464 EVOLUTION OF COMPUTER BUILDING BLOCKS
system degrades as its size increases. If systems were, instead, configured using microprocessors, and if there was no additional cost in interconnecting the microprocessors, then the points would fall along an ideal multiprocessors line such as shown in the figure. In reality, both costs associated with the physical interconnect and performance degradation due to synchronization overhead will cause the price/performance curve to have a negative slope (the realistic multiprocessors line in the figure). In terms of Figure 1, the critical question facing multiprocessors is whether the realistic multiprocessors price/performance line falls above or below the line for conventional uniprocessor systems.
Figure 1. Cost performance as a function of system cost.
Another important attribute of a multiple-processor computer system is its potential for reliability. Computers are being applied increasingly in situations where a failure might have serious economic and even life-endangering consequences. Since the basic ingredient in the design of a reliable system using real components is redundancy in one form or another, a structure consisting of large numbers of identical processors represents the natural framework in which to design reliable computers. Prior to the advent of the microprocessor, it was unrealistic to consider multiprocessor structures involving more than a few processors because the cost of building the individual processors themselves was high.
Yet another factor that favors the use of multiple processors is the resulting modularity of the system. There has always been a motivation for making computer systems modular for reasons of incremental expandability, ease of maintenance, and enhanced production. A computer system that is built using identical processors, and a small set of interconnection elements that have clean, well-defined interfaces would benefit fully from a modularity in processing power that is currently seen only in memory units of computer systems.
In spite of the advantages offered by multiprocessor organizations, there have been few commercially viable systems constructed to date.* The reason for this is that a number of problems and open issues remain to be resolved before such systems are a practical alternative to more conventional organizations. The major problems currently facing such systems are as follows.
* While the authors know of no commercially available multi-microprocessor systems, Pluribus [Heart et al., 1973] and Tandem  are two multiple-processor systems based on a processor of minicomputer size that are commercially available.