Timeouts in Chapel

Timeouts are important for programs that connect to external resources or that otherwise need to bound execution time. Implementing timeouts in Chapel is possible using the Timer module and task-based parallelism.

use Time;
use IO;

proc main() {
    // For our example, suppose we're executing an external
    // call that returns its result after 2 seconds.
    // We'll use a synchronization variable to simulate this.
    var result1: sync string;
    begin {
        sleep(2);
        result1.writeXF("result 1");
    }

    // Here we implement a timeout using a timer.
    // We'll wait for either the result or a 1-second timeout.
    var timer = new Timer();
    timer.start();
    
    while (timer.elapsed() < 1.0) {
        if result1.isFull {
            writeln(result1.readFF());
            break;
        }
        sleep(0.1);  // Small sleep to avoid busy waiting
    }
    
    if (!result1.isFull) {
        writeln("timeout 1");
    }

    // If we allow a longer timeout of 3 seconds, then we'll
    // be able to get the result.
    var result2: sync string;
    begin {
        sleep(2);
        result2.writeXF("result 2");
    }

    timer.clear();
    timer.start();
    
    while (timer.elapsed() < 3.0) {
        if result2.isFull {
            writeln(result2.readFF());
            break;
        }
        sleep(0.1);
    }
    
    if (!result2.isFull) {
        writeln("timeout 2");
    }
}

Running this program shows the first operation timing out and the second succeeding.

$ chpl timeouts.chpl
$ ./timeouts
timeout 1
result 2

In this Chapel version:

1. We use the Time module for timing and sleeping, and the IO module for output.

2. Instead of channels, we use sync variables (sync string) to communicate between tasks.

3. We create separate tasks using the begin statement, which is Chapel’s way of spawning asynchronous tasks (similar to goroutines).

4. We implement the timeout using a Timer and a while loop, checking periodically if the result is available.

5. The isFull method checks if a value has been written to the sync variable, and readFF() reads the value (blocking if necessary).

6. We use sleep() both to simulate the delay in the “external” calls and to avoid busy waiting in our timeout loops.

This approach provides similar functionality to the original example, demonstrating how to implement timeouts in Chapel. However, it’s worth noting that Chapel’s concurrency model is different from Go’s, so the exact semantics and best practices may differ.