運行多線程代碼時出現內存不足異常

Question

我正在開展一個項目，我將在其中使用不同的Bundles。 讓我們舉個例子，假設我有5個Bundle，每個bundle都有一個方法名稱process 。

現在，我正在使用下面的多線程代碼並行調用所有這5個bundle的process方法。

但不知何故，每當我運行下面的多線程代碼時，它總是會讓我出現內存異常。 但是，如果我按順序運行它意味着，逐個調用進程方法，那么它不會給我任何Out Of memory異常。

以下是代碼 -

public void callBundles(final Map<String, Object> eventData) {

    // Three threads: one thread for the database writer, two threads for the plugin processors
    final ExecutorService executor = Executors.newFixedThreadPool(3);

    final Map<String, String> outputs = (Map<String, String>)eventData.get(Constants.EVENT_HOLDER);

    for (final BundleRegistration.BundlesHolderEntry entry : BundleRegistration.getInstance()) {
        executor.submit(new Runnable () {
            public void run() {
                try {
                    final Map<String, String> response = entry.getPlugin().process(outputs);

                    //process the response and update database.
                    System.out.println(response);
                } catch (Exception e) {
                    e.printStackTrace();
                }               
            }
        });
    }
}

以下是例外情況，每當我在多線程代碼上運行時，我都會得到。

JVMDUMP006I Processing dump event "systhrow", detail "java/lang/OutOfMemoryError" - please wait.
JVMDUMP032I JVM requested Heap dump using 'S:\GitViews\Stream\goldseye\heapdump.20130904.175256.12608.0001.phd' in response to an event
JVMDUMP010I Heap dump written to S:\GitViews\Stream\goldseye\heapdump.20130904.175256.12608.0001.phd
JVMDUMP032I JVM requested Java dump using 'S:\GitViews\Stream\goldseye\javacore.20130904.175256.12608.0002.txt' in response to an event
UTE430: can't allocate buffer
UTE437: Unable to load formatStrings for j9mm
JVMDUMP010I Java dump written to S:\GitViews\Stream\goldseye\javacore.20130904.175256.12608.0002.txt
JVMDUMP032I JVM requested Snap dump using 'S:\GitViews\Stream\goldseye\Snap.20130904.175256.12608.0003.trc' in response to an event
UTE001: Error starting trace thread for "Snap Dump Thread": -1
JVMDUMP010I Snap dump written to S:\GitViews\Stream\goldseye\Snap.20130904.175256.12608.0003.trc
JVMDUMP013I Processed dump event "systhrow", detail "java/lang/OutOfMemoryError".
ERROR: Bundle BullseyeModellingFramework [1] EventDispatcher: Error during dispatch. (java.lang.OutOfMemoryError: Failed to create a thread: retVal -1073741830, errno 12)
java.lang.OutOfMemoryError: Failed to create a thread: retVal -1073741830, errno 12

JVMDUMP006I Processing dump event "systhrow", detail "java/lang/OutOfMemoryError" - please wait.
JVMDUMP032I JVM requested Heap dump using 'S:\GitViews\Stream\goldseye\heapdump.20130904.175302.12608.0004.phd' in response to an event
JVMDUMP010I Heap dump written to S:\GitViews\Stream\goldseye\heapdump.20130904.175302.12608.0004.phd
JVMDUMP032I JVM requested Java dump using 'S:\GitViews\Stream\goldseye\javacore.20130904.175302.12608.0005.txt' in response to an event

我在我的eclipse中使用JDK1.6.0_26作為已安裝的JRE。

Answer 1

每次調用callBundles()都會通過創建一個自己的執行程序來創建一個新的callBundles() 。 每個線程都有自己的堆棧空間！ 因此，如果你說你啟動JVM，第一次調用將創建三個線程，總和為3M堆（1024k是64位JVM的默認堆棧大小），下一次調用另一個3M等1000個調用/ s將需要3GB / S！

第二個問題是你永遠不會shutdown()創建的執行器服務，所以線程將繼續運行，直到垃圾收集器刪除執行程序（ finalize()也調用shutdown() ）。 但是GC永遠不會清除堆棧內存，所以如果堆棧內存是問題並且堆未滿，GC將永遠無法幫助！

你需要使用一個 ExecutorService ，比方說10到30個線程或一個帶有3-30個緩存線程和LinkedBlockingQueue的自定義ThreadPoolExecutor 。 如果可能，請在應用程序停止之前調用服務上的shutdown() 。

檢查應用程序的物理RAM，加載和響應時間，以調整參數堆大小，最大線程數並保持池中線程的活動時間。 查看代碼的其他鎖定部分（數據庫連接池的大小，......）以及服務器的CPU /核心數。 線程池大小的起始點可能是CPU數/核數加1.，更多I / O等待更有用。

Answer 2

主要問題是你沒有真正正確地使用線程池。 如果所有“進程”線程具有相同的優先級，則沒有充分的理由不創建一個大型線程池並將所有Runnable任務提交給它。 注意 - 在這種情況下，“大”是通過實驗和分析確定的：調整它直到您在速度和記憶方面的表現符合您的預期。

這是我所描述的一個例子：

// Using 10000 purely as a concrete example - you should define the correct number
public static final LARGE_NUMBER_OF_THREADS = 10000;
// Elsewhere in code, you defined a static thread pool
public static final ExecutorService EXECUTOR = 
    Executors.newFixedThreadPool(LARGE_NUMBER_OF_THREADS);

public void callBundles(final Map<String, Object> eventData) {

final Map<String, String> outputs = 
    (Map<String, String>)eventData.get(Constants.EVENT_HOLDER);

for (final BundleRegistration.BundlesHolderEntry entry : BundleRegistration.getInstance()) {
    // "Three threads: one thread for the database writer, 
    // two threads for the plugin processors"
    // so you'll need to repeat this future = E.submit() pattern two more times
    Future<?> processFuture = EXECUTOR.submit(new Runnable() {
        public void run() {
            final Map<String, String> response = 
                entry.getPlugin().process(outputs);
            //process the response and update database.
            System.out.println(response);
        }
    }
    // Note, I'm catching the exception out here instead of inside the task
    // This also allows me to force order on the three component threads
    try {
        processFuture.get();
    } catch (Exception e) {
        System.err.println("Should really do something more useful");
        e.printStackTrace();
    }
    // If you wanted to ensure that the three component tasks run in order, 
    // you could future = executor.submit(); future.get(); 
    // for each one of them
}

為了完整起見，您還可以使用緩存的線程池來避免重復創建短期線程。 但是，如果您已經擔心內存消耗，那么固定池可能會更好。

當你進入Java 7時，你可能會發現Fork-Join比一系列Futures更好。 不過，最適合您的需求。