  To achieve scalability with today's heterogeneous HPC resources, we need a
dramatic shift in our thinking; MPI+X is not enough. Asynchronous Many Task
(AMT) runtime systems break down the global barriers imposed by the Bulk
Synchronous Programming model. HPX is an open-source, C++ Standards compliant
AMT runtime system that is developed by a diverse international community of
collaborators called The Ste||ar Group. HPX provides features which allow
application developers to naturally use key design patterns, such as
overlapping communication and computation, decentralizing of control flow,
oversubscribing execution resources and sending work to data instead of data to
work. The Ste||ar Group comprises physicists, engineers, and computer
scientists; men and women from many different institutions and affiliations,
and over a dozen different countries. We are committed to advancing the
development of scalable parallel applications by providing a platform for
collaborating and exchanging ideas. In this paper, we give a detailed
description of the features HPX provides and how they help achieve scalability
and programmability, a list of applications of HPX including two large NSF
funded collaborations (STORM, for storm surge forecasting; and STAR (OctoTiger)
an astro-physics project which runs at 96.8% parallel efficiency on 643,280
cores), and we end with a description of how HPX and the Ste||ar Group fit into
the open source community.
