The maximal utilization of processor co-allocation in multicluster systems

In systems consisting of multiple clusters of processors which employ space sharing for scheduling jobs, such as our distributed ASCI supercomputer (DAS), co-allocation, i.e., the simultaneous allocation of processors to single jobs in multiple clusters, may be required. In studies of scheduling in single clusters it has been shown that the achievable (maximal) utilization may be much less than 100%, a problem that may be aggravated in multicluster systems. In this paper we study the maximal utilization when co-allocating jobs in multicluster systems, both with analytic means (we derive exact and approximate formulas when the service-time distribution is exponential), and with simulations with synthetic workloads and with workloads derived from the logs of actual systems.