Utilization and Predictability in Scheduling the IBM SP2 with Back lling

Scheduling jobs on the IBM SP2 system is usually done by giving each job a partition of the machine for its exclusive use. Allocating such partitions in the order that the jobs arrive (FCFS scheduling) is fair and predictable, but su ers from severe fragmentation, leading to low utilization. This motivated Argonne National Lab, where the rst large SP1 was installed, to develop the EASY scheduler. This scheduler, which has since been adopted by many other SP2 sites, uses aggressive back lling: small jobs are moved ahead to ll in holes in the schedule, provided they do not delay the rst job in the queue. We show that a more conservative approach, in which small jobs move ahead only if they do not delay any job in the queue, produces essentially the same bene ts in terms of utilization. Our conservative scheme has the added advantage that queueing times can be predicted in advance, whereas in EASY the queueing time is