Asynchronous execution with actions#

This example extends the previous example by introducing actions: functions that can be run remotely. In this example, however, we will still only run the action locally. The mechanism to execute actions stays the same: hpx::async. Later examples will demonstrate running actions on remote localities (e.g. Remote execution with actions).


The source code for this example can be found here: fibonacci.cpp.

To compile this program, go to your HPX build directory (see Building HPX for information on configuring and building HPX) and enter:

$ make examples.quickstart.fibonacci

To run the program type:

$ ./bin/fibonacci

This should print (time should be approximate):

fibonacci(10) == 55
elapsed time: 0.00186288 [s]

This run used the default settings, which calculate the tenth element of the Fibonacci sequence. To declare which Fibonacci value you want to calculate, use the --n-value option. Additionally you can use the --hpx:threads option to declare how many OS-threads you wish to use when running the program. For instance, running:

$ ./bin/fibonacci --n-value 20 --hpx:threads 4

Will yield:

fibonacci(20) == 6765
elapsed time: 0.233827 [s]


The code needed to initialize the HPX runtime is the same as in the previous example:

int main(int argc, char* argv[])
    // Configure application-specific options
    hpx::program_options::options_description desc_commandline(
        "Usage: " HPX_APPLICATION_STRING " [options]");

        "n value for the Fibonacci function");

    // Initialize and run HPX
    hpx::init_params init_args;
    init_args.desc_cmdline = desc_commandline;

    return hpx::init(argc, argv, init_args);

The hpx::init function in main() starts the runtime system, and invokes hpx_main() as the first HPX-thread. The command line option --n-value is read in, a timer (hpx::chrono::high_resolution_timer) is set up to record the time it takes to do the computation, the fibonacci action is invoked synchronously, and the answer is printed out.

int hpx_main(hpx::program_options::variables_map& vm)
    // extract command line argument, i.e. fib(N)
    std::uint64_t n = vm["n-value"].as<std::uint64_t>();

        // Keep track of the time required to execute.
        hpx::chrono::high_resolution_timer t;

        // Wait for fib() to return the value
        fibonacci_action fib;
        std::uint64_t r = fib(hpx::find_here(), n);

        char const* fmt = "fibonacci({1}) == {2}\nelapsed time: {3} [s]\n";
        hpx::util::format_to(std::cout, fmt, n, r, t.elapsed());

    return hpx::finalize();    // Handles HPX shutdown

Upon a closer look we see that we’ve created a std::uint64_t to store the result of invoking our fibonacci_action fib. This action will launch synchronously (as the work done inside of the action will be asynchronous itself) and return the result of the Fibonacci sequence. But wait, what is an action? And what is this fibonacci_action? For starters, an action is a wrapper for a function. By wrapping functions, HPX can send packets of work to different processing units. These vehicles allow users to calculate work now, later, or on certain nodes. The first argument to our action is the location where the action should be run. In this case, we just want to run the action on the machine that we are currently on, so we use hpx::find_here. To further understand this we turn to the code to find where fibonacci_action was defined:

// forward declaration of the Fibonacci function
std::uint64_t fibonacci(std::uint64_t n);

// This is to generate the required boilerplate we need for the remote
// invocation to work.
HPX_PLAIN_ACTION(fibonacci, fibonacci_action)

A plain action is the most basic form of action. Plain actions wrap simple global functions which are not associated with any particular object (we will discuss other types of actions in Components and actions). In this block of code the function fibonacci() is declared. After the declaration, the function is wrapped in an action in the declaration HPX_PLAIN_ACTION. This function takes two arguments: the name of the function that is to be wrapped and the name of the action that you are creating.

This picture should now start making sense. The function fibonacci() is wrapped in an action fibonacci_action, which was run synchronously but created asynchronous work, then returns a std::uint64_t representing the result of the function fibonacci(). Now, let’s look at the function fibonacci():

std::uint64_t fibonacci(std::uint64_t n)
    if (n < 2)
        return n;

    // We restrict ourselves to execute the Fibonacci function locally.
    hpx::id_type const locality_id = hpx::find_here();

    // Invoking the Fibonacci algorithm twice is inefficient.
    // However, we intentionally demonstrate it this way to create some
    // heavy workload.

    fibonacci_action fib;
    hpx::future<std::uint64_t> n1 = hpx::async(fib, locality_id, n - 1);
    hpx::future<std::uint64_t> n2 = hpx::async(fib, locality_id, n - 2);

    return n1.get() +
        n2.get();    // wait for the Futures to return their values

This block of code is much more straightforward and should look familiar from the previous example. First, if (n < 2), meaning n is 0 or 1, then we return 0 or 1 (recall the first element of the Fibonacci sequence is 0 and the second is 1). If n is larger than 1 we spawn two tasks using hpx::async. Each of these futures represents an asynchronous, recursive call to fibonacci. As previously we wait for both futures to finish computing, get the results, add them together, and return that value as our result. The recursive call tree will continue until n is equal to 0 or 1, at which point the value can be returned because it is implicitly known. When this termination condition is reached, the futures can then be added up, producing the n-th value of the Fibonacci sequence.