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                ## Chapter 11. Concurrency(并發) ### Item 84: Don’t depend on the thread scheduler(不要依賴線程調度器) When many threads are runnable, the thread scheduler determines which ones get to run and for how long. Any reasonable operating system will try to make this determination fairly, but the policy can vary. Therefore, well-written programs shouldn’t depend on the details of this policy. **Any program that relies on the thread scheduler for correctness or performance is likely to be nonportable.** 當許多線程可以運行時,線程調度器決定哪些線程可以運行以及運行多長時間。任何合理的操作系統都會嘗試公平地做出這個決定,但是策略可能會有所不同。因此,編寫良好的程序不應該依賴于此策略的細節。**任何依賴線程調度器來保證正確性或性能的程序都可能是不可移植的。** The best way to write a robust, responsive, portable program is to ensure that the average number of runnable threads is not significantly greater than the number of processors. This leaves the thread scheduler with little choice: it simply runs the runnable threads till they’re no longer runnable. The program’s behavior doesn’t vary too much, even under radically different thread-scheduling policies. Note that the number of runnable threads isn’t the same as the total number of threads, which can be much higher. Threads that are waiting are not runnable. 編寫健壯、響應快、可移植程序的最佳方法是確保可運行線程的平均數量不顯著大于處理器的數量。這使得線程調度器幾乎沒有選擇:它只運行可運行線程,直到它們不再可運行為止。即使在完全不同的線程調度策略下,程序的行為也沒有太大的變化。注意,可運行線程的數量與線程總數不相同,后者可能更高。正在等待的線程不可運行。 The main technique for keeping the number of runnable threads low is to have each thread do some useful work, and then wait for more. **Threads should not run if they aren’t doing useful work.** In terms of the Executor Framework (Item 80), this means sizing thread pools appropriately [Goetz06, 8.2] and keeping tasks short, but not too short, or dispatching overhead will harm performance. 保持可運行線程數量低的主要技術是讓每個線程做一些有用的工作,然后等待更多的工作。**如果線程沒有做有用的工作,它們就不應該運行。** 對于 Executor 框架([Item-80](/Chapter-11/Chapter-11-Item-80-Prefer-executors,-tasks,-and-streams-to-threads.md)),這意味著適當調整線程池的大小 [Goetz06, 8.2],并保持任務短小(但不要太短),否則分派開銷依然會損害性能。 Threads should not busy-wait, repeatedly checking a shared object waiting for its state to change. Besides making the program vulnerable to the vagaries of the thread scheduler, busy-waiting greatly increases the load on the processor, reducing the amount of useful work that others can accomplish. As an extreme example of what not to do, consider this perverse reimplementation of CountDownLatch: 線程不應該處于循環檢查共享對象狀態變化。除了使程序容易受到線程調度器變化無常的影響之外,循環檢查狀態變化還大大增加了處理器的負載,還影響其他線程獲取處理器進行工作。作為反面的極端例子,考慮一下 CountDownLatch 的不正確的重構實現: ``` // Awful CountDownLatch implementation - busy-waits incessantly! public class SlowCountDownLatch { private int count; public SlowCountDownLatch(int count) { if (count < 0) throw new IllegalArgumentException(count + " < 0"); this.count = count; } public void await() { while (true) { synchronized(this) { if (count == 0) return; } } } public synchronized void countDown() { if (count != 0) count--; } } ``` On my machine, SlowCountDownLatch is about ten times slower than Java’s CountDownLatch when 1,000 threads wait on a latch. While this example may seem a bit far-fetched, it’s not uncommon to see systems with one or more threads that are unnecessarily runnable. Performance and portability are likely to suffer. 在我的機器上,當 1000 個線程等待一個鎖存器時,SlowCountDownLatch 的速度大約是 Java 的 CountDownLatch 的 10 倍。雖然這個例子看起來有點牽強,但是具有一個或多個不必要運行的線程的系統并不少見。性能和可移植性可能會受到影響。 When faced with a program that barely works because some threads aren’t getting enough CPU time relative to others, **resist the temptation to “fix” the program by putting in calls to Thread.yield.** You may succeed in getting the program to work after a fashion, but it will not be portable. The same yield invocations that improve performance on one JVM implementation might make it worse on a second and have no effect on a third. **Thread.yield has no testable semantics.** A better course of action is to restructure the application to reduce the number of concurrently runnable threads. 當面對一個幾乎不能工作的程序時,而原因是由于某些線程相對于其他線程沒有獲得足夠的 CPU 時間,那么 **通過調用 `Thread.yield` 來「修復」程序** 你也許能勉強讓程序運行起來,但它是不可移植的。在一個 JVM 實現上提高性能的相同的 yield 調用,在一些JVM 實現上可能會使性能變差,而在其他 JVM 實現上可能沒有任何影響。**`Thread.yield` 沒有可測試的語義。** 更好的做法是重構應用程序,以減少并發運行線程的數量。 A related technique, to which similar caveats apply, is adjusting thread priorities. **Thread priorities are among the least portable features of Java.** It is not unreasonable to tune the responsiveness of an application by tweaking a few thread priorities, but it is rarely necessary and is not portable. It is unreasonable to attempt to solve a serious liveness problem by adjusting thread priorities. The problem is likely to return until you find and fix the underlying cause. 一個相關的技術是調整線程優先級,類似的警告也適用于此技術,即,線程優先級是 Java 中最不可移植的特性之一。通過調整線程優先級來調優應用程序的響應性并非不合理,但很少情況下是必要的,而且不可移植。試圖通過調整線程優先級來解決嚴重的活性問題是不合理的。在找到并修復潛在原因之前,問題很可能會再次出現。 In summary, do not depend on the thread scheduler for the correctness of your program. The resulting program will be neither robust nor portable. As a corollary, do not rely on Thread.yield or thread priorities. These facilities are merely hints to the scheduler. Thread priorities may be used sparingly to improve the quality of service of an already working program, but they should never be used to “fix” a program that barely works. 總之,不要依賴線程調度器來判斷程序的正確性。生成的程序既不健壯也不可移植。因此,不要依賴 `Thread.yield` 或線程優先級。這些工具只是對調度器的提示。線程優先級可以少量地用于提高已經工作的程序的服務質量,但絕不應該用于「修復」幾乎不能工作的程序。 --- **[Back to contents of the chapter(返回章節目錄)](/Chapter-11/Chapter-11-Introduction.md)** - **Previous Item(上一條目):[Item 83: Use lazy initialization judiciously(明智地使用延遲初始化)](/Chapter-11/Chapter-11-Item-83-Use-lazy-initialization-judiciously.md)** - **Next Item(下一條目):[Chapter 12 Introduction(章節介紹)](/Chapter-12/Chapter-12-Introduction.md)**
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