Question

I’ve just started working with threads in java. I have a simple algorithm that does a lot of calculations. What I need to do is to divide those calculations among different threads. It looks like this:

while(...) {
      ....
      doCalculations(rangeStart, rangeEnd);
}

And what I want to do is something like this:

while(...) {
     ...
     // Notify N threads to start calculations in specific range

     // Wait for them to finish calculating

     // Check results

     ... Repeat

}

Calculating threads don’t have to have a critical section or be synchronized between each other, because they don’t change any shared variables.

What I can’t figure out is how to order threads to start and wait them to finish.

thread[n].start() and thread[n].join() throws an exception.

Thank you!

Answer 1

I use an ExecutorService

private static final int procs = Runtime.getRuntime().availableProcessors();
private final ExecutorService es = new Executors.newFixedThreadPool(procs);

int tasks = ....
int blockSize = (tasks + procss -1) / procs;
List<Future<Results>> futures = new ArrayList<>();

for(int i = 0; i < procs; i++) {
    int start = i * blockSize;
    int end = Math.min(tasks, (i + 1) * blockSize);
    futures.add(es.submit(new Task(start, end));
}

for(Future<Result> future: futures) {
    Result result = future.get();
    // check/accumulate result.
}