Is there a way to force parallelStream() to go parallel?

Question

If the input size is too small the library automatically serializes the execution of the maps in the stream, but this automation doesn't and can't take in account how heavy is the map operation. Is there a way to force parallelStream() to actually parallelize CPU heavy maps?

Answer 1

There seems to be a fundamental misunderstanding. The linked Q&A discusses that the stream apparently doesn’t work in parallel, due to the OP not seeing the expected speedup. The conclusion is that there is no benefit in parallel processing if the workload is too small, not that there was an automatic fallback to sequential execution.

It’s actually the opposite. If you request parallel, you get parallel, even if it actually reduces the performance. The implementation does not switch to the potentially more efficient sequential execution in such cases.

So if you are confident that the per-element workload is high enough to justify the use of a parallel execution regardless of the small number of elements, you can simply request a parallel execution.

As can easily demonstrated:

Stream.of(1, 2).parallel()
      .peek(x -> System.out.println("processing "+x+" in "+Thread.currentThread()))
      .forEach(System.out::println);

On Ideone, it prints

processing 2 in Thread[main,5,main]
2
processing 1 in Thread[ForkJoinPool.commonPool-worker-1,5,main]
1

but the order of messages and details may vary. It may even be possible that in some environments, both task may happen to get executed by the same thread, if it can steel the second task before another thread gets started to pick it up. But of course, if the tasks are expensive enough, this won’t happen. The important point is that the overall workload has been split and enqueued to be potentially picked up by other worker threads.

If execution by a single thread happens in your environment for the simple example above, you may insert simulated workload like this:

Stream.of(1, 2).parallel()
      .peek(x -> System.out.println("processing "+x+" in "+Thread.currentThread()))
      .map(x -> {
           LockSupport.parkNanos("simulated workload", TimeUnit.SECONDS.toNanos(3));
           return x;
        })
      .forEach(System.out::println);

Then, you may also see that the overall execution time will be shorter than “number of elements”דprocessing time per element” if the “processing time per element” is high enough.

---

Update: the misunderstanding might be cause by Brian Goetz’ misleading statement: “In your case, your input set is simply too small to be decomposed”.

It must be emphasized that this is not a general property of the Stream API, but the Map that has been used. A HashMap has a backing array and the entries are distributed within that array depending on their hash code. It might be the case that splitting the array into n ranges doesn’t lead to a balanced split of the contained element, especially, if there are only two. The implementors of the HashMap’s Spliterator considered searching the array for elements to get a perfectly balanced split to be too expensive, not that splitting two elements was not worth it.

Since the HashMap’s default capacity is 16 and the example had only two elements, we can say that the map was oversized. Simply fixing that would also fix the example:

long start = System.nanoTime();

Map<String, Supplier<String>> input = new HashMap<>(2);
input.put("1", () -> {
    System.out.println(Thread.currentThread());
    LockSupport.parkNanos("simulated workload", TimeUnit.SECONDS.toNanos(2));
    return "a";
});
input.put("2", () -> {
    System.out.println(Thread.currentThread());
    LockSupport.parkNanos("simulated workload", TimeUnit.SECONDS.toNanos(2));
    return "b";
});
Map<String, String> results = input.keySet()
        .parallelStream().collect(Collectors.toConcurrentMap(
    key -> key,
    key -> input.get(key).get()));

System.out.println("Time: " + TimeUnit.NANOSECONDS.toMillis(System.nanoTime()- start));

on my machine, it prints

Thread[main,5,main]
Thread[ForkJoinPool.commonPool-worker-1,5,main]
Time: 2058

The conclusion is that the Stream implementation always tries to use parallel execution, if you request it, regardless of the input size. But it depends on the input’s structure how well the workload can be distributed to the worker threads. Things could be even worse, e.g. if you stream lines from a file.

If you think that the benefit of a balanced splitting is worth the cost of a copying step, you could also use new ArrayList<>(input.keySet()).parallelStream() instead of input.keySet().parallelStream(), as the distribution of elements within ArrayList always allows a perflectly balanced split.