  Simulations of galaxy formation follow the gravitational and hydrodynamical
interactions between gas, stars and dark matter through cosmic time. The huge
dynamic range of such calculations severely limits strong scaling behaviour of
the community codes in use, with load-imbalance, cache inefficiencies and poor
vectorisation limiting performance. The new swift code exploits task-based
parallelism designed for many-core compute nodes interacting via MPI using
asynchronous communication to improve speed and scaling. A graph-based domain
decomposition schedules interdependent tasks over available resources. Strong
scaling tests on realistic particle distributions yield excellent parallel
efficiency, and efficient cache usage provides a large speed-up compared to
current codes even on a single core. SWIFT is designed to be easy to use by
shielding the astronomer from computational details such as the construction of
the tasks or MPI communication. The techniques and algorithms used in SWIFT may
benefit other computational physics areas as well, for example that of
compressible hydrodynamics. For details of this open-source project, see
www.swiftsim.com
