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 <boost/noncopyable.hpp>
24 #include <type_traits>
27 #include <folly/detail/TurnSequencer.h>
28 #include <folly/Portability.h>
34 template<typename> class Atom>
38 /// LockFreeRingBuffer<T> is a fixed-size, concurrent ring buffer with the
39 /// following semantics:
41 /// 1. Writers cannot block on other writers UNLESS they are <capacity> writes
42 /// apart from each other (writing to the same slot after a wrap-around)
43 /// 2. Writers cannot block on readers
44 /// 3. Readers can wait for writes that haven't occurred yet
45 /// 4. Readers can detect if they are lagging behind
47 /// In this sense, reads from this buffer are best-effort but writes
50 /// Another way to think about this is as an unbounded stream of writes. The
51 /// buffer contains the last <capacity> writes but readers can attempt to read
52 /// any part of the stream, even outside this window. The read API takes a
53 /// Cursor that can point anywhere in this stream of writes. Reads from the
54 /// "future" can optionally block but reads from the "past" will always fail.
57 template<typename T, template<typename> class Atom = std::atomic>
58 class LockFreeRingBuffer: boost::noncopyable {
60 static_assert(std::is_nothrow_default_constructible<T>::value,
61 "Element type must be nothrow default constructible");
63 static_assert(FOLLY_IS_TRIVIALLY_COPYABLE(T),
64 "Element type must be trivially copyable");
67 /// Opaque pointer to a past or future write.
68 /// Can be moved relative to its current location but not in absolute terms.
70 explicit Cursor(uint64_t initialTicket) noexcept : ticket(initialTicket) {}
72 void moveForward(uint64_t steps = 1) noexcept {
76 void moveBackward(uint64_t steps = 1) noexcept {
84 protected: // for test visibility reasons
86 friend class LockFreeRingBuffer;
89 explicit LockFreeRingBuffer(size_t capacity) noexcept
91 , slots_(new detail::RingBufferSlot<T,Atom>[capacity])
95 /// Perform a single write of an object of type T.
96 /// Writes can block iff a previous writer has not yet completed a write
97 /// for the same slot (before the most recent wrap-around).
98 void write(T& value) noexcept {
99 uint64_t ticket = ticket_.fetch_add(1);
100 slots_[idx(ticket)].write(turn(ticket), value);
103 /// Read the value at the cursor.
104 /// Returns true if the read succeeded, false otherwise. If the return
105 /// value is false, dest is to be considered partially read and in an
106 /// inconsistent state. Readers are advised to discard it.
107 bool tryRead(T& dest, const Cursor& cursor) noexcept {
108 return slots_[idx(cursor.ticket)].tryRead(dest, turn(cursor.ticket));
111 /// Read the value at the cursor or block if the write has not occurred yet.
112 /// Returns true if the read succeeded, false otherwise. If the return
113 /// value is false, dest is to be considered partially read and in an
114 /// inconsistent state. Readers are advised to discard it.
115 bool waitAndTryRead(T& dest, const Cursor& cursor) noexcept {
116 return slots_[idx(cursor.ticket)].waitAndTryRead(dest, turn(cursor.ticket));
119 /// Returns a Cursor pointing to the first write that has not occurred yet.
120 Cursor currentHead() noexcept {
121 return Cursor(ticket_.load());
124 /// Returns a Cursor pointing to a currently readable write.
125 /// skipFraction is a value in the [0, 1] range indicating how far into the
126 /// currently readable window to place the cursor. 0 means the
127 /// earliest readable write, 1 means the latest readable write (if any).
128 Cursor currentTail(double skipFraction = 0.0) noexcept {
129 assert(skipFraction >= 0.0 && skipFraction <= 1.0);
130 uint64_t ticket = ticket_.load();
132 uint64_t backStep = std::llround((1.0 - skipFraction) * capacity_);
134 // always try to move at least one step backward to something readable
135 backStep = std::max<uint64_t>(1, backStep);
137 // can't go back more steps than we've taken
138 backStep = std::min(ticket, backStep);
140 return Cursor(ticket - backStep);
143 ~LockFreeRingBuffer() {
147 const size_t capacity_;
149 const std::unique_ptr<detail::RingBufferSlot<T,Atom>[]> slots_;
151 Atom<uint64_t> ticket_;
153 uint32_t idx(uint64_t ticket) noexcept {
154 return ticket % capacity_;
157 uint32_t turn(uint64_t ticket) noexcept {
158 return (ticket / capacity_);
160 }; // LockFreeRingBuffer
163 template<typename T, template<typename> class Atom>
164 class RingBufferSlot {
166 explicit RingBufferSlot() noexcept
172 void write(const uint32_t turn, T& value) noexcept {
173 Atom<uint32_t> cutoff(0);
174 sequencer_.waitForTurn(turn * 2, cutoff, false);
176 // Change to an odd-numbered turn to indicate write in process
177 sequencer_.completeTurn(turn * 2);
179 data = std::move(value);
180 sequencer_.completeTurn(turn * 2 + 1);
184 bool waitAndTryRead(T& dest, uint32_t turn) noexcept {
185 uint32_t desired_turn = (turn + 1) * 2;
186 Atom<uint32_t> cutoff(0);
187 if(!sequencer_.tryWaitForTurn(desired_turn, cutoff, false)) {
190 memcpy(&dest, &data, sizeof(T));
192 // if it's still the same turn, we read the value successfully
193 return sequencer_.isTurn(desired_turn);
196 bool tryRead(T& dest, uint32_t turn) noexcept {
197 // The write that started at turn 0 ended at turn 2
198 if (!sequencer_.isTurn((turn + 1) * 2)) {
201 memcpy(&dest, &data, sizeof(T));
203 // if it's still the same turn, we read the value successfully
204 return sequencer_.isTurn((turn + 1) * 2);
209 TurnSequencer<Atom> sequencer_;
213 } // namespace detail