PORTS: Experiences with a Scheduler for Dynamic Real-Time Systems

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Ghosh, Kaushik
Fujimoto, Richard M.
Schwan, Karsten
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This paper describes several of our experiences with a real-time scheduler. Using a robot control application program, we motivate the importance of supporting multiple schedulers within the same application program. We demonstrate the utility of speculative task execution in dynamic real-time systems, and describe the implementation of a scheduler for performing speculative execution and recovery. We show that existing real-time scheduler interfaces have scope for improvement, especially when scheduling latency must be low and when multiple schedulers used by a single application must co-exist on a single processor. A new scheduler interface is specified and its basic costs are evaluated experimentally. Preliminary measurements on a KSR-1 machine are quoted. The measurements demonstrate how the execution times of temporal queries may be reduced by use of access structures to scheduler data structures. Finally, there are several overheads associated with speculative execution, and multiple schedulers in a single application. We consider the problem of on-line reconfiguration of the several overheads associated with the speculative-execution paradigm for optimal performance in the face of these overheads. Initial performance measurements of the PORTS scheduler indicate that it is possible to perform real-time scheduling with latencies approximating those of proposed specialized scheduling co-processors.
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