Research and Advances

Interference in multiprocessor computer systems with interleaved memory

This paper analyzes the memory interference caused by several processors simultaneously using several memory modules. Exact results are computed for a simple model of such a system. The limiting value is derived for the relative degree of memory interference as the system size increases. The model of the limiting behavior of the system yields approximate results for the simple model and also suggests that the results are valid for a much larger class of models, including those more nearly like real systems than the simple model. The assumptions and results of the simple model are tested against some measurements of program behavior and simulations of systems using memory references from real programs. The model results provide a good indication of the performance that should be expected from real systems of this type.


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Research and Advances

Trace-driven modeling and analysis of CPU scheduling in a multiprogramming system

Microscopic level job stream data obtained in a production environment by an event-driven software probe is used to drive a model of a multiprogramming computer system. The CPU scheduling algorithm of the model is systematically varied. This technique, called trace-driven modeling, provides an accurate replica of a production environment for the testing of variations in the system. At the same time alterations in scheduling methods can be easily carried out in a controlled way with cause and effects relationships being isolated. The scheduling methods tested included the best possible and worst possible methods, the traditional methods of multiprogramming theory, round-robin, first-come-first-served, etc., and dynamic predictors. The relative and absolute performances of these scheduling methods are given. It is concluded that a successful CPU scheduling method must be preemptive and must prevent a given job from holding the CPU for too long a period.

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