Task-parallel hello world

View hello5-taskpar.chpl on GitHub

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).

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.

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

For further examples of using task parallelism, refer to examples/primers/taskParallel.chpl.