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UET scheduling with unit interprocessor communication delays. (English) Zbl 0634.90031
We consider the problem of scheduling a partially ordered set of unit execution time tasks (UET) on $$m>1$$ processors where there is a communication delay of unit time between any pair of distinct processors. We show that the problem of finding an optimal schedule is NP-hard. A greedy schedule is one where no processor remains idle if there is some task available which it could process. We establish that the length of an arbitrary greedy schedule, $$\omega^ c_ g$$ satisfies $\omega^ c_ g\leq (3-\frac{2}{m})\omega^ c_{opt}-(1-\frac{1}{m})$ where $$\omega^ c_{opt}$$ is the length of the optimal schedule. We define a generalized list schedule (a type of greedy schedule) and discuss anomalous behaviour of such schedules with respect to speed-up. The relevance of these results to the implementation of parallel languages is discussed.

##### MSC:
 90B35 Deterministic scheduling theory in operations research 68Q25 Analysis of algorithms and problem complexity 65K05 Numerical mathematical programming methods
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##### References:
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