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Today’s largest supercomputers have over two hundred thousand CPU cores and even larger systems are under development. Typically, these systems are programmed using message passing. Over the past decade, there has been considerable interest in developing simpler and more expressive programming models for them. Partitioned global address space (PGAS) languages are viewed as perhaps the most promising alternative. In this paper, we report on our experience developing a set of PGAS extensions to Fortran that we call Coarray Fortran 2.0 (CAF 2.0). Our design for CAF 2.0 goes well beyond the original 1998 design of Coarray Fortran (CAF) by Numrich and Reid. CAF 2.0 includes language support for many features including teams, collective communication, asynchronous communication, function shipping, and synchronization. We describe the implementation of these features and our experiences using them to implement the High Performance Computing Challenge (HPCC) benchmarks, including High Performance Linpack (HPL), RandomAccess, Fast Fourier Transform (FFT), and STREAM triad. On 4096 CPU cores of a Cray XT with 2.3 GHz single socket quad-core Opteron processors, we achieved 18.3 TFLOP/s with HPL, 2.01 GUP/s with RandomAccess, 125 GFLOP/s with FFT, and a bandwidth of 8.73 TByte/s with STREAM triad.

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