2 * Copyright 2016 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.
20 #include <folly/detail/Futex.h>
21 #include <folly/experimental/fibers/TimeoutController.h>
23 namespace folly { namespace fibers {
31 * Primitive which allows to put current Fiber to sleep and wake it from another
43 * Puts active fiber to sleep. Returns when post is called.
48 * Put active fiber to sleep indefinitely. However, timeoutHandler may
49 * be used elsewhere on the same thread in order to schedule a wakeup
50 * for the active fiber. Users of timeoutHandler must be on the same thread
51 * as the active fiber and may only schedule one timeout, which must occur
52 * after the active fiber calls wait.
54 void wait(TimeoutHandler& timeoutHandler);
57 * Puts active fiber to sleep. Returns when post is called.
59 * @param mainContextFunc this function is immediately executed on the main
63 void wait(F&& mainContextFunc);
66 * This is here only not break tao/locks. Please don't use it, because it is
67 * inefficient when used on Fibers.
69 template<typename C, typename D = typename C::duration>
70 bool timed_wait(const std::chrono::time_point<C,D>& timeout);
73 * Puts active fiber to sleep. Returns when post is called.
75 * @param timeout Baton will be automatically awaken if timeout is hit
77 * @return true if was posted, false if timeout expired
79 bool timed_wait(TimeoutController::Duration timeout);
82 * Puts active fiber to sleep. Returns when post is called.
84 * @param timeout Baton will be automatically awaken if timeout is hit
85 * @param mainContextFunc this function is immediately executed on the main
88 * @return true if was posted, false if timeout expired
91 bool timed_wait(TimeoutController::Duration timeout, F&& mainContextFunc);
94 * Checks if the baton has been posted without blocking.
95 * @return true iff the baton has been posted.
100 * Wakes up Fiber which was waiting on this Baton (or if no Fiber is waiting,
101 * next wait() call will return immediately).
106 * Reset's the baton (equivalent to destroying the object and constructing
107 * another one in place).
108 * Caller is responsible for making sure no one is waiting on/posting the
109 * baton when reset() is called.
114 * Provides a way to schedule a wakeup for a wait()ing fiber.
115 * A TimeoutHandler must be passed to Baton::wait(TimeoutHandler&)
116 * before a timeout is scheduled. It is only safe to use the
117 * TimeoutHandler on the same thread as the wait()ing fiber.
118 * scheduleTimeout() may only be called once prior to the end of the
119 * associated Baton's life.
121 class TimeoutHandler {
123 void scheduleTimeout(TimeoutController::Duration timeoutMs);
128 void cancelTimeout();
130 std::function<void()> timeoutFunc_{nullptr};
131 FiberManager* fiberManager_{nullptr};
133 intptr_t timeoutPtr_{0};
139 * Must be positive. If multiple threads are actively using a
140 * higher-level data structure that uses batons internally, it is
141 * likely that the post() and wait() calls happen almost at the same
142 * time. In this state, we lose big 50% of the time if the wait goes
143 * to sleep immediately. On circa-2013 devbox hardware it costs about
144 * 7 usec to FUTEX_WAIT and then be awoken (half the t/iter as the
145 * posix_sem_pingpong test in BatonTests). We can improve our chances
146 * of early post by spinning for a bit, although we have to balance
147 * this against the loss if we end up sleeping any way. Spins on this
148 * hw take about 7 nanos (all but 0.5 nanos is the pause instruction).
149 * We give ourself 300 spins, which is about 2 usec of waiting. As a
150 * partial consolation, since we are using the pause instruction we
151 * are giving a speed boost to the colocated hyperthread.
153 PreBlockAttempts = 300,
156 explicit Baton(intptr_t state) : waitingFiber_(state) {};
158 void postHelper(intptr_t new_value);
162 template <typename F>
163 inline void waitFiber(FiberManager& fm, F&& mainContextFunc);
165 * Spin for "some time" (see discussion on PreBlockAttempts) waiting
167 * @return true if we received a post the spin wait, false otherwise. If the
168 * function returns true then Baton state is guaranteed to be POSTED
170 bool spinWaitForEarlyPost();
172 bool timedWaitThread(TimeoutController::Duration timeout);
174 static constexpr intptr_t NO_WAITER = 0;
175 static constexpr intptr_t POSTED = -1;
176 static constexpr intptr_t TIMEOUT = -2;
177 static constexpr intptr_t THREAD_WAITING = -3;
180 std::atomic<intptr_t> waitingFiber_;
182 folly::detail::Futex<> futex;
183 int32_t _unused_packing;
190 #include <folly/experimental/fibers/Baton-inl.h>