  Fast multipole methods (FMM) on distributed mem- ory have traditionally used
a bulk-synchronous model of com- municating the local essential tree (LET) and
overlapping it with computation of the local data. This could be perceived as
an extreme case of data aggregation, where the whole LET is communicated at
once. Charm++ allows a much finer control over the granularity of
communication, and has a asynchronous execution model that fits well with the
structure of our FMM code. Unlike previous work on asynchronous fast N-body
methods such as ChaNGa and PEPC, the present work performs a direct comparison
against the traditional bulk-synchronous approach and the asynchronous approach
using Charm++. Furthermore, the serial performance of our FMM code is over an
order of magnitude better than these previous codes, so it is much more
challenging to hide the overhead of Charm++.
