HPX - High Performance ParalleX

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Integrating HPX into existing applications

Having your application work with HPX is unfortunately not as simple as just linking against it. However it can be rather straightforward or rather complex based on the type of application that is trying to be integrated. The complexity derives from the overall design of the application. If the application was designed to be parallel, then integrating should be relatively straightforward. However HPX is not some magic wand that will make your application work in parallel. Applications that are serial or that rely too heavily on global variables will have a more difficult time in integrating. Most parallel applications have already identified the sections of code that do the most work and have abstracted them out to use the parallel library of their choice.

One of the big advantages of HPX is the concept of moving work to data. Data-heavy applications can use components to create remote objects that encapsulate that data, and call methods on these component-objects. Computation-heavy applications do not care so much about moving work to data, but instead try to get as much parallel computation done as possible. HPX futures provide a nice tool for accomplishing this. Using futures, one can set up a lot of work to be done, fire it off and wait for them to be finished. HPX takes care of creating the threads, moving the work across node boundaries, and making sure the calling thread suspends when it wants the value from a future that is still executing. A lot of boring code that you do not need to write. The one caveat is that for any function you wrap with a future, any parameters of those functions need to be serializable. This can be a rather complicated procedure if the data structures are complex. HPX uses Boost.Serialization for the serialization of the data to be used in futures. That really is the hardest part, because once the serialization functions are completed, you can use futures anywhere in your code. If you plan on only running on an SMP machine, then HPX never calls the serialization functions and they may be blank (but the signatures are still required).

The next stumbling block to integrating HPX is having it actually link against your program. If your application uses a CMake build system, you can set the CMake Module Path to the path where HPX has the FindHPX.cmake module. Here is a code snippet demonstrating this. It adds the preprocessor definition HPX_ENABLED to the compiler command line if HPX is found.

# This adds the HPX cmake configuration directory to the search path.
set(CMAKE_MODULE_PATH
    $(CMAKE_MODULE_PATH)
    ${HPX_INSTALL_DIR}/share/cmake-2.8/Modules)

# Instruct cmake to find the HPX installation
find_package(HPX)
IF(HPX_FOUND)
    # Add a special option to you compiler command line.
    MESSAGE("HPX was found.")
    SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -D HPX_ENABLED")
ENDIF()

Now to actually build the application you should have a code snippet similar to this in your CMakeLists.txt file:

IF(HPX_FOUND)
    # build your application using HPX
    add_hpx_executable(your_app
        ESSENTIAL
        SOURCES ${your_app_SRCS})
ELSE()
    # build your application with out using HPX
    add_executable(your_app ${your_app_SRCS})
ENDIF()

When you configure your application, all you need to do is set the HPX_INSTALL_DIR variable to point to the installation of HPX! Now all you have to do is type make and the application should build using the HPX installation you specified.

If you do not use CMake, you can still build against HPX but you should refer to the section on How to Build HPX Components with pkg-config.

Those two things, the serialization and actually integrating with HPX, are the biggest stumbling blocks that you should encountered when trying to integrate your application with HPX. For an idea on how to integrate HPX constructs into your application, you should take a look at some of the examples that HPX includes.


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