An experiment with the interchangeability of processes and monitors

Two styles of operating system implementation based on the use of monitors and processes, respectively, are identified, and arguments for a basic equivalence of these systems despite large stylistic differences are presented. The ‘Lauer‐Needham Duality Hypothesis’ states that the two styles are equivalent, both in terms of ease of programming and in efficiency of the resulting systems. A domain for which the first part of this claim holds is outlined, and data affirming the essential equivalence of performance within that domain are presented. An operating system based on the Cambridge CAP system, called SIMOS, was simulated for a wide range of hardware configurations and job loads. SIMOS is written using a module concept that allows an individual module to be interpreted as a monitor in one run and as a process in another. Runs using a monitor to control access to some data can be compared with runs using a process to control access to the same data. The throughput and response time for the two styles of system were found to be identical in most cases. However, a degradation in response time occurred in a process‐based system when the job load and the low level scheduling policy were poorly matched.