在Swift中使用Grand Central Dispatch来并行化并加速"for"循环？

Question

我试图围绕如何使用GCD来并行化和加速蒙特卡罗模拟.大多数/所有简单示例都是针对Objective C提供的,我真的需要一个Swift的简单示例,因为Swift是我的第一个"真正的"编程语言.

Swift中蒙特卡罗模拟的最小工作版本将是这样的:

import Foundation

import Cocoa
var winner = 0
var j = 0
var i = 0
var chance = 0
var points = 0
for j=1;j<1000001;++j{
    var ability = 500

    var player1points = 0

    for i=1;i<1000;++i{
        chance = Int(arc4random_uniform(1001))
        if chance<(ability-points) {++points}
        else{points = points - 1}
    }
    if points > 0{++winner}
}
    println(winner)

代码可以直接粘贴到xcode 6.1中的命令行程序项目中

最内层的循环不能并行化,因为变量"points"的新值在下一个循环中使用.但最外面的只是运行最里面的模拟1000000次并计算结果,应该是并行化的理想候选者.

所以我的问题是如何使用GCD并行化最外层的for循环？

Answer 1

"多线程迭代"可以通过以下方式完成dispatch_apply():

let outerCount = 100    // # of concurrent block iterations
let innerCount = 10000  // # of iterations within each block

let the_queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_apply(UInt(outerCount), the_queue) { outerIdx -> Void in
    for innerIdx in 1 ... innerCount {
       // ...
    }
}

(你必须弄清楚外部和内部计数之间的最佳关系.)

有两件事需要注意:

• arc4random()使用内部互斥锁,这使得从多个并行线程调用时非常慢,请参阅使用dispatch_group_async执行并发代码的性能比单线程版本慢很多.从那里给出的答案, rand_r()(每个线程有单独的种子)似乎是更快的替代方案.

• winner不能同时从多个线程修改结果变量.您可以使用数组,而不是每个线程更新其自己的元素,然后添加结果./sf/answers/1875301361/中描述了一种线程安全的方法.

然后它大致如下:

let outerCount = 100     // # of concurrent block iterations
let innerCount = 10000   // # of iterations within each block

let the_queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

var winners = [Int](count: outerCount, repeatedValue: 0)
winners.withUnsafeMutableBufferPointer { winnersPtr -> Void in

    dispatch_apply(UInt(outerCount), the_queue) { outerIdx -> Void in
        var seed = arc4random() // seed for rand_r() in this "thread"

        for innerIdx in 1 ... innerCount {
            var points = 0
            var ability = 500

            for i in 1 ... 1000 {
                let chance = Int(rand_r(&seed) % 1001)
                if chance < (ability-points) { ++points }
                else {points = points - 1}
            }
            if points > 0 {
                winnersPtr[Int(outerIdx)] += 1
            }
        }
    }
}

// Add results:
let winner = reduce(winners, 0, +)
println(winner)