2 * Copyright 2017 Facebook, Inc.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
19 #include <folly/Optional.h>
20 #include <folly/io/async/AsyncTimeout.h>
21 #include <folly/io/async/DelayedDestruction.h>
23 #include <boost/intrusive/list.hpp>
24 #include <glog/logging.h>
34 * Hashed Hierarchical Wheel Timer
36 * We model timers as the number of ticks until the next
37 * due event. We allow 32-bits of space to track this
38 * due interval, and break that into 4 regions of 8 bits.
39 * Each region indexes into a bucket of 256 lists.
41 * Bucket 0 represents those events that are due the soonest.
42 * Each tick causes us to look at the next list in a bucket.
43 * The 0th list in a bucket is special; it means that it is time to
44 * flush the timers from the next higher bucket and schedule them
45 * into a different bucket.
47 * This technique results in a very cheap mechanism for
48 * maintaining time and timers.
50 * Unlike the original timer wheel paper, this implementation does
51 * *not* tick constantly, and instead calculates the exact next wakeup
54 class HHWheelTimer : private folly::AsyncTimeout,
55 public folly::DelayedDestruction {
57 // This type has always been a misnomer, because it is not a unique pointer.
58 using UniquePtr = std::unique_ptr<HHWheelTimer, Destructor>;
59 using SharedPtr = std::shared_ptr<HHWheelTimer>;
61 template <typename... Args>
62 static UniquePtr newTimer(Args&&... args) {
63 return UniquePtr(new HHWheelTimer(std::forward<Args>(args)...));
67 * A callback to be notified when a timeout has expired.
70 : public boost::intrusive::list_base_hook<
71 boost::intrusive::link_mode<boost::intrusive::auto_unlink>> {
80 * timeoutExpired() is invoked when the timeout has expired.
82 virtual void timeoutExpired() noexcept = 0;
84 /// This callback was canceled. The default implementation is to just
85 /// proxy to `timeoutExpired` but if you care about the difference between
86 /// the timeout finishing or being canceled you can override this.
87 virtual void callbackCanceled() noexcept {
92 * Cancel the timeout, if it is running.
94 * If the timeout is not scheduled, cancelTimeout() does nothing.
96 void cancelTimeout() {
97 if (wheel_ == nullptr) {
98 // We're not scheduled, so there's nothing to do.
105 * Return true if this timeout is currently scheduled, and false otherwise.
107 bool isScheduled() const {
108 return wheel_ != nullptr;
113 * Don't override this unless you're doing a test. This is mainly here so
114 * that we can override it to simulate lag in steady_clock.
116 virtual std::chrono::milliseconds getCurTime() {
117 return std::chrono::duration_cast<std::chrono::milliseconds>(
118 std::chrono::steady_clock::now().time_since_epoch());
122 // Get the time remaining until this timeout expires
123 std::chrono::milliseconds getTimeRemaining(
124 std::chrono::milliseconds now) const {
125 if (now >= expiration_) {
126 return std::chrono::milliseconds(0);
128 return expiration_ - now;
131 void setScheduled(HHWheelTimer* wheel,
132 std::chrono::milliseconds);
133 void cancelTimeoutImpl();
135 HHWheelTimer* wheel_;
136 std::chrono::milliseconds expiration_;
139 typedef boost::intrusive::list<
141 boost::intrusive::constant_time_size<false> > List;
143 std::shared_ptr<RequestContext> context_;
145 // Give HHWheelTimer direct access to our members so it can take care
146 // of scheduling/cancelling.
147 friend class HHWheelTimer;
151 * Create a new HHWheelTimer with the specified interval and the
152 * default timeout value set.
154 * Objects created using this version of constructor can be used
155 * to schedule both variable interval timeouts using
156 * scheduleTimeout(callback, timeout) method, and default
157 * interval timeouts using scheduleTimeout(callback) method.
159 static int DEFAULT_TICK_INTERVAL;
160 explicit HHWheelTimer(
161 folly::TimeoutManager* timeoutManager,
162 std::chrono::milliseconds intervalMS =
163 std::chrono::milliseconds(DEFAULT_TICK_INTERVAL),
164 AsyncTimeout::InternalEnum internal = AsyncTimeout::InternalEnum::NORMAL,
165 std::chrono::milliseconds defaultTimeoutMS =
166 std::chrono::milliseconds(-1));
169 * Cancel all outstanding timeouts
171 * @returns the number of timeouts that were cancelled.
176 * Get the tick interval for this HHWheelTimer.
178 * Returns the tick interval in milliseconds.
180 std::chrono::milliseconds getTickInterval() const {
185 * Get the default timeout interval for this HHWheelTimer.
187 * Returns the timeout interval in milliseconds.
189 std::chrono::milliseconds getDefaultTimeout() const {
190 return defaultTimeout_;
194 * Schedule the specified Callback to be invoked after the
195 * specified timeout interval.
197 * If the callback is already scheduled, this cancels the existing timeout
198 * before scheduling the new timeout.
200 void scheduleTimeout(Callback* callback,
201 std::chrono::milliseconds timeout);
202 void scheduleTimeoutImpl(Callback* callback,
203 std::chrono::milliseconds timeout);
206 * Schedule the specified Callback to be invoked after the
207 * default timeout interval.
209 * If the callback is already scheduled, this cancels the existing timeout
210 * before scheduling the new timeout.
212 * This method uses CHECK() to make sure that the default timeout was
213 * specified on the object initialization.
215 void scheduleTimeout(Callback* callback);
218 void scheduleTimeoutFn(F fn, std::chrono::milliseconds timeout) {
219 struct Wrapper : Callback {
220 Wrapper(F f) : fn_(std::move(f)) {}
221 void timeoutExpired() noexcept override {
224 } catch (std::exception const& e) {
225 LOG(ERROR) << "HHWheelTimer timeout callback threw an exception: "
228 LOG(ERROR) << "HHWheelTimer timeout callback threw a non-exception.";
234 Wrapper* w = new Wrapper(std::move(fn));
235 scheduleTimeout(w, timeout);
239 * Return the number of currently pending timeouts
241 uint64_t count() const {
245 bool isDetachable() const {
246 return !folly::AsyncTimeout::isScheduled();
249 using folly::AsyncTimeout::attachEventBase;
250 using folly::AsyncTimeout::detachEventBase;
251 using folly::AsyncTimeout::getTimeoutManager;
255 * Protected destructor.
257 * Use destroy() instead. See the comments in DelayedDestruction for more
260 virtual ~HHWheelTimer();
263 // Forbidden copy constructor and assignment operator
264 HHWheelTimer(HHWheelTimer const &) = delete;
265 HHWheelTimer& operator=(HHWheelTimer const &) = delete;
267 // Methods inherited from AsyncTimeout
268 virtual void timeoutExpired() noexcept;
270 std::chrono::milliseconds interval_;
271 std::chrono::milliseconds defaultTimeout_;
273 static constexpr int WHEEL_BUCKETS = 4;
274 static constexpr int WHEEL_BITS = 8;
275 static constexpr unsigned int WHEEL_SIZE = (1 << WHEEL_BITS);
276 static constexpr unsigned int WHEEL_MASK = (WHEEL_SIZE - 1);
277 static constexpr uint32_t LARGEST_SLOT = 0xffffffffUL;
279 typedef Callback::List CallbackList;
280 CallbackList buckets_[WHEEL_BUCKETS][WHEEL_SIZE];
281 std::vector<uint64_t> bitmap_;
283 int64_t timeToWheelTicks(std::chrono::milliseconds t) {
284 return t.count() / interval_.count();
287 bool cascadeTimers(int bucket, int tick);
291 std::chrono::milliseconds startTime_;
293 int64_t calcNextTick();
295 void scheduleNextTimeout();
297 bool* processingCallbacksGuard_;
298 CallbackList timeouts; // Timeouts queued to run
300 std::chrono::milliseconds getCurTime() {
301 return std::chrono::duration_cast<std::chrono::milliseconds>(
302 std::chrono::steady_clock::now().time_since_epoch());