并行添加到数组

Question

我正在使用 Grand Central Dispatch 将一个数组的元素转换为另一个数组。我在源数组上调用 dispatch_apply，将其转换为零个或多个项目，然后将它们添加到目标数组。这是一个简化的例子：

let src = Array(0..<1000)
var dst = [UInt32]()

let queue = dispatch_queue_create("myqueue", DISPATCH_QUEUE_CONCURRENT)
dispatch_apply(src.count, queue) { i in
    dst.append(arc4random_uniform(UInt32(i))) // <-- potential error here
}

print(dst)

我有时在 append 行上遇到错误。错误始终是以下之一：

1. malloc: *** error for object 0x107508f00: pointer being freed was not allocated
2. fatal error: UnsafeMutablePointer.destroy with negative count
3. fatal error: Can't form Range with end < start

我猜这是因为 append 不是线程安全的。我做错了什么以及如何解决？

Answer 1

如您所见，Swift 可能会重新定位数组以提高内存效率。当您运行多个 append 时，它必然会发生。我的猜测是，与您的转换相比，附加到数组是一种成本非常低的操作。您可以创建一个串行队列来扩展 dst 数组：

let src = Array(0..<1000)
var dst = [UInt32]()

let queue = dispatch_queue_create("myqueue", DISPATCH_QUEUE_CONCURRENT)
let serialQueue = dispatch_queue_create("mySerialQueue", DISPATCH_QUEUE_SERIAL)
let serialGroup = dispatch_group_create()

dispatch_apply(src.count, queue) { i in
    let result = arc4random_uniform(UInt32(i)) // Your long-running transformation here
    dispatch_group_async(serialGroup, serialQueue) {
        dst.append(result)
    }
}

// Wait until all append operations are complete
dispatch_group_wait(serialGroup, DISPATCH_TIME_FOREVER)

Answer 2

如果有人对 Swift 4（我想是 Swift 3 之后）解决方案感兴趣（方法略有不同）：

let source = Array(0..<1000)

// If inside class/struct, make this lazy or execute the block inside init
var destination: [UInt32] = {
    var copy = Array<UInt32>.init(repeating: 0, count: source.count)

    DispatchQueue.concurrentPerform(iterations: source.count) { i in
        // Do stuff to get the i'th element 
        copy[i] = UInt32.random(in: 0..<i) 
    }

    return copy
}()

如果您选择避免数组大小与并发操作数之间的联系：

（例如，您可以选择 0..<500 与 500..<1000 并行进行两次迭代）

let iterations = 2  

DispatchQueue.concurrentPerform(iterations: iterations) { index in

    // First iteration would start at 0 whereas second (index = 1) starts at 500
    let start = index * source.count / iterations

    // First and second iterations will have 500 and 1000 upper bounds respectively 
    let end = (index + 1) * source.count / iterations

    for i in start..<end {
        // Do stuff to get the i'th element
        copy[i] = UInt32.random(in: 0..<source.count)
    }
}

并行添加到数组

Adding to array in parallel

grand-central-dispatch

swift

swift2