What price virtual memory
Maria C. Davis, George E. Richmond · Annual Simulation Symposium · 1974
Virtual memory offered a long term solution to a company requirement for a large memory system, but at what price in other computer resources? This study sought to predict the effect on compute load of moving from an IBM System 370 Model 165 with OS/MVT to a Model 165-2 or Model 168 with VS2. The compute load on the two virtual memory systems will increase in different ways relative to a Model 165 as a function of both the input/output activity and program locality.A GPSS simulation was performed for the 165, 165-2 and 168 central processor interaction with memory. The data flow between the instruction decode unit, execution unit, high-speed buffer and storage was modeled and parameterized, resulting in predicted bounds on the performance of a major time-critical application on a 165-2 and 168. Data collected later by a hardware monitor during 168 benchmarks verified the model.For the company's large, varying batch load, software monitor data, performance data for the new operating system and characterizations of the job stream based on hardware monitoring were combined to project the compute price of moving to the virtual systems.Thus with a combination of simulation, benchmarks, and hardware and software monitors, management was offered valuable technical inputs on the predicted effects of changes in computer model and operating system.