生产者/消费者只允许一个线程完成工作

Question

我正在尝试使用线程和NSCondition在Swift 2中实现生产者/消费者类。到目前为止，我已经提出了这个代码：

import Foundation

class ProducterConsumer {

    private let numberOfProducts = 1000
    private var products: [Int] = []
    private var condition = NSCondition()

    private func producer() {
        for var i = 0; i < numberOfProducts; ++i {
            condition.lock()
            products.append(i)
            condition.broadcast()
            condition.unlock()
        }
    }

    private func consumer(id: Int) {
        while true {
            condition.lock()
            while products.count == 0 {
                condition.wait()
            }
            let product = products.popLast()!
            print("[\(id)] takes \(product)")
            condition.unlock()
        }
    }

    func runAndWait() {
        let queue = NSOperationQueue()
        for var i = 0; i < 2; ++i {
            queue.addOperationWithBlock {
                self.consumer(i)
            }
        }
        queue.addOperationWithBlock {
            self.producer()
        }
        queue.waitUntilAllOperationsAreFinished()
    }

}

let pc = ProducterConsumer()
pc.runAndWait()

基本上我正在使用操作队列创建3个新线程：1个生成器填充Int的共享数组（从0到1000）和2个弹出值并显示它们的消费者。

结果显示只有消费者线程＃0“有效”并且似乎是弹出值。我无法弄清楚为什么消费者线程＃1不会在控制台中显示。我预计两个消费者之间或多或少会有50％的工作时间。我缺少什么？

[0] takes 0
[0] takes 1
...
[0] takes 998
[0] takes 999

谢谢！

Answer 1

问题似乎是lazy var is not thread-safe，您的代码甚至在我的测试中崩溃，并显示错误消息，如

*** -[NSCondition dealloc]: condition (<NSCondition: 0x10080b310> '(null)') deallocated while still in use
*** Break on _NSLockError() to debug.
*** -[NSCondition wait]: condition (<NSCondition: 0x100e00920> '(null)') not locked
*** -[NSCondition unlock]: condition (<NSCondition: 0x100e00920> '(null)') unlocked from thread which did not lock it

如您所见，（至少）有两种不同的NSCondition 实例

如果您将属性更改为

private var condition = NSCondition()

然后在ProducterConsumer()时创建单个条件变量 实例已创建，一切都按预期运行。

另一个问题是

for var i = 0; i < 2; ++i {
    queue.addOperationWithBlock {
        self.consumer(i)
    }
}

该块通过引用捕获变量i 。因此 即使是不同的运行线程也可以报告相同的数字i。 要按值捕获变量，请使用显式捕获列表：

for var i = 0; i < 5; ++i {
    queue.addOperationWithBlock { [i] in
        self.consumer(i)
    }
}

我还建议在多线程程序中使用NSLog()进行调试输出，因为

与NSLog()和相比，
• print()是线程安全的

• NSLog()打印进程ID和线程ID，以便您可以 验证输出是否来自不同的线程。例如：

  2015-11-09 15:21:43.711 MyProgram[8569:537106] [0] takes 19
  2015-11-09 15:21:43.788 MyProgram[8569:537108] [2] takes 18
  

Answer 2

我稍微修改你的消费者阻止（NSOperation）

private func consumer(id: Int) {
        while true {
            condition.lock()
            while products.count == 0 {
                condition.wait()
            }
            let product = products.popLast()!
            condition.unlock()
            NSLog("[\(id)] takes \(product)")
            usleep(500000)
        }
    }

检查NSLog报告的内容

2015-11-09 20:26:06.249 Untitled Page 10[90128:4829072] [0] takes 0
2015-11-09 20:26:06.262 Untitled Page 10[90128:4829076] [1] takes 1
2015-11-09 20:26:06.760 Untitled Page 10[90128:4829072] [0] takes 9
2015-11-09 20:26:06.783 Untitled Page 10[90128:4829076] [1] takes 8
2015-11-09 20:26:07.272 Untitled Page 10[90128:4829072] [0] takes 7
2015-11-09 20:26:07.295 Untitled Page 10[90128:4829076] [1] takes 6
2015-11-09 20:26:07.775 Untitled Page 10[90128:4829072] [0] takes 5
2015-11-09 20:26:07.803 Untitled Page 10[90128:4829076] [1] takes 4
2015-11-09 20:26:08.282 Untitled Page 10[90128:4829072] [0] takes 3
2015-11-09 20:26:08.311 Untitled Page 10[90128:4829076] [1] takes 2

特别是消费者消费的价值顺序。

NSCondition对象在给定线程中充当锁和检查点。

您不应该依赖队列语义来确保操作对象的特定执行顺序

产品数量100 消费者数量20 没有延误

2015-11-09 21:16:04.687 Untitled Page 10[90457:4863959] [0] takes 0
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