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>> {
77 * timeoutExpired() is invoked when the timeout has expired.
79 virtual void timeoutExpired() noexcept = 0;
81 /// This callback was canceled. The default implementation is to just
82 /// proxy to `timeoutExpired` but if you care about the difference between
83 /// the timeout finishing or being canceled you can override this.
84 virtual void callbackCanceled() noexcept {
89 * Cancel the timeout, if it is running.
91 * If the timeout is not scheduled, cancelTimeout() does nothing.
93 void cancelTimeout() {
94 if (wheel_ == nullptr) {
95 // We're not scheduled, so there's nothing to do.
102 * Return true if this timeout is currently scheduled, and false otherwise.
104 bool isScheduled() const {
105 return wheel_ != nullptr;
110 * Don't override this unless you're doing a test. This is mainly here so
111 * that we can override it to simulate lag in steady_clock.
113 virtual std::chrono::steady_clock::time_point getCurTime() {
114 return std::chrono::steady_clock::now();
118 // Get the time remaining until this timeout expires
119 std::chrono::milliseconds getTimeRemaining(
120 std::chrono::steady_clock::time_point now) const {
121 if (now >= expiration_) {
122 return std::chrono::milliseconds(0);
124 return std::chrono::duration_cast<std::chrono::milliseconds>(
128 void setScheduled(HHWheelTimer* wheel,
129 std::chrono::milliseconds);
130 void cancelTimeoutImpl();
132 HHWheelTimer* wheel_{nullptr};
133 std::chrono::steady_clock::time_point expiration_{};
136 typedef boost::intrusive::list<
138 boost::intrusive::constant_time_size<false> > List;
140 std::shared_ptr<RequestContext> context_;
142 // Give HHWheelTimer direct access to our members so it can take care
143 // of scheduling/cancelling.
144 friend class HHWheelTimer;
148 * Create a new HHWheelTimer with the specified interval and the
149 * default timeout value set.
151 * Objects created using this version of constructor can be used
152 * to schedule both variable interval timeouts using
153 * scheduleTimeout(callback, timeout) method, and default
154 * interval timeouts using scheduleTimeout(callback) method.
156 static int DEFAULT_TICK_INTERVAL;
157 explicit HHWheelTimer(
158 folly::TimeoutManager* timeoutManager,
159 std::chrono::milliseconds intervalMS =
160 std::chrono::milliseconds(DEFAULT_TICK_INTERVAL),
161 AsyncTimeout::InternalEnum internal = AsyncTimeout::InternalEnum::NORMAL,
162 std::chrono::milliseconds defaultTimeoutMS =
163 std::chrono::milliseconds(-1));
166 * Cancel all outstanding timeouts
168 * @returns the number of timeouts that were cancelled.
173 * Get the tick interval for this HHWheelTimer.
175 * Returns the tick interval in milliseconds.
177 std::chrono::milliseconds getTickInterval() const {
182 * Get the default timeout interval for this HHWheelTimer.
184 * Returns the timeout interval in milliseconds.
186 std::chrono::milliseconds getDefaultTimeout() const {
187 return defaultTimeout_;
191 * Schedule the specified Callback to be invoked after the
192 * specified timeout interval.
194 * If the callback is already scheduled, this cancels the existing timeout
195 * before scheduling the new timeout.
197 void scheduleTimeout(Callback* callback,
198 std::chrono::milliseconds timeout);
199 void scheduleTimeoutImpl(Callback* callback,
200 std::chrono::milliseconds timeout);
203 * Schedule the specified Callback to be invoked after the
204 * default timeout interval.
206 * If the callback is already scheduled, this cancels the existing timeout
207 * before scheduling the new timeout.
209 * This method uses CHECK() to make sure that the default timeout was
210 * specified on the object initialization.
212 void scheduleTimeout(Callback* callback);
215 void scheduleTimeoutFn(F fn, std::chrono::milliseconds timeout) {
216 struct Wrapper : Callback {
217 Wrapper(F f) : fn_(std::move(f)) {}
218 void timeoutExpired() noexcept override {
221 } catch (std::exception const& e) {
222 LOG(ERROR) << "HHWheelTimer timeout callback threw an exception: "
225 LOG(ERROR) << "HHWheelTimer timeout callback threw a non-exception.";
231 Wrapper* w = new Wrapper(std::move(fn));
232 scheduleTimeout(w, timeout);
236 * Return the number of currently pending timeouts
238 uint64_t count() const {
242 bool isDetachable() const {
243 return !folly::AsyncTimeout::isScheduled();
246 using folly::AsyncTimeout::attachEventBase;
247 using folly::AsyncTimeout::detachEventBase;
248 using folly::AsyncTimeout::getTimeoutManager;
252 * Protected destructor.
254 * Use destroy() instead. See the comments in DelayedDestruction for more
257 virtual ~HHWheelTimer();
260 // Forbidden copy constructor and assignment operator
261 HHWheelTimer(HHWheelTimer const &) = delete;
262 HHWheelTimer& operator=(HHWheelTimer const &) = delete;
264 // Methods inherited from AsyncTimeout
265 virtual void timeoutExpired() noexcept;
267 std::chrono::milliseconds interval_;
268 std::chrono::milliseconds defaultTimeout_;
270 static constexpr int WHEEL_BUCKETS = 4;
271 static constexpr int WHEEL_BITS = 8;
272 static constexpr unsigned int WHEEL_SIZE = (1 << WHEEL_BITS);
273 static constexpr unsigned int WHEEL_MASK = (WHEEL_SIZE - 1);
274 static constexpr uint32_t LARGEST_SLOT = 0xffffffffUL;
276 typedef Callback::List CallbackList;
277 CallbackList buckets_[WHEEL_BUCKETS][WHEEL_SIZE];
278 std::vector<uint64_t> bitmap_;
280 int64_t timeToWheelTicks(std::chrono::milliseconds t) {
281 return t.count() / interval_.count();
284 bool cascadeTimers(int bucket, int tick);
288 std::chrono::steady_clock::time_point startTime_;
290 int64_t calcNextTick();
292 void scheduleNextTimeout();
294 bool* processingCallbacksGuard_;
295 CallbackList timeouts; // Timeouts queued to run
297 std::chrono::steady_clock::time_point getCurTime() {
298 return std::chrono::steady_clock::now();