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Jagtenberg, C. and Schwiegelshohn, U. and Uetz, M.J. (2010) Lower bounds for Smith's rule in stochastic machine scheduling. Technical Report TR-CTIT-10-30, Centre for Telematics and Information Technology University of Twente, Enschede. ISSN 1381-3625
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We consider the problem to minimize the weighted sum of completion times in nonpreemptive parallel machine scheduling. In a landmark paper from 1986, Kawaguchi and Kyan  showed that scheduling the jobs according to the WSPT rule -also known as Smith's rule- has a performance guarantee of . They also gave an instance to show that this bound is tight. We consider the stochastic variant of this problem in which the processing times are exponentially distributed random variables. We show,somehow counterintuitively, that the performance guarantee of the WSEPT rule, the stochastic analogue of WSPT, is not better than 1.229. This constitutes the first lower bound for WSEPT in this setting, and in particular, it shows that even with exponentially distributed processing times, stochastic scheduling has somewhat nastier worst-case examples than deterministic scheduling. In that respect, our analysis sheds new light on the fundamental differences between deterministic and stochastic scheduling.
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