# Distributed memory task parallel hello world¶

View hello6-taskpar-dist.chpl on GitHub

This program shows how to use Chapel's task parallelism and locality features to create a concurrent "Hello, world" program that spans distributed memory locales (compute nodes) as well as (optionally) the processing units (cores) within each locale. The number of locales on which to run is specified on the executable's command line using the -nl or --numLocales flag (e.g., ./hello -nl 64).

The following config const specifies whether or not the locale name should be printed (for ease-of-testing purposes); the default can be overridden on the execution command line (e.g., --printLocaleName=false).

config const printLocaleName = true;


This one specifies the number of tasks to use per locale:

config const tasksPerLocale = 1;


Use a coforall-loop to create a distinct task per locale on which the program is executing. Here, we're iterating over the built-in Locales array which stores an array of locale values corresponding 1:1 with the system's compute nodes on which the program is executing. Thus, each iteration corresponds to one of the locales, represented by the loop index variable loc.

coforall loc in Locales {


Migrate each task to its corresponding locale, loc. This is done using an 'on'-clause, which moves execution of the current task to the locale corresponding to the expression following it.

on loc {


Now use a second coforall-loop to create a number of tasks corresponding to the tasksPerLocale configuration constant. Since this loop body doesn't contain any on-clauses, all tasks will remain local to the current locale.

coforall tid in 0..#tasksPerLocale {


Start building up the message to print using a string variable, message.

var message = "Hello, world! (from ";


If we're running more than one task per locale, specialize the message based on the task ID.

if (tasksPerLocale > 1) then
message += "task " + tid + " of " + tasksPerLocale + " on ";


Specialize the message based on the locale on which this task is running:

• here refers to the locale on which this task is running (same as loc)
• .id queries its unique ID in 0..numLocales-1
• .name queries its name (similar to UNIX hostname)
• numLocales refers to the number of locales (as specified by -nl)
message += "locale " + here.id + " of " + numLocales;

if printLocaleName then message += " named " + loc.name;


Terminate the message

message += ")";


Print out the message. Since each message is being printed by a distinct task, they may appear in an arbitrary order.

      writeln(message);
}
}
}


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

For further examples of using locales and on-clauses, refer to examples/primers/locales.chpl.