聊聊Elasticsearch的TaskScheduler

序

本文主要研究一下Elasticsearch的TaskScheduler

TaskScheduler

elasticsearch-7.0.1/libs/nio/src/main/java/org/elasticsearch/nio/TaskScheduler.java

public class TaskScheduler {

    private final PriorityQueue<DelayedTask> tasks = new PriorityQueue<>(Comparator.comparingLong(DelayedTask::getDeadline));

    /**
     * Schedule a task at the defined relative nanotime. When {@link #pollTask(long)} is called with a
     * relative nanotime after the scheduled time, the task will be returned. This method returns a
     * {@link Runnable} that can be run to cancel the scheduled task.
     *
     * @param task to schedule
     * @param relativeNanos defining when to execute the task
     * @return runnable that will cancel the task
     */
    public Runnable scheduleAtRelativeTime(Runnable task, long relativeNanos) {
        DelayedTask delayedTask = new DelayedTask(relativeNanos, task);
        tasks.offer(delayedTask);
        return delayedTask;
    }

    Runnable pollTask(long relativeNanos) {
        DelayedTask task;
        while ((task = tasks.peek()) != null) {
            if (relativeNanos - task.deadline >= 0) {
                tasks.remove();
                if (task.cancelled == false) {
                    return task.runnable;
                }
            } else {
                return null;
            }
        }
        return null;
    }

    long nanosUntilNextTask(long relativeNanos) {
        DelayedTask nextTask = tasks.peek();
        if (nextTask == null) {
            return Long.MAX_VALUE;
        } else {
            return Math.max(nextTask.deadline - relativeNanos, 0);
        }
    }

    private static class DelayedTask implements Runnable {

        private final long deadline;
        private final Runnable runnable;
        private boolean cancelled = false;

        private DelayedTask(long deadline, Runnable runnable) {
            this.deadline = deadline;
            this.runnable = runnable;
        }

        private long getDeadline() {
            return deadline;
        }

        @Override
        public void run() {
            cancelled = true;
        }
    }
}

• TaskScheduler定義了DelayedTask，它實現了Runnable接口，它包含deadline、runnable、cancelled三個屬性
• TaskScheduler定義了DelayedTask類型的PriorityQueue，其comparator爲Comparator.comparingLong(DelayedTask::getDeadline)
• scheduleAtRelativeTime方法將runnable包裝爲delayedTask，而後offer到priorityQueue中；pollTask則peek出來task，若是不爲null則判斷relativeNanos是否大於等於task.deadline，條件成立的話則將其從tasks中移除，而後在cancelled爲false的時候返回task.runnable

SSLChannelContext

elasticsearch-7.0.1/x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/transport/nio/SSLChannelContext.java

public final class SSLChannelContext extends SocketChannelContext {
    //......

    @Override
    public void queueWriteOperation(WriteOperation writeOperation) {
        getSelector().assertOnSelectorThread();
        if (writeOperation instanceof CloseNotifyOperation) {
            sslDriver.initiateClose();
            long relativeNanos = CLOSE_TIMEOUT_NANOS + System.nanoTime();
            closeTimeoutCanceller = getSelector().getTaskScheduler().scheduleAtRelativeTime(this::channelCloseTimeout, relativeNanos);
        } else {
            super.queueWriteOperation(writeOperation);
        }
    }

    private void channelCloseTimeout() {
        closeTimeoutCanceller = DEFAULT_TIMEOUT_CANCELLER;
        setCloseNow();
        getSelector().queueChannelClose(channel);
    }

    //......
}

• SSLChannelContext的queueWriteOperation方法會使用taskScheduler的scheduleAtRelativeTime註冊一個channelCloseTimeout的延時任務

NioSelector

elasticsearch-7.0.1/libs/nio/src/main/java/org/elasticsearch/nio/NioSelector.java

public class NioSelector implements Closeable {

    //......

    public void runLoop() {
        if (runLock.tryLock()) {
            isRunningFuture.complete(null);
            try {
                setThread();
                while (isOpen()) {
                    singleLoop();
                }
            } finally {
                try {
                    cleanupAndCloseChannels();
                } finally {
                    try {
                        selector.close();
                    } catch (IOException e) {
                        eventHandler.selectorException(e);
                    } finally {
                        runLock.unlock();
                        exitedLoop.countDown();
                    }
                }
            }
        } else {
            throw new IllegalStateException("selector is already running");
        }
    }

    void singleLoop() {
        try {
            closePendingChannels();
            preSelect();
            long nanosUntilNextTask = taskScheduler.nanosUntilNextTask(System.nanoTime());
            int ready;
            if (nanosUntilNextTask == 0) {
                ready = selector.selectNow();
            } else {
                long millisUntilNextTask = TimeUnit.NANOSECONDS.toMillis(nanosUntilNextTask);
                // Only select until the next task needs to be run. Do not select with a value of 0 because
                // that blocks without a timeout.
                ready = selector.select(Math.min(300, Math.max(millisUntilNextTask, 1)));
            }
            if (ready > 0) {
                Set<SelectionKey> selectionKeys = selector.selectedKeys();
                Iterator<SelectionKey> keyIterator = selectionKeys.iterator();
                while (keyIterator.hasNext()) {
                    SelectionKey sk = keyIterator.next();
                    keyIterator.remove();
                    if (sk.isValid()) {
                        try {
                            processKey(sk);
                        } catch (CancelledKeyException cke) {
                            eventHandler.genericChannelException((ChannelContext<?>) sk.attachment(),  cke);
                        }
                    } else {
                        eventHandler.genericChannelException((ChannelContext<?>) sk.attachment(),  new CancelledKeyException());
                    }
                }
            }

            handleScheduledTasks(System.nanoTime());
        } catch (ClosedSelectorException e) {
            if (isOpen()) {
                throw e;
            }
        } catch (IOException e) {
            eventHandler.selectorException(e);
        } catch (Exception e) {
            eventHandler.uncaughtException(e);
        }
    }

    private void handleScheduledTasks(long nanoTime) {
        Runnable task;
        while ((task = taskScheduler.pollTask(nanoTime)) != null) {
            try {
                task.run();
            } catch (Exception e) {
                eventHandler.taskException(e);
            }
        }
    }

    //......
}

• NioSelector的runLoop方法調用了singleLoop方法，後者調用了handleScheduledTasks方法，而handleScheduledTasks方法則是從taskScheduler.pollTask，而後執行task.run()

小結

• TaskScheduler定義了DelayedTask，它實現了Runnable接口，它包含deadline、runnable、cancelled三個屬性
• TaskScheduler定義了DelayedTask類型的PriorityQueue，其comparator爲Comparator.comparingLong(DelayedTask::getDeadline)
• scheduleAtRelativeTime方法將runnable包裝爲delayedTask，而後offer到priorityQueue中；pollTask則peek出來task，若是不爲null則判斷relativeNanos是否大於等於task.deadline，條件成立的話則將其從tasks中移除，而後在cancelled爲false的時候返回task.runnable

doc

• TaskScheduler