Tradeoffs Between Coupling Small and Large Processors for Transaction Processing

Abstract

The prospect of coupling a large number of small inexpensive microprocessor-based systems to deliver the performance of a large transaction processing system at lower cost has not been realized to date. A methodology is developed to determine the number of processors needed to satisfy transaction throughput and response time requirements for processors of different MIPS (sizes). The minimum MIPS per processor required to satisfy response time and throughput constraints in a transaction processing complex of N coupled systems is also determined, by using an approximate analytical model driven by measured workload parameters. For realistic overhead assumptions, despite large assumed cost advantages on a per MIPS basis, we find that very small systems may not match up to the cost/ performance of some larger systems, when required to meet the same throughput and response time constraints. If transactions running on smaller systems were allowed a larger response time constraint, then we may be able to construct a lower cost system from smaller and less expensive processors, generally with lower supportable maximum throughput. Besides multisystem's coupling degradation, there is a small systems effect. Because of the increased transaction execution time in smaller systems, transactions hold on to resources longer, thereby causing increased intersystem interference manifested in our models as increased resource contention. Our cost criterion indicates that there is an optimum processor size below which total system costs would increase appreciably. Ways to reduce the intersystem interference and coupling protocol overheads are investigated and shown to decrease this optimum processor size. Sensitivity to transaction size, response time constraint, and resource contention have been carried out. © 1988 IEEE