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>
25 #include <type_traits>
28 #include <folly/detail/TurnSequencer.h>
29 #include <folly/Portability.h>
35 template<typename> class Atom>
39 /// LockFreeRingBuffer<T> is a fixed-size, concurrent ring buffer with the
40 /// following semantics:
42 /// 1. Writers cannot block on other writers UNLESS they are <capacity> writes
43 /// apart from each other (writing to the same slot after a wrap-around)
44 /// 2. Writers cannot block on readers
45 /// 3. Readers can wait for writes that haven't occurred yet
46 /// 4. Readers can detect if they are lagging behind
48 /// In this sense, reads from this buffer are best-effort but writes
51 /// Another way to think about this is as an unbounded stream of writes. The
52 /// buffer contains the last <capacity> writes but readers can attempt to read
53 /// any part of the stream, even outside this window. The read API takes a
54 /// Cursor that can point anywhere in this stream of writes. Reads from the
55 /// "future" can optionally block but reads from the "past" will always fail.
58 template<typename T, template<typename> class Atom = std::atomic>
59 class LockFreeRingBuffer: boost::noncopyable {
61 static_assert(std::is_nothrow_default_constructible<T>::value,
62 "Element type must be nothrow default constructible");
64 static_assert(FOLLY_IS_TRIVIALLY_COPYABLE(T),
65 "Element type must be trivially copyable");
68 /// Opaque pointer to a past or future write.
69 /// Can be moved relative to its current location but not in absolute terms.
71 explicit Cursor(uint64_t initialTicket) noexcept : ticket(initialTicket) {}
73 void moveForward(uint64_t steps = 1) noexcept {
77 void moveBackward(uint64_t steps = 1) noexcept {
85 protected: // for test visibility reasons
87 friend class LockFreeRingBuffer;
90 explicit LockFreeRingBuffer(uint32_t capacity) noexcept
92 , slots_(new detail::RingBufferSlot<T,Atom>[capacity])
96 /// Perform a single write of an object of type T.
97 /// Writes can block iff a previous writer has not yet completed a write
98 /// for the same slot (before the most recent wrap-around).
99 void write(T& value) noexcept {
100 uint64_t ticket = ticket_.fetch_add(1);
101 slots_[idx(ticket)].write(turn(ticket), value);
104 /// Read the value at the cursor.
105 /// Returns true if the read succeeded, false otherwise. If the return
106 /// value is false, dest is to be considered partially read and in an
107 /// inconsistent state. Readers are advised to discard it.
108 bool tryRead(T& dest, const Cursor& cursor) noexcept {
109 return slots_[idx(cursor.ticket)].tryRead(dest, turn(cursor.ticket));
112 /// Read the value at the cursor or block if the write has not occurred yet.
113 /// Returns true if the read succeeded, false otherwise. If the return
114 /// value is false, dest is to be considered partially read and in an
115 /// inconsistent state. Readers are advised to discard it.
116 bool waitAndTryRead(T& dest, const Cursor& cursor) noexcept {
117 return slots_[idx(cursor.ticket)].waitAndTryRead(dest, turn(cursor.ticket));
120 /// Returns a Cursor pointing to the first write that has not occurred yet.
121 Cursor currentHead() noexcept {
122 return Cursor(ticket_.load());
125 /// Returns a Cursor pointing to a currently readable write.
126 /// skipFraction is a value in the [0, 1] range indicating how far into the
127 /// currently readable window to place the cursor. 0 means the
128 /// earliest readable write, 1 means the latest readable write (if any).
129 Cursor currentTail(double skipFraction = 0.0) noexcept {
130 assert(skipFraction >= 0.0 && skipFraction <= 1.0);
131 uint64_t ticket = ticket_.load();
133 uint64_t backStep = llround((1.0 - skipFraction) * capacity_);
135 // always try to move at least one step backward to something readable
136 backStep = std::max<uint64_t>(1, backStep);
138 // can't go back more steps than we've taken
139 backStep = std::min(ticket, backStep);
141 return Cursor(ticket - backStep);
144 ~LockFreeRingBuffer() {
148 const uint32_t capacity_;
150 const std::unique_ptr<detail::RingBufferSlot<T,Atom>[]> slots_;
152 Atom<uint64_t> ticket_;
154 uint32_t idx(uint64_t ticket) noexcept {
155 return ticket % capacity_;
158 uint32_t turn(uint64_t ticket) noexcept {
159 return (uint32_t)(ticket / capacity_);
161 }; // LockFreeRingBuffer
164 template<typename T, template<typename> class Atom>
165 class RingBufferSlot {
167 explicit RingBufferSlot() noexcept
173 void write(const uint32_t turn, T& value) noexcept {
174 Atom<uint32_t> cutoff(0);
175 sequencer_.waitForTurn(turn * 2, cutoff, false);
177 // Change to an odd-numbered turn to indicate write in process
178 sequencer_.completeTurn(turn * 2);
180 data = std::move(value);
181 sequencer_.completeTurn(turn * 2 + 1);
185 bool waitAndTryRead(T& dest, uint32_t turn) noexcept {
186 uint32_t desired_turn = (turn + 1) * 2;
187 Atom<uint32_t> cutoff(0);
188 if(!sequencer_.tryWaitForTurn(desired_turn, cutoff, false)) {
191 memcpy(&dest, &data, sizeof(T));
193 // if it's still the same turn, we read the value successfully
194 return sequencer_.isTurn(desired_turn);
197 bool tryRead(T& dest, uint32_t turn) noexcept {
198 // The write that started at turn 0 ended at turn 2
199 if (!sequencer_.isTurn((turn + 1) * 2)) {
202 memcpy(&dest, &data, sizeof(T));
204 // if it's still the same turn, we read the value successfully
205 return sequencer_.isTurn((turn + 1) * 2);
210 TurnSequencer<Atom> sequencer_;
214 } // namespace detail