2 * Copyright 2015 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/io/async/AsyncTimeout.h>
20 #include <folly/io/async/DelayedDestruction.h>
22 #include <boost/intrusive/list.hpp>
23 #include <glog/logging.h>
33 * Hashed Hierarchical Wheel Timer
36 * AsyncTimeout - a single timeout.
37 * HHWheelTimer - a set of efficient timeouts with different interval,
38 * but timeouts are not exact.
40 * All of the above are O(1) in insertion, tick update and cancel
42 * This implementation ticks once every 10ms.
43 * We model timers as the number of ticks until the next
44 * due event. We allow 32-bits of space to track this
45 * due interval, and break that into 4 regions of 8 bits.
46 * Each region indexes into a bucket of 256 lists.
48 * Bucket 0 represents those events that are due the soonest.
49 * Each tick causes us to look at the next list in a bucket.
50 * The 0th list in a bucket is special; it means that it is time to
51 * flush the timers from the next higher bucket and schedule them
52 * into a different bucket.
54 * This technique results in a very cheap mechanism for
55 * maintaining time and timers, provided that we can maintain
56 * a consistent rate of ticks.
58 class HHWheelTimer : private folly::AsyncTimeout,
59 public folly::DelayedDestruction {
61 typedef std::unique_ptr<HHWheelTimer, Destructor> UniquePtr;
63 template <typename... Args>
64 static UniquePtr newTimer(Args&&... args) {
65 return UniquePtr(new HHWheelTimer(std::forward<Args>(args)...));
69 * A callback to be notified when a timeout has expired.
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_;
138 typedef boost::intrusive::list_member_hook<
139 boost::intrusive::link_mode<boost::intrusive::auto_unlink> > ListHook;
143 typedef boost::intrusive::list<
145 boost::intrusive::member_hook<Callback, ListHook, &Callback::hook_>,
146 boost::intrusive::constant_time_size<false> > List;
148 std::shared_ptr<RequestContext> context_;
150 // Give HHWheelTimer direct access to our members so it can take care
151 // of scheduling/cancelling.
152 friend class HHWheelTimer;
156 * Create a new HHWheelTimer with the specified interval and the
157 * default timeout value set.
159 * Objects created using this version of constructor can be used
160 * to schedule both variable interval timeouts using
161 * scheduleTimeout(callback, timeout) method, and default
162 * interval timeouts using scheduleTimeout(callback) method.
164 static int DEFAULT_TICK_INTERVAL;
165 explicit HHWheelTimer(folly::EventBase* eventBase,
166 std::chrono::milliseconds intervalMS =
167 std::chrono::milliseconds(DEFAULT_TICK_INTERVAL),
168 AsyncTimeout::InternalEnum internal =
169 AsyncTimeout::InternalEnum::NORMAL,
170 std::chrono::milliseconds defaultTimeoutMS =
171 std::chrono::milliseconds(-1));
174 * Destroy the HHWheelTimer.
176 * A HHWheelTimer should only be destroyed when there are no more
177 * callbacks pending in the set. (If it helps you may use cancelAll() to
178 * cancel all pending timeouts explicitly before calling this.)
180 virtual void destroy();
183 * Cancel all outstanding timeouts
185 * @returns the number of timeouts that were cancelled.
190 * Get the tick interval for this HHWheelTimer.
192 * Returns the tick interval in milliseconds.
194 std::chrono::milliseconds getTickInterval() const {
199 * Get the default timeout interval for this HHWheelTimer.
201 * Returns the timeout interval in milliseconds.
203 std::chrono::milliseconds getDefaultTimeout() const {
204 return defaultTimeout_;
208 * Schedule the specified Callback to be invoked after the
209 * specified timeout interval.
211 * If the callback is already scheduled, this cancels the existing timeout
212 * before scheduling the new timeout.
214 void scheduleTimeout(Callback* callback,
215 std::chrono::milliseconds timeout);
216 void scheduleTimeoutImpl(Callback* callback,
217 std::chrono::milliseconds timeout);
220 * Schedule the specified Callback to be invoked after the
221 * fefault timeout interval.
223 * If the callback is already scheduled, this cancels the existing timeout
224 * before scheduling the new timeout.
226 * This method uses CHECK() to make sure that the default timeout was
227 * specified on the object initialization.
229 void scheduleTimeout(Callback* callback);
232 void scheduleTimeoutFn(F fn, std::chrono::milliseconds timeout) {
233 struct Wrapper : Callback {
234 Wrapper(F f) : fn_(std::move(f)) {}
235 void timeoutExpired() noexcept override {
238 } catch (std::exception const& e) {
239 LOG(ERROR) << "HHWheelTimer timeout callback threw an exception: "
242 LOG(ERROR) << "HHWheelTimer timeout callback threw a non-exception.";
248 Wrapper* w = new Wrapper(std::move(fn));
249 scheduleTimeout(w, timeout);
253 * Return the number of currently pending timeouts
255 uint64_t count() const {
260 * This turns on more exact timing. By default the wheel timer
261 * increments its cached time only once everyN (default) ticks.
263 * With catchupEveryN at 1, timeouts will only be delayed until the
264 * next tick, at which point all overdue timeouts are called. The
265 * wheel timer is approximately 2x slower with this set to 1.
267 * Load testing in opt mode showed skew was about 1% with no catchup.
269 void setCatchupEveryN(uint32_t everyN) {
270 catchupEveryN_ = everyN;
273 bool isDetachable() const {
274 return !folly::AsyncTimeout::isScheduled();
277 using folly::AsyncTimeout::attachEventBase;
278 using folly::AsyncTimeout::detachEventBase;
279 using folly::AsyncTimeout::getTimeoutManager;
283 * Protected destructor.
285 * Use destroy() instead. See the comments in DelayedDestruction for more
288 virtual ~HHWheelTimer();
291 // Forbidden copy constructor and assignment operator
292 HHWheelTimer(HHWheelTimer const &) = delete;
293 HHWheelTimer& operator=(HHWheelTimer const &) = delete;
295 // Methods inherited from AsyncTimeout
296 virtual void timeoutExpired() noexcept;
298 std::chrono::milliseconds interval_;
299 std::chrono::milliseconds defaultTimeout_;
301 static constexpr int WHEEL_BUCKETS = 4;
302 static constexpr int WHEEL_BITS = 8;
303 static constexpr unsigned int WHEEL_SIZE = (1 << WHEEL_BITS);
304 static constexpr unsigned int WHEEL_MASK = (WHEEL_SIZE - 1);
305 static constexpr uint32_t LARGEST_SLOT = 0xffffffffUL;
307 typedef Callback::List CallbackList;
308 CallbackList buckets_[WHEEL_BUCKETS][WHEEL_SIZE];
310 int64_t timeToWheelTicks(std::chrono::milliseconds t) {
311 return t.count() / interval_.count();
314 bool cascadeTimers(int bucket, int tick);
317 std::chrono::milliseconds now_;
319 static constexpr uint32_t DEFAULT_CATCHUP_EVERY_N = 10;
321 uint32_t catchupEveryN_;
322 uint32_t expirationsSinceCatchup_;
323 bool processingCallbacksGuard_;