Fri May 29 06:56:33 UTC 2020 test phlog post 1 -- Network contention frequently dominates the run time of parallel algorithms and limits scaling performance. Most previous studies mitigate or eliminate contention by utilizing one of several ap- proaches: communication-minimizing algorithms; hotspot-avoiding routing schemes; topology-aware task mapping; or improving global network properties, such as bisection bandwidth, edge-expansion, partitioning, and network diameter. In practice, parallel jobs often use only a fraction of a host system. How do processor al- location policies affect contention within a partition? We uti- lize edge-isoperimetric analysis of network graphs to determine whether a network partition has optimal internal bisection. In- creasing the bisection allows a more efficient use of the network resources, decreasing or completely eliminating the link con- tention. We first study torus networks and characterize partition geometries that maximize internal bisection bandwidth. We examine the allocation policies of Mira and JUQUEEN, the two largest pub- licly-accessible Blue Gene/Q torus-based supercomputers. Our analysis demonstrates that the bisection bandwidth of their cur- rent partitions can often be improved by changing the partitions' geometries. These can yield up to a X2 speedup for contention- bound workloads. Benchmarking experiments validate the predic- tions. Our analysis applies to allocation policies of other net- works.