Non-clairvoyant multiprocessor scheduling of jobs with changing execution characteristics (extended abstract)

A multiprocessor system is unlikely to have access to information about the execution characteristics of the jobs it is to schedule. In this work, we are interested in scheduling algorithms for batch jobs that require no such knowledge (such algorithms are called nonclairvoyant). Preemptive scheduling (i.e., redistribution of processors) is important to reduce mean response time in multiprocessor systems, especially in the widely available network of workstations. Preemption is a method to adapt to the uncertain and changing nature of jobs and workloads. Unfortunately, preemption may incur large overheads if it is applied frequently. To account for the cost preemptions, we consider a number of simple scheduling algorithms classified by the number of preemptions they rue allowed, ranging from none to an iniinite number. The Equi-partition algorithm [18], which partitions the processors evenly between the uncompleted jobs, is an example of a simple scheduler that is non-clairvoyant and preempts only when jobs complete. Motwani et al. [15] show that the mean response time of jobs is within two of optimal for fully parallelizable jobs. Since parallel programs can have a wide variety of execution characteristics in practice, we consider a number of classifications of jobs accordhg to how well they are able to utilize processors. Moreover a job may have both sequential and parallel phases in its computation. Hence, we allow jobs to have multiple phases, each of which may have different execution characteristics. *A full version of this paper can be found at http: //www.cs. yorku.ca/ jeff. t { jeff, d~i, brecht, d.ng}@cs . yorku. IX. Edmonds, Brecht, and Deng are supported by NSERC Canada. Chinn was supported in part by NSERC as a Postdoctoral Fellow at York University. Chinn’s current address: One Microsoft Way, Redmond, WA 9S052; dchirmtiicrom.ft. corn,Deng can also be reached at: Department of Computer Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong. ~emlission10make digilalflmrdcopies 0(011 or parl o~lhis nlnleri~i I& personal or classroom me is Wnled WilhOot Itc provided thal lhe copies ,arenot made or dislribil ted for prolil or Commcl-ci,lt.ndv.nnt:ig c,. the L.op\,right notice, the title 01 (he p~lblicaliotl md ifs dale nppmr. md nolice is given thflt copyT@ is by pwmission ol’the A~hl. Inc. TO copy o[lmwise. tO republish. to post w swvers or to redis(rihtlte to Iisis. rc(ltl ires specitic pemlission and/orfix sir(xz ‘ 97 El Paso,Texls IISA Copyrighl 1997 ACM 0-89791 .fW8-6/97/fJ5 .,$3,.5(J For each of these preemptive models and for each of these job classifications, we provide asymptotically tight bounds on the mean response time of non-clairvoyant scheduling algorithms. For example, we show that in the worst case, the mean job response time obtained with Equi-partition is 2 + ~ z 3.74 times that obtained with an optimal algorithm (which may preempt processors any number of times and may use job characteristics to make its scheduling decisions) for a large class of jobs, characterized by multiple phases of arbitrary nondecreaaing and sublinear speedup functions.