suppose that the parallel application uses CAS operations to communicate among threads. These CAS operations will typically involve a cache miss, that is, assuming that the threads are communicating primarily with each other rather than with themselves. Suppose further that the unit of work corresponding to each CAS communication operation takes 300ns, which is sufficient time to compute several floating-point transcendental functions. Then about half of the execution time will be consumed by the CAS communication operations! This in turn means that a two-CPU system running such a parallel program would run no faster than one a sequential implementation running on a single CPU.

The situation is even worse in the distributed-system case, where the latency of a single communications operation might take as long as thousands or even millions of floating-point operations. This illustrates how important it is for communications operations to be extremely infrequent and to enable very large quantities of processing.

Quick Quiz 4.7: Given that distributed-systems communication is so horribly expensive, why does anyone bother with them? End Quick Quiz

The lesson should be quite clear: parallel algorithms must be explicitly designed to run nearly independent threads. The less frequently the threads communicate, whether by atomic operations, locks, or explicit messages, the better the application's performance and scalability will be. In short, achieving excellent parallel performance and scalability means striving for embarrassingly parallel algorithms and implementations, whether by careful choice of data structures and algorithms, use of existing parallel applications and environments, or transforming the problem into one for which an embarrassingly parallel solution exists.

Chapter will discuss design disciplines that promote performance and scalability.