Multicast Pull Scheduling: When Fairness Is Fine

We investigate server scheduling policies to minimize average user perceived latency in pull‐based client‐server systems (systems where multiple clients request data from a server) where the server answers requests on a multicast/ broadcast channel. We first show that there is no O(1) ‐competitive algorithm for this problem. We then give a method to convert any nonclairvoyant unicast scheduling algorithm A to nonclairvoyant multicast scheduling algorithm B . We show that if A works well, when jobs can have parallel and sequential phases, then B works well if it is given twice the resources. More formally, if A is an s ‐speed c ‐approximation unicast algorithm, then its counterpart algorithm B is a 2s ‐speed c ‐approximation multicast algorithm. It is already known [5] that Equi‐partition, which devotes an equal amount of processing power to each job, is a (2 + ϵ) ‐speed O(1 + 1/ϵ) ‐approximation algorithm for unicast scheduling of such jobs. Hence, it follows that the algorithm {BEQUI}, which broadcasts all requested files at a rate proportional to the number of outstanding requests for that file, is a (4 + ϵ) ‐speed O(1 + 1/ϵ) ‐approximation algorithm. We give another algorithm BEQUI‐EDF and show that BEQUI‐EDF is also a (4 + ϵ) ‐speed O(1 + 1/ϵ) ‐approximation algorithm. However, BEQUI‐EDF has the advantage that the maximum number of preemptions is linear in the number of requests, and the advantage that no preemptions occur if the data items have unit size.