Sorting is a commonly used process with a wide breadth of applications in the high performance computing field. Early research in parallel processing has provided us with comprehensive analysis and theory for parallel sorting algorithms. However, modern supercomputers have advanced rapidly in size and changed significantly in architecture, forcing new adaptations to these algorithms. To fully utilize the potential of highly parallel machines, tens of thousands of processors are used. Efficiently scaling parallel sorting on machines of this magnitude is inhibited by the communication-intensive problem of migrating large amounts of data between processors. The challenge is to design a highly scalable sorting algorithm that uses minimal communication, maximizes overlap between computation and communication, and uses memory efficiently. This paper presents a scalable extension of the Histogram Sorting method, making fundamental modifications to the original algorithm in order to minimize message contention and exploit overlap. We implement Histogram Sort, Sample Sort, and Radix Sort in Charm++ and compare their performance. The choice of algorithm as well as the importance of the optimizations is validated by performance tests on two predominant modern supercomputer architectures: XT4 at ORNL (Jaguar) and Blue Gene/P at ANL (Intrepid).