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.
20 #include <folly/detail/Futex.h>
21 #include <folly/experimental/fibers/TimeoutController.h>
23 namespace folly { namespace fibers {
30 * Primitive which allows to put current Fiber to sleep and wake it from another
40 * Puts active fiber to sleep. Returns when post is called.
45 * Puts active fiber to sleep. Returns when post is called.
47 * @param mainContextFunc this function is immediately executed on the main
51 void wait(F&& mainContextFunc);
54 * This is here only not break tao/locks. Please don't use it, because it is
55 * inefficient when used on Fibers.
57 template<typename C, typename D = typename C::duration>
58 bool timed_wait(const std::chrono::time_point<C,D>& timeout);
61 * Puts active fiber to sleep. Returns when post is called.
63 * @param timeout Baton will be automatically awaken if timeout is hit
65 * @return true if was posted, false if timeout expired
67 bool timed_wait(TimeoutController::Duration timeout);
70 * Puts active fiber to sleep. Returns when post is called.
72 * @param timeout Baton will be automatically awaken if timeout is hit
73 * @param mainContextFunc this function is immediately executed on the main
76 * @return true if was posted, false if timeout expired
79 bool timed_wait(TimeoutController::Duration timeout, F&& mainContextFunc);
82 * Checks if the baton has been posted without blocking.
83 * @return true iff the baton has been posted.
88 * Wakes up Fiber which was waiting on this Baton (or if no Fiber is waiting,
89 * next wait() call will return immediately).
94 * Reset's the baton (equivalent to destroying the object and constructing
95 * another one in place).
96 * Caller is responsible for making sure no one is waiting on/posting the
97 * baton when reset() is called.
104 * Must be positive. If multiple threads are actively using a
105 * higher-level data structure that uses batons internally, it is
106 * likely that the post() and wait() calls happen almost at the same
107 * time. In this state, we lose big 50% of the time if the wait goes
108 * to sleep immediately. On circa-2013 devbox hardware it costs about
109 * 7 usec to FUTEX_WAIT and then be awoken (half the t/iter as the
110 * posix_sem_pingpong test in BatonTests). We can improve our chances
111 * of early post by spinning for a bit, although we have to balance
112 * this against the loss if we end up sleeping any way. Spins on this
113 * hw take about 7 nanos (all but 0.5 nanos is the pause instruction).
114 * We give ourself 300 spins, which is about 2 usec of waiting. As a
115 * partial consolation, since we are using the pause instruction we
116 * are giving a speed boost to the colocated hyperthread.
118 PreBlockAttempts = 300,
121 explicit Baton(intptr_t state) : waitingFiber_(state) {};
123 void postHelper(intptr_t new_value);
127 template <typename F>
128 inline void waitFiber(FiberManager& fm, F&& mainContextFunc);
130 * Spin for "some time" (see discussion on PreBlockAttempts) waiting
132 * @return true if we received a post the spin wait, false otherwise. If the
133 * function returns true then Baton state is guaranteed to be POSTED
135 bool spinWaitForEarlyPost();
137 bool timedWaitThread(TimeoutController::Duration timeout);
139 static constexpr intptr_t NO_WAITER = 0;
140 static constexpr intptr_t POSTED = -1;
141 static constexpr intptr_t TIMEOUT = -2;
142 static constexpr intptr_t THREAD_WAITING = -3;
145 std::atomic<intptr_t> waitingFiber_;
147 folly::detail::Futex<> futex;
148 int32_t _unused_packing;
155 #include <folly/experimental/fibers/Baton-inl.h>