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/*
* Copyright (c) Facebook, Inc. and its affiliates.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#pragma once
#include <algorithm>
#include <atomic>
#include <cassert>
#include <cstring>
#include <limits>
#include <type_traits>
#include <folly/Traits.h>
#include <folly/concurrency/CacheLocality.h>
#include <folly/detail/TurnSequencer.h>
#include <folly/portability/Unistd.h>
namespace folly {
namespace detail {
template <typename T, template <typename> class Atom>
struct SingleElementQueue;
template <typename T>
class MPMCPipelineStageImpl;
/// MPMCQueue base CRTP template
template <typename>
class MPMCQueueBase;
} // namespace detail
/// MPMCQueue<T> is a high-performance bounded concurrent queue that
/// supports multiple producers, multiple consumers, and optional blocking.
/// The queue has a fixed capacity, for which all memory will be allocated
/// up front. The bulk of the work of enqueuing and dequeuing can be
/// performed in parallel.
///
/// MPMCQueue is linearizable. That means that if a call to write(A)
/// returns before a call to write(B) begins, then A will definitely end up
/// in the queue before B, and if a call to read(X) returns before a call
/// to read(Y) is started, that X will be something from earlier in the
/// queue than Y. This also means that if a read call returns a value, you
/// can be sure that all previous elements of the queue have been assigned
/// a reader (that reader might not yet have returned, but it exists).
///
/// The underlying implementation uses a ticket dispenser for the head and
/// the tail, spreading accesses across N single-element queues to produce
/// a queue with capacity N. The ticket dispensers use atomic increment,
/// which is more robust to contention than a CAS loop. Each of the
/// single-element queues uses its own CAS to serialize access, with an
/// adaptive spin cutoff. When spinning fails on a single-element queue
/// it uses futex()'s _BITSET operations to reduce unnecessary wakeups
/// even if multiple waiters are present on an individual queue (such as
/// when the MPMCQueue's capacity is smaller than the number of enqueuers
/// or dequeuers).
///
/// In benchmarks (contained in tao/queues/ConcurrentQueueTests)
/// it handles 1 to 1, 1 to N, N to 1, and N to M thread counts better
/// than any of the alternatives present in fbcode, for both small (~10)
/// and large capacities. In these benchmarks it is also faster than
/// tbb::concurrent_bounded_queue for all configurations. When there are
/// many more threads than cores, MPMCQueue is _much_ faster than the tbb
/// queue because it uses futex() to block and unblock waiting threads,
/// rather than spinning with sched_yield.
///
/// NOEXCEPT INTERACTION: tl;dr; If it compiles you're fine. Ticket-based
/// queues separate the assignment of queue positions from the actual
/// construction of the in-queue elements, which means that the T
/// constructor used during enqueue must not throw an exception. This is
/// enforced at compile time using type traits, which requires that T be
/// adorned with accurate noexcept information. If your type does not
/// use noexcept, you will have to wrap it in something that provides
/// the guarantee. We provide an alternate safe implementation for types
/// that don't use noexcept but that are marked folly::IsRelocatable
/// and std::is_nothrow_constructible, which is common for folly types.
/// In particular, if you can declare FOLLY_ASSUME_FBVECTOR_COMPATIBLE
/// then your type can be put in MPMCQueue.
///
/// If you have a pool of N queue consumers that you want to shut down
/// after the queue has drained, one way is to enqueue N sentinel values
/// to the queue. If the producer doesn't know how many consumers there
/// are you can enqueue one sentinel and then have each consumer requeue
/// two sentinels after it receives it (by requeuing 2 the shutdown can
/// complete in O(log P) time instead of O(P)).
template <
typename T,
template <typename> class Atom = std::atomic,
bool Dynamic = false>
class MPMCQueue : public detail::MPMCQueueBase<MPMCQueue<T, Atom, Dynamic>> {
friend class detail::MPMCPipelineStageImpl<T>;
using Slot = detail::SingleElementQueue<T, Atom>;
public:
explicit MPMCQueue(size_t queueCapacity)
: detail::MPMCQueueBase<MPMCQueue<T, Atom, Dynamic>>(queueCapacity) {
this->stride_ = this->computeStride(queueCapacity);
this->slots_ = new Slot[queueCapacity + 2 * this->kSlotPadding];
}
MPMCQueue() noexcept {}
};
/// *** The dynamic version of MPMCQueue is deprecated. ***
/// Use UnboundedQueue instead.
/// The dynamic version of MPMCQueue allows dynamic expansion of queue
/// capacity, such that a queue may start with a smaller capacity than
/// specified and expand only if needed. Users may optionally specify
/// the initial capacity and the expansion multiplier.
///
/// The design uses a seqlock to enforce mutual exclusion among
/// expansion attempts. Regular operations read up-to-date queue
/// information (slots array, capacity, stride) inside read-only
/// seqlock sections, which are unimpeded when no expansion is in
/// progress.
///
/// An expansion computes a new capacity, allocates a new slots array,
/// and updates stride. No information needs to be copied from the
/// current slots array to the new one. When this happens, new slots
/// will not have sequence numbers that match ticket numbers. The
/// expansion needs to compute a ticket offset such that operations
/// that use new arrays can adjust the calculations of slot indexes
/// and sequence numbers that take into account that the new slots
/// start with sequence numbers of zero. The current ticket offset is
/// packed with the seqlock in an atomic 64-bit integer. The initial
/// offset is zero.
///
/// Lagging write and read operations with tickets lower than the
/// ticket offset of the current slots array (i.e., the minimum ticket
/// number that can be served by the current array) must use earlier
/// closed arrays instead of the current one. Information about closed
/// slots arrays (array address, capacity, stride, and offset) is
/// maintained in a logarithmic-sized structure. Each entry in that
/// structure never needs to be changed once set. The number of closed
/// arrays is half the value of the seqlock (when unlocked).
///
/// The acquisition of the seqlock to perform an expansion does not
/// prevent the issuing of new push and pop tickets concurrently. The
/// expansion must set the new ticket offset to a value that couldn't
/// have been issued to an operation that has already gone through a
/// seqlock read-only section (and hence obtained information for
/// older closed arrays).
///
/// Note that the total queue capacity can temporarily exceed the
/// specified capacity when there are lagging consumers that haven't
/// yet consumed all the elements in closed arrays. Users should not
/// rely on the capacity of dynamic queues for synchronization, e.g.,
/// they should not expect that a thread will definitely block on a
/// call to blockingWrite() when the queue size is known to be equal
/// to its capacity.
///
/// Note that some writeIfNotFull() and tryWriteUntil() operations may
/// fail even if the size of the queue is less than its maximum
/// capacity and despite the success of expansion, if the operation
/// happens to acquire a ticket that belongs to a closed array. This
/// is a transient condition. Typically, one or two ticket values may
/// be subject to such condition per expansion.
///
/// The dynamic version is a partial specialization of MPMCQueue with
/// Dynamic == true
template <typename T, template <typename> class Atom>
class MPMCQueue<T, Atom, true>
: public detail::MPMCQueueBase<MPMCQueue<T, Atom, true>> {
friend class detail::MPMCQueueBase<MPMCQueue<T, Atom, true>>;
using Slot = detail::SingleElementQueue<T, Atom>;
struct ClosedArray {
uint64_t offset_{0};
Slot* slots_{nullptr};
size_t capacity_{0};
int stride_{0};
};
public:
explicit MPMCQueue(size_t queueCapacity)
: detail::MPMCQueueBase<MPMCQueue<T, Atom, true>>(queueCapacity) {
size_t cap = std::min<size_t>(kDefaultMinDynamicCapacity, queueCapacity);
initQueue(cap, kDefaultExpansionMultiplier);
}
explicit MPMCQueue(
size_t queueCapacity,
size_t minCapacity,
size_t expansionMultiplier)
: detail::MPMCQueueBase<MPMCQueue<T, Atom, true>>(queueCapacity) {
minCapacity = std::max<size_t>(1, minCapacity);
size_t cap = std::min<size_t>(minCapacity, queueCapacity);
expansionMultiplier = std::max<size_t>(2, expansionMultiplier);
initQueue(cap, expansionMultiplier);
}
MPMCQueue() noexcept {
dmult_ = 0;
closed_ = nullptr;
}
MPMCQueue(MPMCQueue<T, Atom, true>&& rhs) noexcept {
this->capacity_ = rhs.capacity_;
new (&this->dslots_)
Atom<Slot*>(rhs.dslots_.load(std::memory_order_relaxed));
new (&this->dstride_)
Atom<int>(rhs.dstride_.load(std::memory_order_relaxed));
this->dstate_.store(
rhs.dstate_.load(std::memory_order_relaxed), std::memory_order_relaxed);
this->dcapacity_.store(
rhs.dcapacity_.load(std::memory_order_relaxed),
std::memory_order_relaxed);
this->pushTicket_.store(
rhs.pushTicket_.load(std::memory_order_relaxed),
std::memory_order_relaxed);
this->popTicket_.store(
rhs.popTicket_.load(std::memory_order_relaxed),
std::memory_order_relaxed);
this->pushSpinCutoff_.store(
rhs.pushSpinCutoff_.load(std::memory_order_relaxed),
std::memory_order_relaxed);
this->popSpinCutoff_.store(
rhs.popSpinCutoff_.load(std::memory_order_relaxed),
std::memory_order_relaxed);
dmult_ = rhs.dmult_;
closed_ = rhs.closed_;
rhs.capacity_ = 0;
rhs.dslots_.store(nullptr, std::memory_order_relaxed);
rhs.dstride_.store(0, std::memory_order_relaxed);
rhs.dstate_.store(0, std::memory_order_relaxed);
rhs.dcapacity_.store(0, std::memory_order_relaxed);
rhs.pushTicket_.store(0, std::memory_order_relaxed);
rhs.popTicket_.store(0, std::memory_order_relaxed);
rhs.pushSpinCutoff_.store(0, std::memory_order_relaxed);
rhs.popSpinCutoff_.store(0, std::memory_order_relaxed);
rhs.dmult_ = 0;
rhs.closed_ = nullptr;
}
MPMCQueue<T, Atom, true> const& operator=(MPMCQueue<T, Atom, true>&& rhs) {
if (this != &rhs) {
this->~MPMCQueue();
new (this) MPMCQueue(std::move(rhs));
}
return *this;
}
~MPMCQueue() {
if (closed_ != nullptr) {
for (int i = getNumClosed(this->dstate_.load()) - 1; i >= 0; --i) {
delete[] closed_[i].slots_;
}
delete[] closed_;
}
using AtomInt = Atom<int>;
this->dstride_.~AtomInt();
using AtomSlot = Atom<Slot*>;
// Sort of a hack to get ~MPMCQueueBase to free dslots_
auto slots = this->dslots_.load();
this->dslots_.~AtomSlot();
this->slots_ = slots;
}
size_t allocatedCapacity() const noexcept {
return this->dcapacity_.load(std::memory_order_relaxed);
}
template <typename... Args>
void blockingWrite(Args&&... args) noexcept {
uint64_t ticket = this->pushTicket_++;
Slot* slots;
size_t cap;
int stride;
uint64_t state;
uint64_t offset;
do {
if (!trySeqlockReadSection(state, slots, cap, stride)) {
asm_volatile_pause();
continue;
}
if (maybeUpdateFromClosed(state, ticket, offset, slots, cap, stride)) {
// There was an expansion after this ticket was issued.
break;
}
if (slots[this->idx((ticket - offset), cap, stride)].mayEnqueue(
this->turn(ticket - offset, cap))) {
// A slot is ready. No need to expand.
break;
} else if (
this->popTicket_.load(std::memory_order_relaxed) + cap > ticket) {
// May block, but a pop is in progress. No need to expand.
// Get seqlock read section info again in case an expansion
// occurred with an equal or higher ticket.
continue;
} else {
// May block. See if we can expand.
if (tryExpand(state, cap)) {
// This or another thread started an expansion. Get updated info.
continue;
} else {
// Can't expand.
break;
}
}
} while (true);
this->enqueueWithTicketBase(
ticket - offset, slots, cap, stride, std::forward<Args>(args)...);
}
void blockingReadWithTicket(uint64_t& ticket, T& elem) noexcept {
ticket = this->popTicket_++;
Slot* slots;
size_t cap;
int stride;
uint64_t state;
uint64_t offset;
while (!trySeqlockReadSection(state, slots, cap, stride)) {
asm_volatile_pause();
}
// If there was an expansion after the corresponding push ticket
// was issued, adjust accordingly
maybeUpdateFromClosed(state, ticket, offset, slots, cap, stride);
this->dequeueWithTicketBase(ticket - offset, slots, cap, stride, elem);
}
private:
enum {
kSeqlockBits = 6,
kDefaultMinDynamicCapacity = 10,
kDefaultExpansionMultiplier = 10,
};
size_t dmult_;
// Info about closed slots arrays for use by lagging operations
ClosedArray* closed_;
void initQueue(const size_t cap, const size_t mult) {
new (&this->dstride_) Atom<int>(this->computeStride(cap));
Slot* slots = new Slot[cap + 2 * this->kSlotPadding];
new (&this->dslots_) Atom<Slot*>(slots);
this->dstate_.store(0);
this->dcapacity_.store(cap);
dmult_ = mult;
size_t maxClosed = 0;
for (size_t expanded = cap; expanded < this->capacity_; expanded *= mult) {
++maxClosed;
}
closed_ = (maxClosed > 0) ? new ClosedArray[maxClosed] : nullptr;
}
bool tryObtainReadyPushTicket(
uint64_t& ticket,
Slot*& slots,
size_t& cap,
int& stride) noexcept {
uint64_t state;
do {
ticket = this->pushTicket_.load(std::memory_order_acquire); // A
if (!trySeqlockReadSection(state, slots, cap, stride)) {
asm_volatile_pause();
continue;
}
// If there was an expansion with offset greater than this ticket,
// adjust accordingly
uint64_t offset;
maybeUpdateFromClosed(state, ticket, offset, slots, cap, stride);
if (slots[this->idx((ticket - offset), cap, stride)].mayEnqueue(
this->turn(ticket - offset, cap))) {
// A slot is ready.
if (this->pushTicket_.compare_exchange_strong(ticket, ticket + 1)) {
// Adjust ticket
ticket -= offset;
return true;
} else {
continue;
}
} else {
if (ticket != this->pushTicket_.load(std::memory_order_relaxed)) { // B
// Try again. Ticket changed.
continue;
}
// Likely to block.
// Try to expand unless the ticket is for a closed array
if (offset == getOffset(state)) {
if (tryExpand(state, cap)) {
// This or another thread started an expansion. Get up-to-date info.
continue;
}
}
return false;
}
} while (true);
}
bool tryObtainPromisedPushTicket(
uint64_t& ticket,
Slot*& slots,
size_t& cap,
int& stride) noexcept {
uint64_t state;
do {
ticket = this->pushTicket_.load(std::memory_order_acquire);
auto numPops = this->popTicket_.load(std::memory_order_acquire);
if (!trySeqlockReadSection(state, slots, cap, stride)) {
asm_volatile_pause();
continue;
}
const auto curCap = cap;
// If there was an expansion with offset greater than this ticket,
// adjust accordingly
uint64_t offset;
maybeUpdateFromClosed(state, ticket, offset, slots, cap, stride);
int64_t n = ticket - numPops;
if (n >= static_cast<ssize_t>(cap)) {
if ((cap == curCap) && tryExpand(state, cap)) {
// This or another thread started an expansion. Start over.
continue;
}
// Can't expand.
ticket -= offset;
return false;
}
if (this->pushTicket_.compare_exchange_strong(ticket, ticket + 1)) {
// Adjust ticket
ticket -= offset;
return true;
}
} while (true);
}
bool tryObtainReadyPopTicket(
uint64_t& ticket,
Slot*& slots,
size_t& cap,
int& stride) noexcept {
uint64_t state;
do {
ticket = this->popTicket_.load(std::memory_order_relaxed);
if (!trySeqlockReadSection(state, slots, cap, stride)) {
asm_volatile_pause();
continue;
}
// If there was an expansion after the corresponding push ticket
// was issued, adjust accordingly
uint64_t offset;
maybeUpdateFromClosed(state, ticket, offset, slots, cap, stride);
if (slots[this->idx((ticket - offset), cap, stride)].mayDequeue(
this->turn(ticket - offset, cap))) {
if (this->popTicket_.compare_exchange_strong(ticket, ticket + 1)) {
// Adjust ticket
ticket -= offset;
return true;
}
} else {
return false;
}
} while (true);
}
bool tryObtainPromisedPopTicket(
uint64_t& ticket,
Slot*& slots,
size_t& cap,
int& stride) noexcept {
uint64_t state;
do {
ticket = this->popTicket_.load(std::memory_order_acquire);
auto numPushes = this->pushTicket_.load(std::memory_order_acquire);
if (!trySeqlockReadSection(state, slots, cap, stride)) {
asm_volatile_pause();
continue;
}
uint64_t offset;
// If there was an expansion after the corresponding push
// ticket was issued, adjust accordingly
maybeUpdateFromClosed(state, ticket, offset, slots, cap, stride);
if (ticket >= numPushes) {
ticket -= offset;
return false;
}
if (this->popTicket_.compare_exchange_strong(ticket, ticket + 1)) {
ticket -= offset;
return true;
}
} while (true);
}
/// Enqueues an element with a specific ticket number
template <typename... Args>
void enqueueWithTicket(const uint64_t ticket, Args&&... args) noexcept {
Slot* slots;
size_t cap;
int stride;
uint64_t state;
uint64_t offset;
while (!trySeqlockReadSection(state, slots, cap, stride)) {
}
// If there was an expansion after this ticket was issued, adjust
// accordingly
maybeUpdateFromClosed(state, ticket, offset, slots, cap, stride);
this->enqueueWithTicketBase(
ticket - offset, slots, cap, stride, std::forward<Args>(args)...);
}
uint64_t getOffset(const uint64_t state) const noexcept {
return state >> kSeqlockBits;
}
int getNumClosed(const uint64_t state) const noexcept {
return (state & ((1 << kSeqlockBits) - 1)) >> 1;
}
/// Try to expand the queue. Returns true if this expansion was
/// successful or a concurent expansion is in progress. Returns
/// false if the queue has reached its maximum capacity or
/// allocation has failed.
bool tryExpand(const uint64_t state, const size_t cap) noexcept {
if (cap == this->capacity_) {
return false;
}
// Acquire seqlock
uint64_t oldval = state;
assert((state & 1) == 0);
if (this->dstate_.compare_exchange_strong(oldval, state + 1)) {
assert(cap == this->dcapacity_.load());
uint64_t ticket =
1 + std::max(this->pushTicket_.load(), this->popTicket_.load());
size_t newCapacity = std::min(dmult_ * cap, this->capacity_);
Slot* newSlots =
new (std::nothrow) Slot[newCapacity + 2 * this->kSlotPadding];
if (newSlots == nullptr) {
// Expansion failed. Restore the seqlock
this->dstate_.store(state);
return false;
}
// Successful expansion
// calculate the current ticket offset
uint64_t offset = getOffset(state);
// calculate index in closed array
int index = getNumClosed(state);
assert((index << 1) < (1 << kSeqlockBits));
// fill the info for the closed slots array
closed_[index].offset_ = offset;
closed_[index].slots_ = this->dslots_.load();
closed_[index].capacity_ = cap;
closed_[index].stride_ = this->dstride_.load();
// update the new slots array info
this->dslots_.store(newSlots);
this->dcapacity_.store(newCapacity);
this->dstride_.store(this->computeStride(newCapacity));
// Release the seqlock and record the new ticket offset
this->dstate_.store((ticket << kSeqlockBits) + (2 * (index + 1)));
return true;
} else { // failed to acquire seqlock
// Someone acaquired the seqlock. Go back to the caller and get
// up-to-date info.
return true;
}
}
/// Seqlock read-only section
bool trySeqlockReadSection(
uint64_t& state,
Slot*& slots,
size_t& cap,
int& stride) noexcept {
state = this->dstate_.load(std::memory_order_acquire);
if (state & 1) {
// Locked.
return false;
}
// Start read-only section.
slots = this->dslots_.load(std::memory_order_relaxed);
cap = this->dcapacity_.load(std::memory_order_relaxed);
stride = this->dstride_.load(std::memory_order_relaxed);
// End of read-only section. Validate seqlock.
std::atomic_thread_fence(std::memory_order_acquire);
return (state == this->dstate_.load(std::memory_order_relaxed));
}
/// If there was an expansion after ticket was issued, update local variables
/// of the lagging operation using the most recent closed array with
/// offset <= ticket and return true. Otherwise, return false;
bool maybeUpdateFromClosed(
const uint64_t state,
const uint64_t ticket,
uint64_t& offset,
Slot*& slots,
size_t& cap,
int& stride) noexcept {
offset = getOffset(state);
if (ticket >= offset) {
return false;
}
for (int i = getNumClosed(state) - 1; i >= 0; --i) {
offset = closed_[i].offset_;
if (offset <= ticket) {
slots = closed_[i].slots_;
cap = closed_[i].capacity_;
stride = closed_[i].stride_;
return true;
}
}
// A closed array with offset <= ticket should have been found
assert(false);
return false;
}
};
namespace detail {
/// CRTP specialization of MPMCQueueBase
template <
template <typename T, template <typename> class Atom, bool Dynamic>
class Derived,
typename T,
template <typename> class Atom,
bool Dynamic>
class MPMCQueueBase<Derived<T, Atom, Dynamic>> {
// Note: Using CRTP static casts in several functions of this base
// template instead of making called functions virtual or duplicating
// the code of calling functions in the derived partially specialized
// template
static_assert(
std::is_nothrow_constructible<T, T&&>::value ||
folly::IsRelocatable<T>::value,
"T must be relocatable or have a noexcept move constructor");
public:
typedef T value_type;
using Slot = detail::SingleElementQueue<T, Atom>;
explicit MPMCQueueBase(size_t queueCapacity)
: capacity_(queueCapacity),
dstate_(0),
dcapacity_(0),
pushTicket_(0),
popTicket_(0),
pushSpinCutoff_(0),
popSpinCutoff_(0) {
if (queueCapacity == 0) {
throw std::invalid_argument(
"MPMCQueue with explicit capacity 0 is impossible"
// Stride computation in derived classes would sigfpe if capacity is 0
);
}
// ideally this would be a static assert, but g++ doesn't allow it
assert(
alignof(MPMCQueue<T, Atom>) >= hardware_destructive_interference_size);
assert(
static_cast<uint8_t*>(static_cast<void*>(&popTicket_)) -
static_cast<uint8_t*>(static_cast<void*>(&pushTicket_)) >=
static_cast<ptrdiff_t>(hardware_destructive_interference_size));
}
/// A default-constructed queue is useful because a usable (non-zero
/// capacity) queue can be moved onto it or swapped with it
MPMCQueueBase() noexcept
: capacity_(0),
slots_(nullptr),
stride_(0),
dstate_(0),
dcapacity_(0),
pushTicket_(0),
popTicket_(0),
pushSpinCutoff_(0),
popSpinCutoff_(0) {}
/// IMPORTANT: The move constructor is here to make it easier to perform
/// the initialization phase, it is not safe to use when there are any
/// concurrent accesses (this is not checked).
MPMCQueueBase(MPMCQueueBase<Derived<T, Atom, Dynamic>>&& rhs) noexcept
: capacity_(rhs.capacity_),
slots_(rhs.slots_),
stride_(rhs.stride_),
dstate_(rhs.dstate_.load(std::memory_order_relaxed)),
dcapacity_(rhs.dcapacity_.load(std::memory_order_relaxed)),
pushTicket_(rhs.pushTicket_.load(std::memory_order_relaxed)),
popTicket_(rhs.popTicket_.load(std::memory_order_relaxed)),
pushSpinCutoff_(rhs.pushSpinCutoff_.load(std::memory_order_relaxed)),
popSpinCutoff_(rhs.popSpinCutoff_.load(std::memory_order_relaxed)) {
// relaxed ops are okay for the previous reads, since rhs queue can't
// be in concurrent use
// zero out rhs
rhs.capacity_ = 0;
rhs.slots_ = nullptr;
rhs.stride_ = 0;
rhs.dstate_.store(0, std::memory_order_relaxed);
rhs.dcapacity_.store(0, std::memory_order_relaxed);
rhs.pushTicket_.store(0, std::memory_order_relaxed);
rhs.popTicket_.store(0, std::memory_order_relaxed);
rhs.pushSpinCutoff_.store(0, std::memory_order_relaxed);
rhs.popSpinCutoff_.store(0, std::memory_order_relaxed);
}
/// IMPORTANT: The move operator is here to make it easier to perform
/// the initialization phase, it is not safe to use when there are any
/// concurrent accesses (this is not checked).
MPMCQueueBase<Derived<T, Atom, Dynamic>> const& operator=(
MPMCQueueBase<Derived<T, Atom, Dynamic>>&& rhs) {
if (this != &rhs) {
this->~MPMCQueueBase();
new (this) MPMCQueueBase(std::move(rhs));
}
return *this;
}
MPMCQueueBase(const MPMCQueueBase&) = delete;
MPMCQueueBase& operator=(const MPMCQueueBase&) = delete;
/// MPMCQueue can only be safely destroyed when there are no
/// pending enqueuers or dequeuers (this is not checked).
~MPMCQueueBase() {
delete[] slots_;
}
/// Returns the number of writes (including threads that are blocked waiting
/// to write) minus the number of reads (including threads that are blocked
/// waiting to read). So effectively, it becomes:
/// elements in queue + pending(calls to write) - pending(calls to read).
/// If nothing is pending, then the method returns the actual number of
/// elements in the queue.
/// The returned value can be negative if there are no writers and the queue
/// is empty, but there is one reader that is blocked waiting to read (in
/// which case, the returned size will be -1).
ssize_t size() const noexcept {
// since both pushes and pops increase monotonically, we can get a
// consistent snapshot either by bracketing a read of popTicket_ with
// two reads of pushTicket_ that return the same value, or the other
// way around. We maximize our chances by alternately attempting
// both bracketings.
uint64_t pushes = pushTicket_.load(std::memory_order_acquire); // A
uint64_t pops = popTicket_.load(std::memory_order_acquire); // B
while (true) {
uint64_t nextPushes = pushTicket_.load(std::memory_order_acquire); // C
if (pushes == nextPushes) {
// pushTicket_ didn't change from A (or the previous C) to C,
// so we can linearize at B (or D)
return ssize_t(pushes - pops);
}
pushes = nextPushes;
uint64_t nextPops = popTicket_.load(std::memory_order_acquire); // D
if (pops == nextPops) {
// popTicket_ didn't chance from B (or the previous D), so we
// can linearize at C
return ssize_t(pushes - pops);
}
pops = nextPops;
}
}
/// Returns true if there are no items available for dequeue
bool isEmpty() const noexcept {
return size() <= 0;
}
/// Returns true if there is currently no empty space to enqueue
bool isFull() const noexcept {
// careful with signed -> unsigned promotion, since size can be negative
return size() >= static_cast<ssize_t>(capacity_);
}
/// Returns is a guess at size() for contexts that don't need a precise
/// value, such as stats. More specifically, it returns the number of writes
/// minus the number of reads, but after reading the number of writes, more
/// writers could have came before the number of reads was sampled,
/// and this method doesn't protect against such case.
/// The returned value can be negative.
ssize_t sizeGuess() const noexcept {
return writeCount() - readCount();
}
/// Doesn't change
size_t capacity() const noexcept {
return capacity_;
}
/// Doesn't change for non-dynamic
size_t allocatedCapacity() const noexcept {
return capacity_;
}
/// Returns the total number of calls to blockingWrite or successful
/// calls to write, including those blockingWrite calls that are
/// currently blocking
uint64_t writeCount() const noexcept {
return pushTicket_.load(std::memory_order_acquire);
}
/// Returns the total number of calls to blockingRead or successful
/// calls to read, including those blockingRead calls that are currently
/// blocking
uint64_t readCount() const noexcept {
return popTicket_.load(std::memory_order_acquire);
}
/// Enqueues a T constructed from args, blocking until space is
/// available. Note that this method signature allows enqueue via
/// move, if args is a T rvalue, via copy, if args is a T lvalue, or
/// via emplacement if args is an initializer list that can be passed
/// to a T constructor.
template <typename... Args>
void blockingWrite(Args&&... args) noexcept {
enqueueWithTicketBase(
pushTicket_++, slots_, capacity_, stride_, std::forward<Args>(args)...);
}
/// If an item can be enqueued with no blocking, does so and returns
/// true, otherwise returns false. This method is similar to
/// writeIfNotFull, but if you don't have a specific need for that
/// method you should use this one.
///
/// One of the common usages of this method is to enqueue via the
/// move constructor, something like q.write(std::move(x)). If write
/// returns false because the queue is full then x has not actually been
/// consumed, which looks strange. To understand why it is actually okay
/// to use x afterward, remember that std::move is just a typecast that
/// provides an rvalue reference that enables use of a move constructor
/// or operator. std::move doesn't actually move anything. It could
/// more accurately be called std::rvalue_cast or std::move_permission.
template <typename... Args>
bool write(Args&&... args) noexcept {
uint64_t ticket;
Slot* slots;
size_t cap;
int stride;
if (static_cast<Derived<T, Atom, Dynamic>*>(this)->tryObtainReadyPushTicket(
ticket, slots, cap, stride)) {
// we have pre-validated that the ticket won't block
enqueueWithTicketBase(
ticket, slots, cap, stride, std::forward<Args>(args)...);
return true;
} else {
return false;
}
}
template <class Clock, typename... Args>
bool tryWriteUntil(
const std::chrono::time_point<Clock>& when,
Args&&... args) noexcept {
uint64_t ticket;
Slot* slots;
size_t cap;
int stride;
if (tryObtainPromisedPushTicketUntil(ticket, slots, cap, stride, when)) {
// we have pre-validated that the ticket won't block, or rather that
// it won't block longer than it takes another thread to dequeue an
// element from the slot it identifies.
enqueueWithTicketBase(
ticket, slots, cap, stride, std::forward<Args>(args)...);
return true;
} else {
return false;
}
}
/// If the queue is not full, enqueues and returns true, otherwise
/// returns false. Unlike write this method can be blocked by another
/// thread, specifically a read that has linearized (been assigned
/// a ticket) but not yet completed. If you don't really need this
/// function you should probably use write.
///
/// MPMCQueue isn't lock-free, so just because a read operation has
/// linearized (and isFull is false) doesn't mean that space has been
/// made available for another write. In this situation write will
/// return false, but writeIfNotFull will wait for the dequeue to finish.
/// This method is required if you are composing queues and managing
/// your own wakeup, because it guarantees that after every successful
/// write a readIfNotEmpty will succeed.
template <typename... Args>
bool writeIfNotFull(Args&&... args) noexcept {
uint64_t ticket;
Slot* slots;
size_t cap;
int stride;
if (static_cast<Derived<T, Atom, Dynamic>*>(this)
->tryObtainPromisedPushTicket(ticket, slots, cap, stride)) {
// some other thread is already dequeuing the slot into which we
// are going to enqueue, but we might have to wait for them to finish
enqueueWithTicketBase(
ticket, slots, cap, stride, std::forward<Args>(args)...);
return true;
} else {
return false;
}
}
/// Moves a dequeued element onto elem, blocking until an element
/// is available
void blockingRead(T& elem) noexcept {
uint64_t ticket;
static_cast<Derived<T, Atom, Dynamic>*>(this)->blockingReadWithTicket(
ticket, elem);
}
/// Same as blockingRead() but also records the ticket nunmer
void blockingReadWithTicket(uint64_t& ticket, T& elem) noexcept {
assert(capacity_ != 0);
ticket = popTicket_++;
dequeueWithTicketBase(ticket, slots_, capacity_, stride_, elem);
}
/// If an item can be dequeued with no blocking, does so and returns
/// true, otherwise returns false.
bool read(T& elem) noexcept {
uint64_t ticket;
return readAndGetTicket(ticket, elem);
}
/// Same as read() but also records the ticket nunmer
bool readAndGetTicket(uint64_t& ticket, T& elem) noexcept {
Slot* slots;
size_t cap;
int stride;
if (static_cast<Derived<T, Atom, Dynamic>*>(this)->tryObtainReadyPopTicket(
ticket, slots, cap, stride)) {
// the ticket has been pre-validated to not block
dequeueWithTicketBase(ticket, slots, cap, stride, elem);
return true;
} else {
return false;
}
}
template <class Clock, typename... Args>
bool tryReadUntil(
const std::chrono::time_point<Clock>& when,
T& elem) noexcept {
uint64_t ticket;
Slot* slots;
size_t cap;
int stride;
if (tryObtainPromisedPopTicketUntil(ticket, slots, cap, stride, when)) {
// we have pre-validated that the ticket won't block, or rather that
// it won't block longer than it takes another thread to enqueue an
// element on the slot it identifies.
dequeueWithTicketBase(ticket, slots, cap, stride, elem);
return true;
} else {
return false;
}
}
/// If the queue is not empty, dequeues and returns true, otherwise
/// returns false. If the matching write is still in progress then this
/// method may block waiting for it. If you don't rely on being able
/// to dequeue (such as by counting completed write) then you should
/// prefer read.
bool readIfNotEmpty(T& elem) noexcept {
uint64_t ticket;
Slot* slots;
size_t cap;
int stride;
if (static_cast<Derived<T, Atom, Dynamic>*>(this)
->tryObtainPromisedPopTicket(ticket, slots, cap, stride)) {
// the matching enqueue already has a ticket, but might not be done
dequeueWithTicketBase(ticket, slots, cap, stride, elem);
return true;
} else {
return false;
}
}
protected:
enum {
/// Once every kAdaptationFreq we will spin longer, to try to estimate
/// the proper spin backoff
kAdaptationFreq = 128,
/// To avoid false sharing in slots_ with neighboring memory
/// allocations, we pad it with this many SingleElementQueue-s at
/// each end
kSlotPadding =
(hardware_destructive_interference_size - 1) / sizeof(Slot) + 1
};
/// The maximum number of items in the queue at once
alignas(hardware_destructive_interference_size) size_t capacity_;
/// Anonymous union for use when Dynamic = false and true, respectively
union {
/// An array of capacity_ SingleElementQueue-s, each of which holds
/// either 0 or 1 item. We over-allocate by 2 * kSlotPadding and don't
/// touch the slots at either end, to avoid false sharing
Slot* slots_;
/// Current dynamic slots array of dcapacity_ SingleElementQueue-s
Atom<Slot*> dslots_;
};
/// Anonymous union for use when Dynamic = false and true, respectively
union {
/// The number of slots_ indices that we advance for each ticket, to
/// avoid false sharing. Ideally slots_[i] and slots_[i + stride_]
/// aren't on the same cache line
int stride_;
/// Current stride
Atom<int> dstride_;
};
/// The following two memebers are used by dynamic MPMCQueue.
/// Ideally they should be in MPMCQueue<T,Atom,true>, but we get
/// better cache locality if they are in the same cache line as
/// dslots_ and dstride_.
///
/// Dynamic state. A packed seqlock and ticket offset
Atom<uint64_t> dstate_;
/// Dynamic capacity
Atom<size_t> dcapacity_;
/// Enqueuers get tickets from here
alignas(hardware_destructive_interference_size) Atom<uint64_t> pushTicket_;
/// Dequeuers get tickets from here
alignas(hardware_destructive_interference_size) Atom<uint64_t> popTicket_;
/// This is how many times we will spin before using FUTEX_WAIT when
/// the queue is full on enqueue, adaptively computed by occasionally
/// spinning for longer and smoothing with an exponential moving average
alignas(
hardware_destructive_interference_size) Atom<uint32_t> pushSpinCutoff_;
/// The adaptive spin cutoff when the queue is empty on dequeue
alignas(hardware_destructive_interference_size) Atom<uint32_t> popSpinCutoff_;
/// Alignment doesn't prevent false sharing at the end of the struct,
/// so fill out the last cache line
char pad_[hardware_destructive_interference_size - sizeof(Atom<uint32_t>)];
/// We assign tickets in increasing order, but we don't want to
/// access neighboring elements of slots_ because that will lead to
/// false sharing (multiple cores accessing the same cache line even
/// though they aren't accessing the same bytes in that cache line).
/// To avoid this we advance by stride slots per ticket.
///
/// We need gcd(capacity, stride) to be 1 so that we will use all
/// of the slots. We ensure this by only considering prime strides,
/// which either have no common divisors with capacity or else have
/// a zero remainder after dividing by capacity. That is sufficient
/// to guarantee correctness, but we also want to actually spread the
/// accesses away from each other to avoid false sharing (consider a
/// stride of 7 with a capacity of 8). To that end we try a few taking
/// care to observe that advancing by -1 is as bad as advancing by 1
/// when in comes to false sharing.
///
/// The simple way to avoid false sharing would be to pad each
/// SingleElementQueue, but since we have capacity_ of them that could
/// waste a lot of space.
static int computeStride(size_t capacity) noexcept {
static const int smallPrimes[] = {2, 3, 5, 7, 11, 13, 17, 19, 23};
int bestStride = 1;
size_t bestSep = 1;
for (int stride : smallPrimes) {
if ((stride % capacity) == 0 || (capacity % stride) == 0) {
continue;
}
size_t sep = stride % capacity;
sep = std::min(sep, capacity - sep);
if (sep > bestSep) {
bestStride = stride;
bestSep = sep;
}
}
return bestStride;
}
/// Returns the index into slots_ that should be used when enqueuing or
/// dequeuing with the specified ticket
size_t idx(uint64_t ticket, size_t cap, int stride) noexcept {
return ((ticket * stride) % cap) + kSlotPadding;
}
/// Maps an enqueue or dequeue ticket to the turn should be used at the
/// corresponding SingleElementQueue
uint32_t turn(uint64_t ticket, size_t cap) noexcept {
assert(cap != 0);
return uint32_t(ticket / cap);
}
/// Tries to obtain a push ticket for which SingleElementQueue::enqueue
/// won't block. Returns true on immediate success, false on immediate
/// failure.
bool tryObtainReadyPushTicket(
uint64_t& ticket,
Slot*& slots,
size_t& cap,
int& stride) noexcept {
ticket = pushTicket_.load(std::memory_order_acquire); // A
slots = slots_;
cap = capacity_;
stride = stride_;
while (true) {
if (!slots[idx(ticket, cap, stride)].mayEnqueue(turn(ticket, cap))) {
// if we call enqueue(ticket, ...) on the SingleElementQueue
// right now it would block, but this might no longer be the next
// ticket. We can increase the chance of tryEnqueue success under
// contention (without blocking) by rechecking the ticket dispenser
auto prev = ticket;
ticket = pushTicket_.load(std::memory_order_acquire); // B
if (prev == ticket) {
// mayEnqueue was bracketed by two reads (A or prev B or prev
// failing CAS to B), so we are definitely unable to enqueue
return false;
}
} else {
// we will bracket the mayEnqueue check with a read (A or prev B
// or prev failing CAS) and the following CAS. If the CAS fails
// it will effect a load of pushTicket_
if (pushTicket_.compare_exchange_strong(ticket, ticket + 1)) {
return true;
}
}
}
}
/// Tries until when to obtain a push ticket for which
/// SingleElementQueue::enqueue won't block. Returns true on success, false
/// on failure.
/// ticket is filled on success AND failure.
template <class Clock>
bool tryObtainPromisedPushTicketUntil(
uint64_t& ticket,
Slot*& slots,
size_t& cap,
int& stride,
const std::chrono::time_point<Clock>& when) noexcept {
bool deadlineReached = false;
while (!deadlineReached) {
if (static_cast<Derived<T, Atom, Dynamic>*>(this)
->tryObtainPromisedPushTicket(ticket, slots, cap, stride)) {
return true;
}
// ticket is a blocking ticket until the preceding ticket has been
// processed: wait until this ticket's turn arrives. We have not reserved
// this ticket so we will have to re-attempt to get a non-blocking ticket
// if we wake up before we time-out.
deadlineReached =
!slots[idx(ticket, cap, stride)].tryWaitForEnqueueTurnUntil(
turn(ticket, cap),
pushSpinCutoff_,
(ticket % kAdaptationFreq) == 0,
when);
}
return false;
}
/// Tries to obtain a push ticket which can be satisfied if all
/// in-progress pops complete. This function does not block, but
/// blocking may be required when using the returned ticket if some
/// other thread's pop is still in progress (ticket has been granted but
/// pop has not yet completed).
bool tryObtainPromisedPushTicket(
uint64_t& ticket,
Slot*& slots,
size_t& cap,
int& stride) noexcept {
auto numPushes = pushTicket_.load(std::memory_order_acquire); // A
slots = slots_;
cap = capacity_;
stride = stride_;
while (true) {
ticket = numPushes;
const auto numPops = popTicket_.load(std::memory_order_acquire); // B
// n will be negative if pops are pending
const int64_t n = int64_t(numPushes - numPops);
if (n >= static_cast<ssize_t>(capacity_)) {
// Full, linearize at B. We don't need to recheck the read we
// performed at A, because if numPushes was stale at B then the
// real numPushes value is even worse
return false;
}
if (pushTicket_.compare_exchange_strong(numPushes, numPushes + 1)) {
return true;
}
}
}
/// Tries to obtain a pop ticket for which SingleElementQueue::dequeue
/// won't block. Returns true on immediate success, false on immediate
/// failure.
bool tryObtainReadyPopTicket(
uint64_t& ticket,
Slot*& slots,
size_t& cap,
int& stride) noexcept {
ticket = popTicket_.load(std::memory_order_acquire);
slots = slots_;
cap = capacity_;
stride = stride_;
while (true) {
if (!slots[idx(ticket, cap, stride)].mayDequeue(turn(ticket, cap))) {
auto prev = ticket;
ticket = popTicket_.load(std::memory_order_acquire);
if (prev == ticket) {
return false;
}
} else {
if (popTicket_.compare_exchange_strong(ticket, ticket + 1)) {
return true;
}
}
}
}
/// Tries until when to obtain a pop ticket for which
/// SingleElementQueue::dequeue won't block. Returns true on success, false
/// on failure.
/// ticket is filled on success AND failure.
template <class Clock>
bool tryObtainPromisedPopTicketUntil(
uint64_t& ticket,
Slot*& slots,
size_t& cap,
int& stride,
const std::chrono::time_point<Clock>& when) noexcept {
bool deadlineReached = false;
while (!deadlineReached) {
if (static_cast<Derived<T, Atom, Dynamic>*>(this)
->tryObtainPromisedPopTicket(ticket, slots, cap, stride)) {
return true;
}
// ticket is a blocking ticket until the preceding ticket has been
// processed: wait until this ticket's turn arrives. We have not reserved
// this ticket so we will have to re-attempt to get a non-blocking ticket
// if we wake up before we time-out.
deadlineReached =
!slots[idx(ticket, cap, stride)].tryWaitForDequeueTurnUntil(
turn(ticket, cap),
pushSpinCutoff_,
(ticket % kAdaptationFreq) == 0,
when);
}
return false;
}
/// Similar to tryObtainReadyPopTicket, but returns a pop ticket whose
/// corresponding push ticket has already been handed out, rather than
/// returning one whose corresponding push ticket has already been
/// completed. This means that there is a possibility that the caller
/// will block when using the ticket, but it allows the user to rely on
/// the fact that if enqueue has succeeded, tryObtainPromisedPopTicket
/// will return true. The "try" part of this is that we won't have
/// to block waiting for someone to call enqueue, although we might
/// have to block waiting for them to finish executing code inside the
/// MPMCQueue itself.
bool tryObtainPromisedPopTicket(
uint64_t& ticket,
Slot*& slots,
size_t& cap,
int& stride) noexcept {
auto numPops = popTicket_.load(std::memory_order_acquire); // A
slots = slots_;
cap = capacity_;
stride = stride_;
while (true) {
ticket = numPops;
const auto numPushes = pushTicket_.load(std::memory_order_acquire); // B
if (numPops >= numPushes) {
// Empty, or empty with pending pops. Linearize at B. We don't
// need to recheck the read we performed at A, because if numPops
// is stale then the fresh value is larger and the >= is still true
return false;
}
if (popTicket_.compare_exchange_strong(numPops, numPops + 1)) {
return true;
}
}
}
// Given a ticket, constructs an enqueued item using args
template <typename... Args>
void enqueueWithTicketBase(
uint64_t ticket,
Slot* slots,
size_t cap,
int stride,
Args&&... args) noexcept {
slots[idx(ticket, cap, stride)].enqueue(
turn(ticket, cap),
pushSpinCutoff_,
(ticket % kAdaptationFreq) == 0,
std::forward<Args>(args)...);
}
// To support tracking ticket numbers in MPMCPipelineStageImpl
template <typename... Args>
void enqueueWithTicket(uint64_t ticket, Args&&... args) noexcept {
enqueueWithTicketBase(
ticket, slots_, capacity_, stride_, std::forward<Args>(args)...);
}
// Given a ticket, dequeues the corresponding element
void dequeueWithTicketBase(
uint64_t ticket,
Slot* slots,
size_t cap,
int stride,
T& elem) noexcept {
assert(cap != 0);
slots[idx(ticket, cap, stride)].dequeue(
turn(ticket, cap),
popSpinCutoff_,
(ticket % kAdaptationFreq) == 0,
elem);
}
};
/// SingleElementQueue implements a blocking queue that holds at most one
/// item, and that requires its users to assign incrementing identifiers
/// (turns) to each enqueue and dequeue operation. Note that the turns
/// used by SingleElementQueue are doubled inside the TurnSequencer
template <typename T, template <typename> class Atom>
struct SingleElementQueue {
~SingleElementQueue() noexcept {
if ((sequencer_.uncompletedTurnLSB() & 1) == 1) {
// we are pending a dequeue, so we have a constructed item
destroyContents();
}
}
/// enqueue using in-place noexcept construction
template <
typename... Args,
typename = typename std::enable_if<
std::is_nothrow_constructible<T, Args...>::value>::type>
void enqueue(
const uint32_t turn,
Atom<uint32_t>& spinCutoff,
const bool updateSpinCutoff,
Args&&... args) noexcept {
sequencer_.waitForTurn(turn * 2, spinCutoff, updateSpinCutoff);
new (&contents_) T(std::forward<Args>(args)...);
sequencer_.completeTurn(turn * 2);
}
/// enqueue using move construction, either real (if
/// is_nothrow_move_constructible) or simulated using relocation and
/// default construction (if IsRelocatable and is_nothrow_constructible)
template <
typename = typename std::enable_if<
(folly::IsRelocatable<T>::value &&
std::is_nothrow_constructible<T>::value) ||
std::is_nothrow_constructible<T, T&&>::value>::type>
void enqueue(
const uint32_t turn,
Atom<uint32_t>& spinCutoff,
const bool updateSpinCutoff,
T&& goner) noexcept {
enqueueImpl(
turn,
spinCutoff,
updateSpinCutoff,
std::move(goner),
typename std::conditional<
std::is_nothrow_constructible<T, T&&>::value,
ImplByMove,
ImplByRelocation>::type());
}
/// Waits until either:
/// 1: the dequeue turn preceding the given enqueue turn has arrived
/// 2: the given deadline has arrived
/// Case 1 returns true, case 2 returns false.
template <class Clock>
bool tryWaitForEnqueueTurnUntil(
const uint32_t turn,
Atom<uint32_t>& spinCutoff,
const bool updateSpinCutoff,
const std::chrono::time_point<Clock>& when) noexcept {
return sequencer_.tryWaitForTurn(
turn * 2, spinCutoff, updateSpinCutoff, &when) !=
TurnSequencer<Atom>::TryWaitResult::TIMEDOUT;
}
bool mayEnqueue(const uint32_t turn) const noexcept {
return sequencer_.isTurn(turn * 2);
}
void dequeue(
uint32_t turn,
Atom<uint32_t>& spinCutoff,
const bool updateSpinCutoff,
T& elem) noexcept {
dequeueImpl(
turn,
spinCutoff,
updateSpinCutoff,
elem,
typename std::conditional<
folly::IsRelocatable<T>::value,
ImplByRelocation,
ImplByMove>::type());
}
/// Waits until either:
/// 1: the enqueue turn preceding the given dequeue turn has arrived
/// 2: the given deadline has arrived
/// Case 1 returns true, case 2 returns false.
template <class Clock>
bool tryWaitForDequeueTurnUntil(
const uint32_t turn,
Atom<uint32_t>& spinCutoff,
const bool updateSpinCutoff,
const std::chrono::time_point<Clock>& when) noexcept {
return sequencer_.tryWaitForTurn(
turn * 2 + 1, spinCutoff, updateSpinCutoff, &when) !=
TurnSequencer<Atom>::TryWaitResult::TIMEDOUT;
}
bool mayDequeue(const uint32_t turn) const noexcept {
return sequencer_.isTurn(turn * 2 + 1);
}
private:
/// Storage for a T constructed with placement new
aligned_storage_for_t<T> contents_;
/// Even turns are pushes, odd turns are pops
TurnSequencer<Atom> sequencer_;
T* ptr() noexcept {
return static_cast<T*>(static_cast<void*>(&contents_));
}
void destroyContents() noexcept {
try {
ptr()->~T();
} catch (...) {
// g++ doesn't seem to have std::is_nothrow_destructible yet
}
if (kIsDebug) {
memset(&contents_, 'Q', sizeof(T));
}
}
/// Tag classes for dispatching to enqueue/dequeue implementation.
struct ImplByRelocation {};
struct ImplByMove {};
/// enqueue using nothrow move construction.
void enqueueImpl(
const uint32_t turn,
Atom<uint32_t>& spinCutoff,
const bool updateSpinCutoff,
T&& goner,
ImplByMove) noexcept {
sequencer_.waitForTurn(turn * 2, spinCutoff, updateSpinCutoff);
new (&contents_) T(std::move(goner));
sequencer_.completeTurn(turn * 2);
}
/// enqueue by simulating nothrow move with relocation, followed by
/// default construction to a noexcept relocation.
void enqueueImpl(
const uint32_t turn,
Atom<uint32_t>& spinCutoff,
const bool updateSpinCutoff,
T&& goner,
ImplByRelocation) noexcept {
sequencer_.waitForTurn(turn * 2, spinCutoff, updateSpinCutoff);
memcpy(
static_cast<void*>(&contents_),
static_cast<void const*>(&goner),
sizeof(T));
sequencer_.completeTurn(turn * 2);
new (&goner) T();
}
/// dequeue by destructing followed by relocation. This version is preferred,
/// because as much work as possible can be done before waiting.
void dequeueImpl(
uint32_t turn,
Atom<uint32_t>& spinCutoff,
const bool updateSpinCutoff,
T& elem,
ImplByRelocation) noexcept {
try {
elem.~T();
} catch (...) {
// unlikely, but if we don't complete our turn the queue will die
}
sequencer_.waitForTurn(turn * 2 + 1, spinCutoff, updateSpinCutoff);
memcpy(
static_cast<void*>(&elem),
static_cast<void const*>(&contents_),
sizeof(T));
sequencer_.completeTurn(turn * 2 + 1);
}
/// dequeue by nothrow move assignment.
void dequeueImpl(
uint32_t turn,
Atom<uint32_t>& spinCutoff,
const bool updateSpinCutoff,
T& elem,
ImplByMove) noexcept {
sequencer_.waitForTurn(turn * 2 + 1, spinCutoff, updateSpinCutoff);
elem = std::move(*ptr());
destroyContents();
sequencer_.completeTurn(turn * 2 + 1);
}
};
} // namespace detail
} // namespace folly
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