Java Iterator Concurrency

Question

I'm trying to loop over a Java iterator concurrently, but am having troubles with the best way to do this.

Here is what I have where I don't try to do anything concurrently.

Long l;    
Iterator<Long> i = getUserIDs();

while (i.hasNext()) {
    l = i.next();

    someObject.doSomething(l);
    anotheObject.doSomething(l);
}

There should be no race conditions between the things I'm doing on the non iterator objects, so I'm not too worried about that. I'd just like to speed up how long it takes to loop through the iterator by not doing it sequentially.

Thanks in advance.

score 6 · Accepted Answer · answered Feb 26 '12 at 17:29

One solution is to use an executor to parallelise your work.

Simple example:

ExecutorService executor = Executors.newCachedThreadPool();

Iterator<Long> i = getUserIDs();
while (i.hasNext()) {
    final Long l = i.next();

    Runnable task = new Runnable() {
        public void run() {
            someObject.doSomething(l);
            anotheObject.doSomething(l);
        }
    }

    executor.submit(task);
}

executor.shutdown();

This will create a new thread for each item in the iterator, which will then do the work. You can tune how many threads are used by using a different method on the Executors class, or subdivide the work as you see fit (e.g. a different Runnable for each of the method calls).

thkala · Answer 2 · 2012-02-26T17:44:54.410

A can offer two possible approaches:

Use a thread pool and dispatch the items received from the iterator to a set of processing threads. This will not accelerate the iterator operations themselves, since those would still happen in a single thread, but it will parallelize the actual processing.
Depending on how the iteration is created, you might be able to split the iteration process to multiple segments, each to be processed by a separate thread via a different Iterator object. For an example, have a look at the List.sublist(int fromIndex, int toIndex) and List.listIterator(int index) methods.

This would allow the iterator operations to happen in parallel, but it is not always possible to segment the iteration like this, usually due to the simple fact that the items to be iterated over are not immediately available.
As a bonus trick, if the iteration operations are expensive or slow, such as those required to access a database, you might see a throughput improvement if you separate them out to a separate thread that will use the iterator to fill in a BlockingQueue. The dispatcher thread will then only have to access the queue, without waiting on the iterator object to retrieve the next item.

The most important advice in this case is this: "Use your profiler", usually to be followed by "Do not optimise prematurely". By using a profiler, such as VisualVM, you should be able to ascertain the exact cause of any performance issues, without taking shots in the dark.

score 1 · Answer 3 · answered Feb 26 '12 at 17:32

If you are using Java 7, you can use the new fork/join; see the tutorial.

Not only does it split automatically the tasks among the threads, but if some thread finishes its tasks earlier than the other threads, it "steals" some tasks from the other threads.

Java Iterator Concurrency

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