Compiling and Executing Multi-Locale Programs

Selecting a Communication Mechanism

All multi-locale Chapel programs are implemented using runtime communication routines which transparently move data and tasks between the locales on which the program is running. The choice of communication runtime is made via the --comm compiler flag (or equivalently, the $CHPL_COMM environment variable). Portable options for communication include:

  • none: specifies that no communication should be performed, restricting the program's execution to a single locale.
  • gasnet: specifies that communication should be implemented using the open-source GASNet library developed at Berkeley.
  • ofi: specifies that communication should be implemented using the open-source libfabric library component of OpenFabrics Interfaces (OFI). This configuration is preliminary; see Using Chapel with libfabric for more information.

When using the Chapel module on Cray systems, a third option is also available:

  • ugni: specifies that communication should be implemented using Cray's uGNI interface.

In order to use any of these options, the Chapel runtime libraries must have been built with the specified communication layer enabled. See Building Chapel for general information on building Chapel and Multilocale Chapel Execution for guidance specific to building the runtime for multi-locale execution.

Compiling for One vs. Multiple Locales

The Chapel compiler supports distinct modes for creating single-locale versus multi-locale executables. This mode can be selected via the --[no-]local flag, whose default setting is based on the choice of communication mechanism above. Specifically, when the communication is none, the compiler defaults to --local compilation. When it is set to anything else, the compiler defaults to --no-local compilation. Due to this default, this flag rarely needs to be used in practice.

Executables compiled in --local mode can only execute using a single locale. Executables compiled in --no-local mode can be executed on one or more locales, but typically incur overhead when run on a single locale as compared to those compiled in --local mode.

Executing Using Multiple Locales

To specify the number of locales on which a program should run, use the --numLocales flag (or -nl for short).

For example, the following command-line says to run the given Chapel program on four locales:

./myChapelProgram -nl 4

Note that all Chapel programs begin as a single task running on locale 0. Control over locality is explicit within Chapel, like parallelism. In order for a program to execute using multiple locales, it must request this either explicitly using on-clauses or implicitly using abstractions that themselves use on-clauses.

Programs compiled with communication mode none can only execute on a single shared-memory locale and therefore need not specify a number of locales since the only valid choice is 1. If a user tries to run the program using more than one locale, an error message will be printed.