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UPC++: a PGAS library for C++

Overview

UPC++ is a parallel programming library for developing C++ applications with the Partitioned Global Address Space (PGAS) model.

UPC++ has three main objectives:

  • Provide an object-oriented PGAS programming model in the context of the popular C++ language

  • Expose useful asynchronous parallel programming idioms unavailable in traditional SPMD models, such as remote function invocation and continuation-based operation completion, to support complex scientific applications

  • Offer an easy on-ramp to PGAS programming through interoperability with other existing parallel programming systems (e.g., MPI, OpenMP, CUDA, ROCm/HIP)

UPC++ Documentation

The rest of this document provides basic information for command-line use of the UPC++ software implementation.

Other topics are covered in the following documents:

Usage information for public installs of UPC++ at certain computing centers is available online.

To report problems or request features: issue tracker.

Compiling Against UPC++ on the Command Line

With UPC++ installed, the easiest way to build a UPC++ application from the command line is to use the upcxx compiler wrapper, installed in <upcxx-install-path>/bin/upcxx. The arguments to this wrapper work just like the C++ compiler used to install UPC++ (analogous to the mpicxx compiler wrapper often provided for MPI/C++ programs).

For example, to build an application consisting of my-app1.cpp and my-app2.cpp:

export PATH="<upcxx-install-path>/bin/:$PATH"
upcxx -O -c my-app1.cpp my-app2.cpp
upcxx -O -o my-app my-app1.o my-app2.o -lm

Be sure that all commands used to build one executable consistently pass either a -O option to select the optimized/production version of UPC++ (for performance runs), or a -g option to select the debugging version of UPC++ (for tracking down bugs in your application).

To select a non-default network backend or thread-safe version of the library, you'll need to pass the -network= or -threadmode= options, or set the UPCXX_NETWORK or UPCXX_THREADMODE variables prior to invoking compilation. See the 'UPC++ Backends' section below.

Compiling Against UPC++ in Makefiles

The simplest way to build UPC++ programs from a Makefile is to use the upcxx compiler wrapper documented in the section above to replace your normal C++ compiler command.

If your Makefile structure prevents this and/or requires extraction of the underlying compiler flags to build against UPC++, your build process can query this information by invoking the <upcxx-install-path>/bin/upcxx-meta <what> script, where <what> indicates which form of flags are desired. Valid values are:

  • CXX: The C++ compiler used to install UPC++, which must also be used for building application code.
  • CPPFLAGS: Preprocessor flags which will put the upcxx headers in the compiler's search path and define macros required by those headers.
  • CXXFLAGS: Compiler flags which set debug/optimization settings, and set the minimum C++ language level required by the UPC++ headers.
  • LDFLAGS: Linker flags usually belonging at the front of the link command line (before the list of object files).
  • LIBS: Linker flags belonging at the end of the link command line. These will make libupcxx and its dependencies available to the linker.

For example, to build an application consisting of my-app1.cpp and my-app2.cpp using extracted arguments:

meta="<upcxx-install-path>/bin/upcxx-meta"
$($meta CXX) $($meta CPPFLAGS) $($meta CXXFLAGS) -c my-app1.cpp
$($meta CXX) $($meta CPPFLAGS) $($meta CXXFLAGS) -c my-app2.cpp
$($meta CXX) $($meta LDFLAGS) -o my-app my-app1.o my-app2.o -lm $($meta LIBS)

For an example of a Makefile which builds UPC++ applications, look at example/prog-guide/Makefile. This directory also has code for running all the examples given in the programmer's guide. To use that Makefile, first set the UPCXX_INSTALL shell variable to the <upcxx-install-path>.

Using UPC++ with CMake

A UPCXX CMake package is provided in the installation directory. To use it in a CMake project, append the UPC++ installation directory to the CMAKE_PREFIX_PATH variable (cmake ... -DCMAKE_PREFIX_PATH=/path/to/upcxx/install/prefix ...), then use find_package(UPCXX) in the CMakeLists.txt file of the project.

If it is able to find a compatible UPC++ installation, the CMake package will define a UPCXX:upcxx target (as well as a UPCXX_LIBRARIES variable for legacy projects) that can be added as dependency to your project.

UPC++ Backends

UPC++ provides multiple "backends" offering the user flexibility to choose the means by which the parallel communication facilities are implemented. Those backends are characterized by three dimensions: conduit, thread-mode, and code-mode. All objects in a given executable must agree on the backend in use. The conduit and thread-mode parameters map directly to the GASNet concepts of the same name (for more explanation, see below). Code-mode selects between highly optimized code and highly debuggable code. The upcxx-meta script will assume sensible defaults for these parameters based on the installation configuration. The following environment variables can be set to influence which backend upcxx-meta selects:

  • UPCXX_NETWORK=<see below>: The GASNet network backend ("conduit") to use for communication (the default and available values are system-dependent). Supported values include:

    • ibv is the high-performance InfiniBand network.
    • smp is the high-performance choice for single-node multi-core runs.
    • ofi the libfabric backend, recommended on Omni-Path networks and HPE Cray EX systems.
    • ucx the experimental UCX backend for certain InfiniBand systems.
    • udp is a portable low-performance alternative for testing and debugging.
    • mpi is a portable low-performance alternative for testing and debugging.

  • UPCXX_THREADMODE=[seq|par]: The value par selects the thread-safe version of the library which permits any upcxx function to be called from any thread, within the parameters set by the specification. The value seq adds thread-safety restrictions on the majority of upcxx routines (mostly that communication can only be initiated by a single thread) at the benefit of lower library-internal overhead. See docs/implementation-defined for detailed requirements. The default value is always seq.

  • UPCXX_CODEMODE=[opt|debug]: opt is for highly compiler-optimized code and is recommended for performance measurement and production runs. debug produces unoptimized code, includes extra error checking assertions to validate the correctness of application calls to UPC++, and is annotated with the symbol tables needed by debuggers; debug mode is recommended for all development/debugging of applications. The default value is opt.

Running UPC++ Programs

To run a parallel UPC++ application, use the upcxx-run launcher provided in the installation.

<upcxx-install-path>/bin/upcxx-run -n <ranks> <exe> <args...>

This will run the executable and arguments <exe> <args...> in a parallel context with <ranks> number of UPC++ processes.

Upon startup, each UPC++ process creates a fixed-size shared memory heap that will never grow. By default, this heap is 128 MB per process. This can be adjusted by passing a -shared-heap parameter to upcxx-run, which takes a suffix of KB, MB or GB; e.g. to reserve 1GB per process, call:

<upcxx-install-path>/bin/upcxx-run -shared-heap 1G -n <ranks> <exe> <args...>

There are several additional options that can be passed to upcxx-run. See upcxx-run -h for a complete list of options.

The canonical version of this document is located here: https://upcxx.lbl.gov/wiki/README

For more information, please visit the UPC++ home page

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