  Modern interconnects offer remote direct memory access (RDMA) features. Yet,
most applications rely on explicit message passing for communications albeit
their unwanted overheads. The MPI-3.0 standard defines a programming interface
for exploiting RDMA networks directly, however, it's scalability and
practicability has to be demonstrated in practice. In this work, we develop
scalable bufferless protocols that implement the MPI-3.0 specification. Our
protocols support scaling to millions of cores with negligible memory
consumption while providing highest performance and minimal overheads. To arm
programmers, we provide a spectrum of performance models for all critical
functions and demonstrate the usability of our library and models with several
application studies with up to half a million processes. We show that our
design is comparable to, or better than UPC and Fortran Coarrays in terms of
latency, bandwidth, and message rate. We also demonstrate application
performance improvements with comparable programming complexity.
