我试图围绕如何使用GCD来并行化和加速蒙特卡罗模拟.大部分/全部简单示例都是针对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？
可以使用dispatch_apply()完成“多线程迭代”：
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,这使得它在调用时非常慢
从几个并行的线程,见Performance of concurrent code using dispatch_group_async is MUCH slower than single-threaded version.从那里给出的答案,
rand_r()(每个线程都有单独的种子)似乎是更快的选择.
>不得同时从多个线程修改结果变量获胜者.
您可以使用数组,而不是每个线程更新其自己的元素和结果
之后加入.在https://stackoverflow.com/a/26790019/1187415中描述了线程安全的方法.
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,+)
println(winner)