  This whitepaper describes the load-balancing performance issues that are
observed and tackled during the petascaling of the Vlasiator codes. Vlasiator
is a Vlasov-hybrid simulation code developed in Finnish Meteorological
Institute (FMI). Vlasiator models the communications associated with the
spatial grid operated on as a hypergraph and partitions the grid using the
parallel hypergraph partitioning scheme (PHG) of the Zoltan partitioning
framework. The result of partitioning determines the distribution of grid cells
to processors. It is observed that the partitioning phase takes a substantial
percentage of the overall computation time. Alternative
(graph-partitioning-based) schemes that perform almost as well as the
hypergraph partitioning scheme and that require less preprocessing overhead and
better balance are proposed and investigated. A comparison in terms of effect
on running time, preprocessing overhead and load-balancing quality of Zoltan's
PHG, ParMeTiS, and PT-SCOTCH are presented. Test results on J\"uelich
BlueGene/P cluster are presented.
