2 * Copyright 2014 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 <glog/logging.h>
20 #include <folly/io/async/AsyncTimeout.h>
21 #include <folly/io/async/TimeoutManager.h>
22 #include <folly/io/async/Request.h>
23 #include <folly/Executor.h>
24 #include <folly/futures/DrivableExecutor.h>
32 #include <boost/intrusive/list.hpp>
33 #include <boost/utility.hpp>
35 #include <event.h> // libevent
42 typedef std::function<void()> Cob;
43 template <typename MessageT>
44 class NotificationQueue;
46 class EventBaseObserver {
48 virtual ~EventBaseObserver() {}
50 virtual uint32_t getSampleRate() const = 0;
52 virtual void loopSample(
53 int64_t busyTime, int64_t idleTime) = 0;
56 // Helper class that sets and retrieves the EventBase associated with a given
57 // request via RequestContext. See Request.h for that mechanism.
58 class RequestEventBase : public RequestData {
60 static EventBase* get() {
61 auto data = dynamic_cast<RequestEventBase*>(
62 RequestContext::get()->getContextData(kContextDataName));
69 static void set(EventBase* eb) {
70 RequestContext::get()->setContextData(
72 std::unique_ptr<RequestEventBase>(new RequestEventBase(eb)));
76 explicit RequestEventBase(EventBase* eb) : eb_(eb) {}
78 static constexpr const char* kContextDataName{"EventBase"};
82 * This class is a wrapper for all asynchronous I/O processing functionality
84 * EventBase provides a main loop that notifies EventHandler callback objects
85 * when I/O is ready on a file descriptor, and notifies AsyncTimeout objects
86 * when a specified timeout has expired. More complex, higher-level callback
87 * mechanisms can then be built on top of EventHandler and AsyncTimeout.
89 * A EventBase object can only drive an event loop for a single thread. To
90 * take advantage of multiple CPU cores, most asynchronous I/O servers have one
91 * thread per CPU, and use a separate EventBase for each thread.
93 * In general, most EventBase methods may only be called from the thread
94 * running the EventBase's loop. There are a few exceptions to this rule, for
95 * methods that are explicitly intended to allow communication with a
96 * EventBase from other threads. When it is safe to call a method from
97 * another thread it is explicitly listed in the method comments.
99 class EventBase : private boost::noncopyable,
100 public TimeoutManager,
101 public DrivableExecutor {
104 * A callback interface to use with runInLoop()
106 * Derive from this class if you need to delay some code execution until the
107 * next iteration of the event loop. This allows you to schedule code to be
108 * invoked from the top-level of the loop, after your immediate callers have
111 * If a LoopCallback object is destroyed while it is scheduled to be run in
112 * the next loop iteration, it will automatically be cancelled.
116 virtual ~LoopCallback() {}
118 virtual void runLoopCallback() noexcept = 0;
119 void cancelLoopCallback() {
123 bool isLoopCallbackScheduled() const {
124 return hook_.is_linked();
128 typedef boost::intrusive::list_member_hook<
129 boost::intrusive::link_mode<boost::intrusive::auto_unlink> > ListHook;
133 typedef boost::intrusive::list<
135 boost::intrusive::member_hook<LoopCallback, ListHook,
136 &LoopCallback::hook_>,
137 boost::intrusive::constant_time_size<false> > List;
139 // EventBase needs access to LoopCallbackList (and therefore to hook_)
140 friend class EventBase;
141 std::shared_ptr<RequestContext> context_;
145 * Create a new EventBase object.
150 * Create a new EventBase object that will use the specified libevent
151 * event_base object to drive the event loop.
153 * The EventBase will take ownership of this event_base, and will call
154 * event_base_free(evb) when the EventBase is destroyed.
156 explicit EventBase(event_base* evb);
160 * Runs the event loop.
162 * loop() will loop waiting for I/O or timeouts and invoking EventHandler
163 * and AsyncTimeout callbacks as their events become ready. loop() will
164 * only return when there are no more events remaining to process, or after
165 * terminateLoopSoon() has been called.
167 * loop() may be called again to restart event processing after a previous
168 * call to loop() or loopForever() has returned.
170 * Returns true if the loop completed normally (if it processed all
171 * outstanding requests, or if terminateLoopSoon() was called). If an error
172 * occurs waiting for events, false will be returned.
177 * Wait for some events to become active, run them, then return.
179 * When EVLOOP_NONBLOCK is set in flags, the loop won't block if there
180 * are not any events to process.
182 * This is useful for callers that want to run the loop manually.
184 * Returns the same result as loop().
186 bool loopOnce(int flags = 0);
189 * Runs the event loop.
191 * loopForever() behaves like loop(), except that it keeps running even if
192 * when there are no more user-supplied EventHandlers or AsyncTimeouts
193 * registered. It will only return after terminateLoopSoon() has been
196 * This is useful for callers that want to wait for other threads to call
197 * runInEventBaseThread(), even when there are no other scheduled events.
199 * loopForever() may be called again to restart event processing after a
200 * previous call to loop() or loopForever() has returned.
202 * Throws a std::system_error if an error occurs.
207 * Causes the event loop to exit soon.
209 * This will cause an existing call to loop() or loopForever() to stop event
210 * processing and return, even if there are still events remaining to be
213 * It is safe to call terminateLoopSoon() from another thread to cause loop()
214 * to wake up and return in the EventBase loop thread. terminateLoopSoon()
215 * may also be called from the loop thread itself (for example, a
216 * EventHandler or AsyncTimeout callback may call terminateLoopSoon() to
217 * cause the loop to exit after the callback returns.) If the loop is not
218 * running, this will cause the next call to loop to terminate soon after
219 * starting. If a loop runs out of work (and so terminates on its own)
220 * concurrently with a call to terminateLoopSoon(), this may cause a race
223 * Note that the caller is responsible for ensuring that cleanup of all event
224 * callbacks occurs properly. Since terminateLoopSoon() causes the loop to
225 * exit even when there are pending events present, there may be remaining
226 * callbacks present waiting to be invoked. If the loop is later restarted
227 * pending events will continue to be processed normally, however if the
228 * EventBase is destroyed after calling terminateLoopSoon() it is the
229 * caller's responsibility to ensure that cleanup happens properly even if
230 * some outstanding events are never processed.
232 void terminateLoopSoon();
235 * Adds the given callback to a queue of things run after the current pass
236 * through the event loop completes. Note that if this callback calls
237 * runInLoop() the new callback won't be called until the main event loop
238 * has gone through a cycle.
240 * This method may only be called from the EventBase's thread. This
241 * essentially allows an event handler to schedule an additional callback to
242 * be invoked after it returns.
244 * Use runInEventBaseThread() to schedule functions from another thread.
246 * The thisIteration parameter makes this callback run in this loop
247 * iteration, instead of the next one, even if called from a
248 * runInLoop callback (normal io callbacks that call runInLoop will
249 * always run in this iteration). This was originally added to
250 * support detachEventBase, as a user callback may have called
251 * terminateLoopSoon(), but we want to make sure we detach. Also,
252 * detachEventBase almost always must be called from the base event
253 * loop to ensure the stack is unwound, since most users of
254 * EventBase are not thread safe.
256 * Ideally we would not need thisIteration, and instead just use
257 * runInLoop with loop() (instead of terminateLoopSoon).
259 void runInLoop(LoopCallback* callback, bool thisIteration = false);
262 * Convenience function to call runInLoop() with a std::function.
264 * This creates a LoopCallback object to wrap the std::function, and invoke
265 * the std::function when the loop callback fires. This is slightly more
266 * expensive than defining your own LoopCallback, but more convenient in
267 * areas that aren't performance sensitive where you just want to use
268 * std::bind. (std::bind is fairly slow on even by itself.)
270 * This method may only be called from the EventBase's thread. This
271 * essentially allows an event handler to schedule an additional callback to
272 * be invoked after it returns.
274 * Use runInEventBaseThread() to schedule functions from another thread.
276 void runInLoop(const Cob& c, bool thisIteration = false);
278 void runInLoop(Cob&& c, bool thisIteration = false);
281 * Adds the given callback to a queue of things run before destruction
282 * of current EventBase.
284 * This allows users of EventBase that run in it, but don't control it,
285 * to be notified before EventBase gets destructed.
287 * Note: will be called from the thread that invoked EventBase destructor,
288 * before the final run of loop callbacks.
290 void runOnDestruction(LoopCallback* callback);
293 * Adds a callback that will run immediately *before* the event loop.
294 * This is very similar to runInLoop(), but will not cause the loop to break:
295 * For example, this callback could be used to get loop times.
297 void runBeforeLoop(LoopCallback* callback);
300 * Run the specified function in the EventBase's thread.
302 * This method is thread-safe, and may be called from another thread.
304 * If runInEventBaseThread() is called when the EventBase loop is not
305 * running, the function call will be delayed until the next time the loop is
308 * If runInEventBaseThread() returns true the function has successfully been
309 * scheduled to run in the loop thread. However, if the loop is terminated
310 * (and never later restarted) before it has a chance to run the requested
311 * function, the function will be run upon the EventBase's destruction.
313 * If two calls to runInEventBaseThread() are made from the same thread, the
314 * functions will always be run in the order that they were scheduled.
315 * Ordering between functions scheduled from separate threads is not
318 * @param fn The function to run. The function must not throw any
320 * @param arg An argument to pass to the function.
322 * @return Returns true if the function was successfully scheduled, or false
323 * if there was an error scheduling the function.
326 bool runInEventBaseThread(void (*fn)(T*), T* arg) {
327 return runInEventBaseThread(reinterpret_cast<void (*)(void*)>(fn),
328 reinterpret_cast<void*>(arg));
331 bool runInEventBaseThread(void (*fn)(void*), void* arg);
334 * Run the specified function in the EventBase's thread
336 * This version of runInEventBaseThread() takes a std::function object.
337 * Note that this is less efficient than the version that takes a plain
338 * function pointer and void* argument, as it has to allocate memory to copy
339 * the std::function object.
341 * If the loop is terminated (and never later restarted) before it has a
342 * chance to run the requested function, the function will be run upon the
343 * EventBase's destruction.
345 * The function must not throw any exceptions.
347 bool runInEventBaseThread(const Cob& fn);
350 * Runs the given Cob at some time after the specified number of
351 * milliseconds. (No guarantees exactly when.)
353 * @return true iff the cob was successfully registered.
358 TimeoutManager::InternalEnum = TimeoutManager::InternalEnum::NORMAL);
361 * Set the maximum desired latency in us and provide a callback which will be
362 * called when that latency is exceeded.
364 void setMaxLatency(int64_t maxLatency, const Cob& maxLatencyCob) {
365 maxLatency_ = maxLatency;
366 maxLatencyCob_ = maxLatencyCob;
370 * Set smoothing coefficient for loop load average; # of milliseconds
371 * for exp(-1) (1/2.71828...) decay.
373 void setLoadAvgMsec(uint32_t ms);
376 * reset the load average to a desired value
378 void resetLoadAvg(double value = 0.0);
381 * Get the average loop time in microseconds (an exponentially-smoothed ave)
383 double getAvgLoopTime() const {
384 return avgLoopTime_.get();
388 * check if the event base loop is running.
390 bool isRunning() const {
391 return loopThread_.load(std::memory_order_relaxed) != 0;
395 * wait until the event loop starts (after starting the event loop thread).
397 void waitUntilRunning();
399 int getNotificationQueueSize() const;
401 void setMaxReadAtOnce(uint32_t maxAtOnce);
404 * Verify that current thread is the EventBase thread, if the EventBase is
407 bool isInEventBaseThread() const {
408 auto tid = loopThread_.load(std::memory_order_relaxed);
409 return tid == 0 || pthread_equal(tid, pthread_self());
412 bool inRunningEventBaseThread() const {
413 return pthread_equal(
414 loopThread_.load(std::memory_order_relaxed), pthread_self());
417 // --------- interface to underlying libevent base ------------
418 // Avoid using these functions if possible. These functions are not
419 // guaranteed to always be present if we ever provide alternative EventBase
420 // implementations that do not use libevent internally.
421 event_base* getLibeventBase() const { return evb_; }
422 static const char* getLibeventVersion();
423 static const char* getLibeventMethod();
426 * only EventHandler/AsyncTimeout subclasses and ourselves should
429 * This is used to mark the beginning of a new loop cycle by the
430 * first handler fired within that cycle.
433 bool bumpHandlingTime();
435 class SmoothLoopTime {
437 explicit SmoothLoopTime(uint64_t timeInterval)
438 : expCoeff_(-1.0/timeInterval)
440 , oldBusyLeftover_(0) {
441 VLOG(11) << "expCoeff_ " << expCoeff_ << " " << __PRETTY_FUNCTION__;
444 void setTimeInterval(uint64_t timeInterval);
445 void reset(double value = 0.0);
447 void addSample(int64_t idle, int64_t busy);
453 void dampen(double factor) {
460 int64_t oldBusyLeftover_;
464 const std::shared_ptr<EventBaseObserver>& observer) {
465 observer_ = observer;
468 const std::shared_ptr<EventBaseObserver>& getObserver() {
473 * Set the name of the thread that runs this event base.
475 void setName(const std::string& name);
478 * Returns the name of the thread that runs this event base.
480 const std::string& getName();
482 /// Implements the Executor interface
483 void add(Cob fn) override {
484 // runInEventBaseThread() takes a const&,
485 // so no point in doing std::move here.
486 runInEventBaseThread(fn);
489 /// Implements the DrivableExecutor interface
490 void drive() override {
497 void attachTimeoutManager(AsyncTimeout* obj,
498 TimeoutManager::InternalEnum internal);
500 void detachTimeoutManager(AsyncTimeout* obj);
502 bool scheduleTimeout(AsyncTimeout* obj, std::chrono::milliseconds timeout);
504 void cancelTimeout(AsyncTimeout* obj);
506 bool isInTimeoutManagerThread() {
507 return isInEventBaseThread();
510 // Helper class used to short circuit runInEventBaseThread
511 class RunInLoopCallback : public LoopCallback {
513 RunInLoopCallback(void (*fn)(void*), void* arg);
514 void runLoopCallback() noexcept;
522 * Helper function that tells us whether we have already handled
523 * some event/timeout/callback in this loop iteration.
525 bool nothingHandledYet();
527 // --------- libevent callbacks (not for client use) ------------
529 static void runFunctionPtr(std::function<void()>* fn);
531 // small object used as a callback arg with enough info to execute the
532 // appropriate client-provided Cob
533 class CobTimeout : public AsyncTimeout {
535 CobTimeout(EventBase* b, const Cob& c, TimeoutManager::InternalEnum in)
536 : AsyncTimeout(b, in), cob_(c) {}
538 virtual void timeoutExpired() noexcept;
544 typedef boost::intrusive::list_member_hook<
545 boost::intrusive::link_mode<boost::intrusive::auto_unlink> > ListHook;
549 typedef boost::intrusive::list<
551 boost::intrusive::member_hook<CobTimeout, ListHook, &CobTimeout::hook>,
552 boost::intrusive::constant_time_size<false> > List;
555 typedef LoopCallback::List LoopCallbackList;
556 class FunctionRunner;
558 bool loopBody(int flags = 0);
560 // executes any callbacks queued by runInLoop(); returns false if none found
561 bool runLoopCallbacks(bool setContext = true);
563 void initNotificationQueue();
565 CobTimeout::List pendingCobTimeouts_;
567 LoopCallbackList loopCallbacks_;
568 LoopCallbackList runBeforeLoopCallbacks_;
569 LoopCallbackList onDestructionCallbacks_;
571 // This will be null most of the time, but point to currentCallbacks
572 // if we are in the middle of running loop callbacks, such that
573 // runInLoop(..., true) will always run in the current loop
575 LoopCallbackList* runOnceCallbacks_;
577 // stop_ is set by terminateLoopSoon() and is used by the main loop
578 // to determine if it should exit
581 // The ID of the thread running the main loop.
582 // 0 if loop is not running.
583 // Note: POSIX doesn't guarantee that 0 is an invalid pthread_t (or
584 // even that atomic<pthread_t> is valid), but that's how it is
585 // everywhere (at least on Linux, FreeBSD, and OSX).
586 std::atomic<pthread_t> loopThread_;
588 // pointer to underlying event_base class doing the heavy lifting
591 // A notification queue for runInEventBaseThread() to use
592 // to send function requests to the EventBase thread.
593 std::unique_ptr<NotificationQueue<std::pair<void (*)(void*), void*>>> queue_;
594 std::unique_ptr<FunctionRunner> fnRunner_;
596 // limit for latency in microseconds (0 disables)
599 // exponentially-smoothed average loop time for latency-limiting
600 SmoothLoopTime avgLoopTime_;
602 // smoothed loop time used to invoke latency callbacks; differs from
603 // avgLoopTime_ in that it's scaled down after triggering a callback
604 // to reduce spamminess
605 SmoothLoopTime maxLatencyLoopTime_;
607 // callback called when latency limit is exceeded
610 // we'll wait this long before running deferred callbacks if the event
612 static const int kDEFAULT_IDLE_WAIT_USEC = 20000; // 20ms
614 // Wrap-around loop counter to detect beginning of each loop
615 uint64_t nextLoopCnt_;
616 uint64_t latestLoopCnt_;
619 // Observer to export counters
620 std::shared_ptr<EventBaseObserver> observer_;
621 uint32_t observerSampleCount_;
623 // Name of the thread running this EventBase