算法技巧-时间轮（一）
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时间轮（Time Wheel）是一种数据结构，用于实现基于时间的事件调度系统。它被广泛应用于计算机网络、操作系统、分布式系统等领域中。
时间轮概念
时间轮的基本思想是将时间划分为固定大小的时间段，并将这些时间段组成一个环形结构。每个时间段对应一个槽（Slot），槽中保存了在该时间段内需要执行的事件列表。时间轮按照固定的时间间隔（通常为1秒）逐个推进，当时间轮的指针指向某个槽时，就执行该槽中所有事件。
优点
高效处理批量任务
时间轮可以高效的利用线程资源来进行批量化调度，把大批量的调度任务全部都绑定时间轮上，通过时间轮进行所有任务的管理，触发以及运行。
降低时间复杂度
时间轮算法可以将插入和删除操作的时间复杂度都降为O(1)，相较于JDK 提供的 java.util.Timer 和 DelayedQueue 等工具类，其底层实现使用的是堆这种数据结构，存取操作的复杂度都是 O(nlog(n))，无法支持大量的定时任务。
缺点
时间精确度的问题
时间轮调度器的时间的精度可能不是很高，对于精度要求特别高的调度任务可能不太适合。因为时间轮算法的精度取决于时间段“指针”单元的最小粒度大小，比如时间轮的格子是一秒跳一次，那么调度精度小于一秒的任务就无法被时间轮所调度。
可以采用多级时间轮提高精度
宕机后无法恢复重新调度
时间轮任务队列存储在内存中，没有做宕机备份，无法在宕机恢复后重新调度。
支持宕机恢复需要做很多额外操作，可以参考携程QMQ实现
经典实现
DUBBO中时间轮实现
Dubbo 的时间轮实现位于 dubbo-common 模块的 org.apache.dubbo.common.timer 包中
核心接口
主要有下面几个实现时间轮
Timer
接口定义了定时器的基本行为
public interface Timer {
//提交一个定时任务（TimerTask）并返回关联的 Timeout 对象
Timeout newTimeout(TimerTask task, long delay, TimeUnit unit);
Set<Timeout> stop();
boolean isStop();
}
Timeout
定义了一个超时处理器的操作接口，由newTimeout方法返回
public interface Timeout {
//定义了一个超时处理器的操作接口，由newTimeout方法返回
Timer timer();
//返回与此Timeout实例关联的TimerTask对象
TimerTask task();
boolean isExpired();
boolean isCancelled();
boolean cancel();
}
TimerTask
具体任务的基类,Timeout 对象与 TimerTask 对象一一对应，两者的关系类似于线程池返回的 Future 对象与提交到线程池中的任务对象之间的关系。通过 Timeout 对象，我们不仅可以查看定时任务的状态，还可以操作定时任务（例如取消关联的定时任务）
public interface TimerTask {
void run(Timeout timeout) throws Exception;
}
HashedWheelTimeout
HashedWheelTimeout 是 Timeout 接口的唯一实现，是 HashedWheelTimer 的内部类。 HashedWheelTimeout 扮演了两个角色：
- 第一个，时间轮中双向链表的节点，即定时任务 TimerTask 在 HashedWheelTimer 中的容器。
- 第二个，定时任务 TimerTask 提交到 HashedWheelTimer 之后返回的句柄（Handle），用于在时间轮外部查看和控制定时任务。
关键字段和核心方法如下：
private static final class HashedWheelTimeout implements Timeout {
//状态
private static final int ST_INIT = 0;
private static final int ST_CANCELLED = 1;
private static final int ST_EXPIRED = 2;
//状态更新
private static final AtomicIntegerFieldUpdater<HashedWheelTimeout> STATE_UPDATER =
AtomicIntegerFieldUpdater.newUpdater(HashedWheelTimeout.class, "state");
@SuppressWarnings({"unused", "FieldMayBeFinal", "RedundantFieldInitialization"})
private volatile int state = ST_INIT;
private final HashedWheelTimer timer;
// 指实际被调度的任务
private final TimerTask task;
// 指定时任务执行的时间。这个时间是在创建 HashedWheelTimeout 时指定的，计算公式是：currentTime（创建 HashedWheelTimeout 的时间） + delay（任务延迟时间） - startTime（HashedWheelTimer 的启动时间），时间单位为纳秒。
private final long deadline;
//指当前任务剩余的时钟周期数。
long remainingRounds;
HashedWheelTimeout next;
HashedWheelTimeout prev;
HashedWheelBucket bucket;
HashedWheelTimeout(HashedWheelTimer timer, TimerTask task, long deadline) {
this.timer = timer;
this.task = task;
this.deadline = deadline;
}
@Override
public Timer timer() {
return timer;
}
@Override
public TimerTask task() {
return task;
}
@Override
public boolean cancel() {
// only update the state it will be removed from HashedWheelBucket on next tick.
if (!compareAndSetState(ST_INIT, ST_CANCELLED)) {
return false;
}
timer.cancelledTimeouts.add(this);
return true;
}
void remove() {
HashedWheelBucket bucket = this.bucket;
if (bucket != null) {
bucket.remove(this);
} else {
timer.pendingTimeouts.decrementAndGet();
}
}
public boolean compareAndSetState(int expected, int state) {
return STATE_UPDATER.compareAndSet(this, expected, state);
}
public int state() {
return state;
}
@Override
public boolean isCancelled() {
return state() == ST_CANCELLED;
}
@Override
public boolean isExpired() {
return state() == ST_EXPIRED;
}
public void expire() {
if (!compareAndSetState(ST_INIT, ST_EXPIRED)) {
return;
}
try {
task.run(this);
} catch (Throwable t) {
if (logger.isWarnEnabled()) {
logger.warn("An exception was thrown by " + TimerTask.class.getSimpleName() + '.', t);
}
}
}
@Override
public String toString() {
...
}
}
HashedWheelBucket
HashedWheelTimer的内部实现类，HashedWheelBucket 是时间轮中的一个槽，时间轮中的槽实际上就是一个用于缓存和管理双向链表的容器，双向链表中的每一个节点就是一个 HashedWheelTimeout 对象，也就关联了一个 TimerTask 定时任务。
HashedWheelBucket 持有双向链表的首尾两个节点，分别是 head 和 tail 两个字段，再加上每个 HashedWheelTimeout 节点均持有前驱和后继的引用，这样就可以正向或是逆向遍历整个双向链表了。
private static final class HashedWheelBucket {
/**
* Used for the linked-list datastructure
*/
private HashedWheelTimeout head;
private HashedWheelTimeout tail;
/**
* Add {@link HashedWheelTimeout} to this bucket.
*/
void addTimeout(HashedWheelTimeout timeout) {
assert timeout.bucket == null;
timeout.bucket = this;
if (head == null) {
head = tail = timeout;
} else {
tail.next = timeout;
timeout.prev = tail;
tail = timeout;
}
}
/**
* Expire all {@link HashedWheelTimeout}s for the given {@code deadline}.
*/
void expireTimeouts(long deadline) {
HashedWheelTimeout timeout = head;
// process all timeouts
while (timeout != null) {
HashedWheelTimeout next = timeout.next;
if (timeout.remainingRounds <= 0) {
next = remove(timeout);
if (timeout.deadline <= deadline) {
timeout.expire();
} else {
// The timeout was placed into a wrong slot. This should never happen.
throw new IllegalStateException(String.format(
"timeout.deadline (%d) > deadline (%d)", timeout.deadline, deadline));
}
} else if (timeout.isCancelled()) {
next = remove(timeout);
} else {
timeout.remainingRounds--;
}
timeout = next;
}
}
public HashedWheelTimeout remove(HashedWheelTimeout timeout) {
HashedWheelTimeout next = timeout.next;
// remove timeout that was either processed or cancelled by updating the linked-list
if (timeout.prev != null) {
timeout.prev.next = next;
}
if (timeout.next != null) {
timeout.next.prev = timeout.prev;
}
if (timeout == head) {
// if timeout is also the tail we need to adjust the entry too
if (timeout == tail) {
tail = null;
head = null;
} else {
head = next;
}
} else if (timeout == tail) {
// if the timeout is the tail modify the tail to be the prev node.
tail = timeout.prev;
}
// null out prev, next and bucket to allow for GC.
timeout.prev = null;
timeout.next = null;
timeout.bucket = null;
timeout.timer.pendingTimeouts.decrementAndGet();
return next;
}
/**
* Clear this bucket and return all not expired / cancelled {@link Timeout}s.
*/
void clearTimeouts(Set<Timeout> set) {
for (; ; ) {
HashedWheelTimeout timeout = pollTimeout();
if (timeout == null) {
return;
}
if (timeout.isExpired() || timeout.isCancelled()) {
continue;
}
set.add(timeout);
}
}
private HashedWheelTimeout pollTimeout() {
HashedWheelTimeout head = this.head;
if (head == null) {
return null;
}
HashedWheelTimeout next = head.next;
if (next == null) {
tail = this.head = null;
} else {
this.head = next;
next.prev = null;
}
// null out prev and next to allow for GC.
head.next = null;
head.prev = null;
head.bucket = null;
return head;
}
}
HashedWheelTimer
时间轮的唯一实现 HashedWheelTimer 会根据当前时间轮指针选定对应的槽（HashedWheelBucket），从双向链表的头部开始迭代，对每个定时任务（HashedWheelTimeout）进行计算，属于当前时钟周期则取出运行，不属于则将其剩余的时钟周期数减一操作。 核心属性
//时间轮的实例数
private static final AtomicInteger INSTANCE_COUNTER = new AtomicInteger();
private static final AtomicBoolean WARNED_TOO_MANY_INSTANCES = new AtomicBoolean();
private static final int INSTANCE_COUNT_LIMIT = 64;
//状态更新原子修改
private static final AtomicIntegerFieldUpdater<HashedWheelTimer> WORKER_STATE_UPDATER =
AtomicIntegerFieldUpdater.newUpdater(HashedWheelTimer.class, "workerState");
//真正执行定时任务的逻辑封装这个 Runnable 对象中
private final Worker worker = new Worker();
//时间轮内部真正执行定时任务的线程。
private final Thread workerThread;
private static final int WORKER_STATE_INIT = 0;
private static final int WORKER_STATE_STARTED = 1;
private static final int WORKER_STATE_SHUTDOWN = 2;
/**
* 0 - init, 1 - started, 2 - shut down
*/
@SuppressWarnings({"unused", "FieldMayBeFinal"})
private volatile int workerState;
//时间指针每次加 1 所代表的实际时间，单位为纳秒
private final long tickDuration;
//该数组就是时间轮的环形队列，每一个元素都是一个槽。当指定时间轮槽数为 n 时，实际上会取大于且最靠近 n 的 2 的幂次方值。
private final HashedWheelBucket[] wheel;
//掩码， mask = wheel.length - 1，执行 ticks & mask 便能定位到对应的时钟槽
private final int mask;
private final CountDownLatch startTimeInitialized = new CountDownLatch(1);
//timeouts 队列用于缓冲外部提交时间轮中的定时任务
private final Queue<HashedWheelTimeout> timeouts = new LinkedBlockingQueue<>();
//cancelledTimeouts 队列用于暂存取消的定时任务 HashedWheelTimer 会在处理 HashedWheelBucket 的双向链表之前，先处理这两个队列中的数据。
private final Queue<HashedWheelTimeout> cancelledTimeouts = new LinkedBlockingQueue<>();
//等待任务数
private final AtomicLong pendingTimeouts = new AtomicLong(0);
//最大任务数
private final long maxPendingTimeouts;
//开始时间
private volatile long startTime;
构造方法
public HashedWheelTimer(
ThreadFactory threadFactory,
long tickDuration, TimeUnit unit, int ticksPerWheel,
long maxPendingTimeouts) {
if (threadFactory == null) {
throw new NullPointerException("threadFactory");
}
if (unit == null) {
throw new NullPointerException("unit");
}
if (tickDuration <= 0) {
throw new IllegalArgumentException("tickDuration must be greater than 0: " + tickDuration);
}
if (ticksPerWheel <= 0) {
throw new IllegalArgumentException("ticksPerWheel must be greater than 0: " + ticksPerWheel);
}
// Normalize ticksPerWheel to power of two and initialize the wheel.
wheel = createWheel(ticksPerWheel);
mask = wheel.length - 1;
// Convert tickDuration to nanos.
this.tickDuration = unit.toNanos(tickDuration);
// Prevent overflow.
if (this.tickDuration >= Long.MAX_VALUE / wheel.length) {
throw new IllegalArgumentException(String.format(
"tickDuration: %d (expected: 0 < tickDuration in nanos < %d",
tickDuration, Long.MAX_VALUE / wheel.length));
}
workerThread = threadFactory.newThread(worker);
this.maxPendingTimeouts = maxPendingTimeouts;
if (INSTANCE_COUNTER.incrementAndGet() > INSTANCE_COUNT_LIMIT &&
WARNED_TOO_MANY_INSTANCES.compareAndSet(false, true)) {
reportTooManyInstances();
}
}
//创建槽
private static HashedWheelBucket[] createWheel(int ticksPerWheel) {
if (ticksPerWheel <= 0) {
throw new IllegalArgumentException(
"ticksPerWheel must be greater than 0: " + ticksPerWheel);
}
if (ticksPerWheel > 1073741824) {
throw new IllegalArgumentException(
"ticksPerWheel may not be greater than 2^30: " + ticksPerWheel);
}
ticksPerWheel = normalizeTicksPerWheel(ticksPerWheel);
HashedWheelBucket[] wheel = new HashedWheelBucket[ticksPerWheel];
for (int i = 0; i < wheel.length; i++) {
wheel[i] = new HashedWheelBucket();
}
return wheel;
}
//取ticksPerWheel往上最接近的2的幂次方
private static int normalizeTicksPerWheel(int ticksPerWheel) {
int normalizedTicksPerWheel = ticksPerWheel - 1;
normalizedTicksPerWheel |= normalizedTicksPerWheel >>> 1;
normalizedTicksPerWheel |= normalizedTicksPerWheel >>> 2;
normalizedTicksPerWheel |= normalizedTicksPerWheel >>> 4;
normalizedTicksPerWheel |= normalizedTicksPerWheel >>> 8;
normalizedTicksPerWheel |= normalizedTicksPerWheel >>> 16;
return normalizedTicksPerWheel + 1;
}
启动时间轮,构造函数中调用
public void start() {
switch (WORKER_STATE_UPDATER.get(this)) {
case WORKER_STATE_INIT:
if (WORKER_STATE_UPDATER.compareAndSet(this, WORKER_STATE_INIT, WORKER_STATE_STARTED)) {
workerThread.start();
}
break;
case WORKER_STATE_STARTED:
break;
case WORKER_STATE_SHUTDOWN:
throw new IllegalStateException("cannot be started once stopped");
default:
throw new Error("Invalid WorkerState");
}
// Wait until the startTime is initialized by the worker.
while (startTime == 0) {
try {
startTimeInitialized.await();
} catch (InterruptedException ignore) {
// Ignore - it will be ready very soon.
}
}
}
停止时间轮
public Set<Timeout> stop() {
if (Thread.currentThread() == workerThread) {
throw new IllegalStateException(
HashedWheelTimer.class.getSimpleName() +
".stop() cannot be called from " +
TimerTask.class.getSimpleName());
}
if (!WORKER_STATE_UPDATER.compareAndSet(this, WORKER_STATE_STARTED, WORKER_STATE_SHUTDOWN)) {
// workerState can be 0 or 2 at this moment - let it always be 2.
if (WORKER_STATE_UPDATER.getAndSet(this, WORKER_STATE_SHUTDOWN) != WORKER_STATE_SHUTDOWN) {
INSTANCE_COUNTER.decrementAndGet();
}
return Collections.emptySet();
}
try {
boolean interrupted = false;
while (workerThread.isAlive()) {
workerThread.interrupt();
try {
//等待100ms不一定执行完
workerThread.join(100);
} catch (InterruptedException ignored) {
interrupted = true;
}
}
if (interrupted) {
//重设标记位
Thread.currentThread().interrupt();
}
} finally {
INSTANCE_COUNTER.decrementAndGet();
}
return worker.unprocessedTimeouts();
}
提交任务
public Timeout newTimeout(TimerTask task, long delay, TimeUnit unit) {
if (task == null) {
throw new NullPointerException("task");
}
if (unit == null) {
throw new NullPointerException("unit");
}
long pendingTimeoutsCount = pendingTimeouts.incrementAndGet();
if (maxPendingTimeouts > 0 && pendingTimeoutsCount > maxPendingTimeouts) {
pendingTimeouts.decrementAndGet();
throw new RejectedExecutionException("Number of pending timeouts ("
+ pendingTimeoutsCount + ") is greater than or equal to maximum allowed pending "
+ "timeouts (" + maxPendingTimeouts + ")");
}
start();
// Add the timeout to the timeout queue which will be processed on the next tick.
// During processing all the queued HashedWheelTimeouts will be added to the correct HashedWheelBucket.
long deadline = System.nanoTime() + unit.toNanos(delay) - startTime;
// Guard against overflow.
if (delay > 0 && deadline < 0) {
deadline = Long.MAX_VALUE;
}
HashedWheelTimeout timeout = new HashedWheelTimeout(this, task, deadline);
timeouts.add(timeout);
return timeout;
}
工作线程正常状态下是个死循环，一致执行
private final class Worker implements Runnable {
private final Set<Timeout> unprocessedTimeouts = new HashSet<Timeout>();
private long tick;
@Override
public void run() {
// Initialize the startTime.
startTime = System.nanoTime();
if (startTime == 0) {
// We use 0 as an indicator for the uninitialized value here, so make sure it's not 0 when initialized.
startTime = 1;
}
// Notify the other threads waiting for the initialization at start().
startTimeInitialized.countDown();
do {
//等待下一个指针
final long deadline = waitForNextTick();
if (deadline > 0) {
int idx = (int) (tick & mask);
//处理取消任务
processCancelledTasks();
HashedWheelBucket bucket =
wheel[idx];
//把timeouts队列中的数据移到槽中，一次10w
transferTimeoutsToBuckets();
//执行槽中到期任务的run方法
bucket.expireTimeouts(deadline);
tick++;
}
} while (WORKER_STATE_UPDATER.get(HashedWheelTimer.this) == WORKER_STATE_STARTED);
// Fill the unprocessedTimeouts so we can return them from stop() method.
for (HashedWheelBucket bucket : wheel) {
bucket.clearTimeouts(unprocessedTimeouts);
}
for (; ; ) {
HashedWheelTimeout timeout = timeouts.poll();
if (timeout == null) {
break;
}
if (!timeout.isCancelled()) {
unprocessedTimeouts.add(timeout);
}
}
processCancelledTasks();
}
private void transferTimeoutsToBuckets() {
// transfer only max. 100000 timeouts per tick to prevent a thread to stale the workerThread when it just
// adds new timeouts in a loop.
for (int i = 0; i < 100000; i++) {
HashedWheelTimeout timeout = timeouts.poll();
if (timeout == null) {
// all processed
break;
}
if (timeout.state() == HashedWheelTimeout.ST_CANCELLED) {
// Was cancelled in the meantime.
continue;
}
long calculated = timeout.deadline / tickDuration;
timeout.remainingRounds = (calculated - tick) / wheel.length;
// Ensure we don't schedule for past.
final long ticks = Math.max(calculated, tick);
int stopIndex = (int) (ticks & mask);
HashedWheelBucket bucket = wheel[stopIndex];
bucket.addTimeout(timeout);
}
}
private void processCancelledTasks() {
for (; ; ) {
HashedWheelTimeout timeout = cancelledTimeouts.poll();
if (timeout == null) {
// all processed
break;
}
try {
timeout.remove();
} catch (Throwable t) {
if (logger.isWarnEnabled()) {
logger.warn("An exception was thrown while process a cancellation task", t);
}
}
}
}
/**
* calculate goal nanoTime from startTime and current tick number,
* then wait until that goal has been reached.
*
* @return Long.MIN_VALUE if received a shutdown request,
* current time otherwise (with Long.MIN_VALUE changed by +1)
*/
private long waitForNextTick() {
long deadline = tickDuration * (tick + 1);
for (; ; ) {
final long currentTime = System.nanoTime() - startTime;
long sleepTimeMs = (deadline - currentTime + 999999) / 1000000;
if (sleepTimeMs <= 0) {
if (currentTime == Long.MIN_VALUE) {
return -Long.MAX_VALUE;
} else {
return currentTime;
}
}
if (isWindows()) {
sleepTimeMs = sleepTimeMs / 10 * 10;
}
try {
Thread.sleep(sleepTimeMs);
} catch (InterruptedException ignored) {
if (WORKER_STATE_UPDATER.get(HashedWheelTimer.this) == WORKER_STATE_SHUTDOWN) {
return Long.MIN_VALUE;
}
}
}
}
Set<Timeout> unprocessedTimeouts() {
return Collections.unmodifiableSet(unprocessedTimeouts);
}
}
使用场景
在 Dubbo 中，时间轮并不直接用于周期性操作，而是只向时间轮提交执行单次的定时任务，在上一次任务执行完成的时候，调用 newTimeout() 方法再次提交当前任务，这样就会在下个周期执行该任务。即使在任务执行过程中出现了 GC、I/O 阻塞等情况，导致任务延迟或卡住，也不会有同样的任务源源不断地提交进来，导致任务堆积。
Dubbo 中对时间轮的应用主要体现在如下两个方面：
- 失败重试， 例如，Provider 向注册中心进行注册失败时的重试操作，或是 Consumer 向注册中心订阅时的失败重试等。
//FailbackRegistry
private void addFailedRegistered(URL url) {
FailedRegisteredTask oldOne = failedRegistered.get(url);
if (oldOne != null) {
return;
}
FailedRegisteredTask newTask = new FailedRegisteredTask(url, this);
oldOne = failedRegistered.putIfAbsent(url, newTask);
if (oldOne == null) {
// never has a retry task. then start a new task for retry.
retryTimer.newTimeout(newTask, retryPeriod, TimeUnit.MILLISECONDS);
}
}
- 周期性定时任务， 例如，定期发送心跳请求，请求超时的处理，或是网络连接断开后的重连机制。
总结
- DUBBO中时间轮使用的较为简单，未考虑的大跨度时间问题和重启恢复，应该主要是其应用场景用不到这些
- 不过其对时间轮中时间加了剩余执行轮数优化remainingRounds，每转一圈会减一
- 具体的任务都是在一个work线程中执行，大批量任务执行时可能会导致执行时间精度不准
剩下部分请见算法技巧-时间轮（二）
参考 1.05 海量定时任务，一个时间轮搞定 2.延时任务一锅端
转载自:https://juejin.cn/post/7258509816691245093