Legion: Programming Distributed Heterogeneous Architectures with Logical Regions

This thesis covers the design and implementation of Legion, a new programming model and runtime system for targeting distributed heterogeneous machine architectures. Legion introduces logical regions as a new abstraction for describing the structure and usage of program data. We describe how logical regions provide a mechanism for applications to express important properties of program data, such as locality and independence, that are often ignored by current programming systems. We also show how logical regions allow programmers to scope the usage of program data by different computations. The explicit nature of logical regions makes these properties of programs manifest, allowing many of the challenging burdens of parallel programming, including dependence analysis and data movement, to be off-loaded from the programmer to the programming system. Logical regions also improve the programmability and portability of applications by decoupling the specification of a program from how it is mapped onto a target architecture. Logical regions abstractly describe sets of program data without requiring any specification regarding the placement or layout of data. To control decisions about the placement of computations and data, we introduce a novel mapping interface that gives an application programmatic control over mapping decisions at runtime. Different implementations of the mapper interface can be used to port applications to new architectures and to explore alternative mapping choices. Legion guarantees that the decisions made through the mapping interface are independent of the correctness of the program, thus facilitating easy porting and tuning of applications to new architectures with different performance characteristics. Using the information provided by logical regions, an implementation of Legion