Having introduced the Chapel language using the first twelve days of AoC 2022 exercises, this completes our Twelve Days of Chapel: Advent of Code 2022 winter spectacular. Of course, Advent of Code 2022 is still going on, but we’re going to take a break as we head into winter shutdowns, holiday vacations, and the like. If a specific exercise from days 13–25 catches our interest, and time permits, we may post additional AoC 2022 articles next month, but we’ll see… We also have other plans for posts and series in the coming year.

Over these first twelve days, we managed to use a lot of Chapel features and find plenty of opportunities for parallel computation, both of which should give you enough material to either be effective or dangerous with the language, depending on how successful we were. In either case, please feel free to ask follow-up questions using the channels noted at the bottom of this article.

That said, because Advent of Code focuses on desktop-ready computations, we never really had the chance to see Chapel’s features supporting distributed parallel computing, which is its whole reason for existing. In this article, we’ll wrap up with a quick preview of such features, introducing them in the context of some computations we’ve already seen.

Summary of This Series

First though, let’s summarize the main features that we used in this series and where they were introduced:

Chapel’s Support for Scalable Distributed Computing: A Preview

As mentioned above, it isn’t terribly surprising that we didn’t encounter Chapel’s support for distributed memory computing in this series given AoC’s focus on desktop computing. Since scalable parallel computing is Chapel’s reason for existing, you can expect to see future articles on this blog cover those aspects of the language in more detail. However, let’s touch on the topic briefly before wrapping up this series.

Chapel’s Locales and their Role in Supporting Distributed Parallelism

A crucial feature for understanding distributed computing in Chapel is the locale. Locales are a type built into the language for representing a portion of the target architecture that can run tasks and store variables. Since that describes a CPU and its memory, your laptop could serve as a locale. In fact, all of the programs written in this series would be considered single-locale Chapel programs since nothing about them has referred to other locales, either explicitly or implicitly. Systems like clusters, the cloud, or supercomputers are considered multi-locale systems for Chapel, where each compute node could be considered a distinct locale.

Data Parallelism Using Distributed Domains and Arrays

One of the simplest ways to get started with multi-locale computing in Chapel is through its support for distributed arrays. These are identical to the arrays we’ve seen in this series, except that rather than storing all of their elements in a single locale’s memory, they are distributed across multiple locales.

Looking back at day 8’s computation, recall that we used a 2D array to find ideal treehouse locations. If desired, we could have distributed the elements of this array across the memories of multiple compute nodes, giving each of them a chunk of the total array. And for a forest that was large enough, this could be a way to run larger problem sizes or to achieve better performance.

To see how this would be done, we could change the 2D domain ForestSpace from its original declaration:

const ForestSpace = {0..<numRows, 0..<numCols};

to this one that uses the Cyclic distribution for arrays:

use CyclicDist;  // we need to use the 'Cyclic Distribution' module

const ForestSpace = {0..<numRows, 0..<numCols} dmapped Cyclic(startIdx=(1,1));

Doing so has the effect of distributing the indices of the ForestSpace domain across the compute nodes of the target system in a round-robin fashion in both of its dimensions. The Forest array declared in terms of ForestSpace would be similarly distributed, such that each locale would store 1/numLocales of the array elements. And then the subsequent loops or slices over Forest would result in parallel computation across all of the locales owning the indices in question.

Thus, by use-ing one additional module and changing just one line, we have turned our desktop AoC code into one that could target all of the processor cores and memories of a supercomputer. This is an example of distributed data parallelism in Chapel since it leverages Chapel’s data-parallel features—its domains, arrays, promotion, and forall loops—to target multiple locales. Like the data-parallel examples we’ve seen in this series, it represents a high-level way of doing distributed computing in Chapel.

Explicit Distributed Computing Using on-clauses

We can also target multiple locales at a lower level using an on statement (or on-clause for short). This can be thought of as a more explicit way of doing distributed computing in Chapel, similar to how Chapel’s task-parallel features were shown in this series to give us lower-level, more explicit control than its high-level data-parallel features.

In our day 11 article, you may recall that the first task-parallel program I introduced was this one:

coforall tid in 1..4 do
  writeln("Hello from task ", tid);
writeln("After the coforall");

Using the on-clause and one additional coforall loop, we can turn this from a 4-task, single-locale program into a multi-locale program:

  coforall loc in Locales do
    on loc do
      coforall tid in 1..4 do
        writeln("Hello from task ", tid, " on locale ", loc.id);

  writeln("After the coforalls");

Here, I’m iterating over a built-in array, Locales, which represents the set of locales on which my program is running. If I specified that I wanted to run my program on 16 locales, or compute nodes, by using the command line:

$ ./hello-coforall --numLocales=16

then the array would have 16 elements, one per node. The coforall will then create a task per element in that array, so 16 tasks in this example, one per locale. Each task will have its own index variable loc that refers to its specific locale. Next, the on-clause says to run the code that it encloses on that specific locale. At this point, I’d have one task running on each of the 16 compute nodes from my distributed system.

Next, each of those tasks encounters the inner coforall, which causes it to create four tasks, mapping them to four local cores on that compute node / locale. And each of those tasks prints a message like before. Note that I’ve added some more arguments to the message to make it unique on each locale. Specifically, I’m using the .id query supported by the locale type to get that locale’s ID from 0..<numLocales.

So, when running on 16 locales, we’d see 64 messages printed to the console. As in the original shared-memory version, these messages would print in an arbitrary order since there is no coordination or synchronization between the tasks. As the tasks complete, they will ‘join’ back together at the ends of their respective coforall loops, and the original task that kicked the rest of them off would print the final message, “After the coforalls”.


Over the course of this “Twelve Days of Chapel” series, we’ve introduced a big cross-section of the features supported by the Chapel language. And in this article, you’ve seen the basis for Chapel’s distributed computing features: locales, distributed arrays, and on-clauses. We hope that this brief taste of distributed computing in Chapel will whet your appetite to learn more about its features for scalable parallel computing beyond the desktop parallelism introduced in this series. And we’ll be coming back to these concepts more in future blog articles and series. Meanwhile, whether you’ve got supercomputer-sized problems to solve, or simply parallel computations you’d like to write on your laptop, you’ve now got a good start on how to express them in Chapel.

We’ll be continuing this blog with new articles and series in 2023, and may return with another Advent of Code series in December next year. Until then, we hope that you’ve enjoyed this series. Please let us know of any follow-up questions or comments you might have in the Blog category of Chapel’s Discourse page, or on any of the other communication mechanisms we support for the community.

With best wishes for the new year,

-Brad, Daniel, Jeremiah, and Michelle