.. default-domain:: chpl

.. _primers-hello5-taskpar:

Task-parallel hello world
=========================

`View hello5-taskpar.chpl on GitHub <https://github.com/chapel-lang/chapel/blob/main/test/release/examples/hello5-taskpar.chpl>`_

This program uses Chapel's `task parallel` features to express an
explicitly concurrent hello world program that utilizes multiple
cores on a single `locale` (compute node).




First, we specify the number of tasks to create via a `config
const`.  By default, set it to the runtime's estimation of maximum
parallelism that the current locale ('`here`') is capable of
executing (``.maxTaskPar``).

.. code-block:: chapel

    config const numTasks = here.maxTaskPar;

Next, we create the specified number tasks using a `coforall-loop`.
This is a parallel loop form that will create a distinct task per
iteration.

This coforall-loop is iterating over the `range` ``0..#numTasks``
which represents the first `numTasks` integers starting at 0
(equivalent to ``0..numTasks-1``).  The result will be `numTasks`
iterations, each of which will be executed as a distinct parallel
task.

Each iteration prints out a message that is unique based on its
value of `tid`.  Due to the task parallelism, the messages may be
printed in any order.  However, the `writeln()` procedure will
prevent finer-grained interleaving of the messages themselves.

.. code-block:: chapel

    coforall tid in 0..#numTasks do
      writeln("Hello, world! (from task ", tid, " of ", numTasks, ")");

For further examples of using task parallelism, refer to
:ref:`examples/primers/taskParallel.chpl <primers-taskParallel>`.