序列與併發排程佇列

Swift 3

序列佇列

func serialQueues () {
    let serialQueue = DispatchQueue(label: "com.example.serial") //default queue type is a serial queue
    let start = Date ()
    for i in 0...3 {                                             //launch a bunch of tasks
        serialQueue.async {                                      //run tasks on a background thread, using our serial queue
            sleep(2)                                             //do some long task eg webservice or database lookup
            let timeTaken = Date().timeIntervalSince(start)
            print("serial long task \(i) done! total time taken: \(timeTaken)")
        }
    }
}

輸出示例：

serial long task 0 done! total time taken: 2.07241100072861
serial long task 1 done! total time taken: 4.16347700357437
serial long task 2 done! total time taken: 6.23209798336029
serial long task 3 done! total time taken: 8.30682599544525

併發佇列

func concurrentQueues () {
    let concurrentQueue = DispatchQueue(label: "com.example.concurrent", attributes: .concurrent) //explicitly specify the queue to be a concurrent queue
    let start = Date ()
    for i in 0...3 {            //launch a bunch of tasks
        concurrentQueue.async { //run tasks on a background thread, using our concurrent queue
            sleep(2)            //do some long task eg webservice or database lookup
            let timeTaken = Date().timeIntervalSince(start)
            print("concurrent long task \(i) done! total time taken: \(timeTaken)")
        }
    }
}

輸出示例：

concurrent long task 3 done! total time taken: 2.07092100381851
concurrent long task 0 done! total time taken: 2.07087397575378
concurrent long task 2 done! total time taken: 2.07086700201035
concurrent long task 1 done! total time taken: 2.07089096307755

討論

從上面的示例中可以看出，序列佇列將按照提交到佇列的順序完成每個任務。每個任務將在執行前等待上一個任務完成。對於併發佇列，每個任務不等待佇列中的其他任務並儘快執行; 優點是佇列上的所有任務將在不同的執行緒上同時執行，這使得併發佇列比序列佇列花費的時間更少。

如果任務執行順序不重要，請始終使用併發佇列以獲得最佳效率。