LMAX Disruptor-什么决定批量大小？

Question

我最近一直在学习LMAX干扰器，并在做一些实验。令我困惑的一件事是EventHandler的onEvent处理程序方法的endOfBatch参数。考虑我的以下代码。首先，我调用的伪消息和使用者类Test1和Test1Worker：

public class Test1 {

}

public class Test1Worker implements EventHandler<Test1>{
    public void onEvent(Test1 event, long sequence, boolean endOfBatch) {
        try{
            Thread.sleep(500);
        }
        catch(Exception e){
            e.printStackTrace();
        }
        System.out.println("Received message with sequence " + sequence + ". "
                + "EndOfBatch = " + endOfBatch);
    }
}

请注意，我设置了500毫秒的延迟，以替代一些实际工作。我还在控制台中打印

然后我的驱动程序类（作为生产者）被称为DisruptrTest：

public class DisruptorTest {

    private static Disruptor<Test1> bus1;

    private static ExecutorService test1Workers;

    public static void main(String[] args){             
        test1Workers = Executors.newFixedThreadPool(1);

        bus1 = new Disruptor<Test1>(new Test1Factory(), 8, test1Workers);           
        bus1.handleEventsWith(new Test1Worker());
        RingBuffer<Test1> buf1 = bus1.start();

        for (int i = 0; i < 10; i++){
            long a = System.currentTimeMillis();
            long next = buf1.next();
            long b = System.currentTimeMillis();
            System.out.println("Delay for claiming slot " + i + " is "+ (b - a));
            try {
                Test1 message = buf1.get(next);
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                buf1.publish(next);
            }
        }
    }

    public static class Test1Factory implements EventFactory<Test1> {
        public Test1 newInstance() {
            return new Test1();
        }

    }   
}

在这里，在初始化所需的内容之后，我将10条消息馈送到环形缓冲区（缓冲区大小8），并尝试监视两件事-生产者在环形缓冲区中申请下一个插槽的延迟，以及消费者端带有序列号的消息，以及特定序列是否被视为批次结束。

现在，有趣的是，处理每条消息需要500毫秒的延迟，这就是我得到的输出：

Delay for claiming slot 0 is 0
Delay for claiming slot 1 is 0
Delay for claiming slot 2 is 0
Delay for claiming slot 3 is 0
Delay for claiming slot 4 is 0
Delay for claiming slot 5 is 0
Delay for claiming slot 6 is 0
Delay for claiming slot 7 is 0
Received message with sequence 0. EndOfBatch = true
Delay for claiming slot 8 is 505
Received message with sequence 1. EndOfBatch = false
Received message with sequence 2. EndOfBatch = false
Received message with sequence 3. EndOfBatch = false
Received message with sequence 4. EndOfBatch = false
Received message with sequence 5. EndOfBatch = false
Received message with sequence 6. EndOfBatch = false
Received message with sequence 7. EndOfBatch = true
Delay for claiming slot 9 is 3519
Received message with sequence 8. EndOfBatch = true
Received message with sequence 9. EndOfBatch = true

但是，如果取消500 ms等待时间，我会得到以下结果：

Delay for claiming slot 0 is 0
Delay for claiming slot 1 is 0
Received message with sequence 0. EndOfBatch = true
Received message with sequence 1. EndOfBatch = true
Delay for claiming slot 2 is 0
Received message with sequence 2. EndOfBatch = true
Delay for claiming slot 3 is 0
Received message with sequence 3. EndOfBatch = true
Delay for claiming slot 4 is 0
Received message with sequence 4. EndOfBatch = true
Delay for claiming slot 5 is 0
Received message with sequence 5. EndOfBatch = true
Delay for claiming slot 6 is 0
Received message with sequence 6. EndOfBatch = true
Delay for claiming slot 7 is 0
Received message with sequence 7. EndOfBatch = true
Delay for claiming slot 8 is 1
Delay for claiming slot 9 is 0
Received message with sequence 8. EndOfBatch = false
Received message with sequence 9. EndOfBatch = true  

因此，某条消息是否被视为在批的末尾（即，批的大小）似乎受到消费者消息处理延迟的影响。也许我在这里很愚蠢，但这就是我应该做的吗？这背后的原因是什么？通常是什么决定了批量大小？提前谢谢。如果我的问题有什么不清楚的地方，请告诉我。

Answer 1

批量大小仅由可用元素的数量决定。因此，如果此时有更多的元素可用，那么它将包含在批次中。例如，若Disruptor调用您的代码，而队列中只有一个元素，那个么您将得到一个endOfBatch=true的调用。若队列中有8个元素，那个么它将收集所有8个元素并在单个批次中发送它们。

您可以在下面的代码中看到，队列中的#个“可用”条目被提取出来，这可能比“下一个”条目多得多。例如，您当前为5，正在等待插槽6，然后3个事件到达，可用的事件数为8，您将在一批中收到多个呼叫（对于6、7、8）。

https://github.com/LMAX-Exchange/disruptor/blob/master/src/main/java/com/lmax/disruptor/BatchEventProcessor.java#L124

final long availableSequence = sequenceBarrier.waitFor(nextSequence);
while (nextSequence <= availableSequence)
{
    event = dataProvider.get(nextSequence);
    eventHandler.onEvent(event, nextSequence, nextSequence == availableSequence);
    nextSequence++;
}

关于元素9处的500ms暂停，请注意，中断器是使用环形缓冲区构建的，并且您已将缓冲区中的插槽数指定为8（请参见此处的第二个参数）：

bus1 = new Disruptor<Test1>(new Test1Factory(), 8, test1Workers);  

如果并非所有使用者都已使用某个元素，并且ringbuffer已满（全部8个元素已满），则生产者将被阻止向缓冲区发布新事件。您可以尝试增加缓冲区大小，比如200万个对象，或者确保您的消费者比生产者快，这样队列就不会填满（删除睡眠，您已经演示过了）。