Introduction to Chapel

Overview

Teaching: 15 min
Exercises: 15 min

Questions

• What is Chapel and why is it useful?

Objectives

• Write and execute our first chapel program.

Chapel is a modern programming language, developed by Cray Inc., that supports HPC via high-level abstractions for data parallelism and task parallelism. These abstractions allow the users to express parallel codes in a natural, almost intuitive, manner. In contrast with other high-level parallel languages, however, Chapel was designed around a multi-resolution philosophy. This means that users can incrementally add more detail to their original code prototype, to optimise it to a particular computer as closely as required.

In a nutshell, with Chapel we can write parallel code with the simplicity and readability of scripting languages such as Python or MATLAB, but achieving performance comparable to compiled languages like C or Fortran (+ traditional parallel libraries such as MPI or OpenMP).

In this lesson we will learn the basic elements and syntax of the language; then we will study task parallelism, the first level of parallelism in Chapel, and finally we will use parallel data structures and data parallelism, which is the higher level of abstraction, in parallel programming, offered by Chapel.

Getting started

Chapel is a compilable language which means that we must compile our source code to generate a binary or executable that we can then run in the computer.

Chapel source code must be written in text files with the extension .chpl. Let’s write a simple “hello world”-type program to demonstrate how we write Chapel code! Using your favourite text editor, create the file hello.chpl with the following content:

writeln('If we can see this, everything works!');

This program can then be compiled with the following bash command:

chpl --fast hello.chpl -o hello.o

The flag --fast indicates the compiler to optimise the binary to run as fast as possible in the given architecture. The -o option tells Chapel what to call the final output program, in this case hello.o.

To run the code, you execute it as you would any other program:

./hello.o

If we can see this, everything works!

Running on a cluster

Depending on the code, it might utilise several or even all cores on the current node. The command above implies that you are allowed to utilise all cores. This might not be the case on an HPC cluster, where a login node is shared by many people at the same time, and where it might not be a good idea to occupy all cores on a login node with CPU-intensive tasks.

On Compute Canada clusters Cedar and Graham we have two versions of Chapel, one is a single-locale (single-node) Chapel, and the other is a multi-locale (multi-node) Chapel. For now, we will start with single-locale Chapel. If you are logged into Cedar or Graham, you’ll need to load the single-locale Chapel module:

module load gcc chapel-multicore

Then, for running a test code on a cluster you would submit an interactive job to the queue

salloc --time=0:30:0 --ntasks=1 --cpus-per-task=3 --mem-per-cpu=1000 --account=def-guest

and then inside that job compile and run the test code

chpl --fast hello.chpl -o hello.o
./hello.o

For production jobs, you would compile the code and then submit a batch script to the queue:

chpl --fast hello.chpl -o hello.o
sbatch script.sh

where the script script.sh would set all Slurm variables and call the executable mybinary.

Case study

Along all the Chapel lessons we will be using the following case study as the leading thread of the discussion. Essentially, we will be building, step by step, a Chapel code to solve the Heat transfer problem described below. Then we will parallelize the code to improve its performance.

Suppose that we have a square metallic plate with some initial heat distribution or initial conditions. We want to simulate the evolution of the temperature across the plate when its border is in contact with a different heat distribution that we call the boundary conditions.

The Laplace equation is the mathematical model for the evolution of the temperature in the plate. To solve this equation numerically, we need to discretise it, i.e. to consider the plate as a grid, or matrix of points, and to evaluate the temperature on each point at each iteration, according to the following difference equation:

T[i,j] = 0.25 (Tp[i-1,j] + Tp[i+1,j] + Tp[i,j-1] + Tp[i,j+1])

Here T stands for the temperature at the current iteration, while Tp contains the temperature calculated at the past iteration (or the initial conditions in case we are at the first iteration). The indices i and j indicate that we are working on the point of the grid located at the ith row and the jth column.

So, our objective is to:

Goals

1. Write a code to implement the difference equation above.The code should have the following requirements:

   • It should work for any given number of rows and columns in the grid.
   • It should run for a given number of iterations, or until the difference between T and Tp is smaller than a given tolerance value.
   • It should output the temperature at a desired position on the grid every given number of iterations.
2. Use task parallelism to improve the performance of the code and run it in the cluster
3. Use data parallelism to improve the performance of the code and run it in the cluster.

Key Points

• Chapel is a compiled language - any programs we make must be compiled with chpl.

• The --fast flag instructs the Chapel compiler to optimise our code.

• The -o flag tells the compiler what to name our output (otherwise it gets named a.out)