Process Array in parallel using GCD

Question

I have a large array that I would like to process by handing slices of it to a few asynchronous tasks. As a proof of concept, I have the written the following code:

Answer 1

I think Nate is right: there are race conditions with the summary variable. To fix it, I used summary's memory directly:

func calcSummary() {
    let group = dispatch_group_create()
    let queue = dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0)

    let summaryMem = UnsafeMutableBufferPointer(start: &summary, count: 10)

    for i in 0 ..< 10 {
        dispatch_group_async(group, queue, {
           let base = i * 50000
           for x in base ..< base + 50000 {
              summaryMem[i] += self.array[x]
           }
        })
    }

    dispatch_group_notify(group, queue, {
        println(self.summary)
    })
}

This works (so far).

EDIT Mike S has a very good point, in his comment below. I have also found this blog post, which sheds some light on the problem.