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Communications of the ACM

Communications of the ACM

A policy-driven scheduler for a time-sharing system

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The services received by a process from a time-sharing operating system can be characterized by a resource count ∑ wiRij where Rij is the number of units of service received by process j from resource i and wi is the cost per unit of the service. Each class of users can be characterized by a policy function which specifies the amount of service a user who belongs to this class should receive as a function of time. Priority changes dynamically as a function of the difference between the service promised to the user by the policy function and the service he actually receives. A scheduling and swapping algorithm which keeps the resource count of each process above its policy function will provide the specified level of service. Overhead can be reduced by avoiding swaps of processes which have received at least this level of service. The algorithm has been implemented in a general purpose operating system, and it has provided significantly better service to interactive and to batch jobs than the previous scheduler.

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