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
/// 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>
-class MPMCQueue : boost::noncopyable {
+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 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 need 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.
+///
+/// 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_;
+ this->slots_ = rhs.slots_;
+ this->stride_ = rhs.stride_;
+ 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.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);
+ 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_;
+ }
+ }
+
+ 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)) {
+ continue;
+ }
+ offset = getOffset(state);
+ if (ticket < offset) {
+ // There was an expansion after this ticket was issued.
+ updateFromClosed(state, ticket, offset, slots, cap, stride);
+ 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));
+ offset = getOffset(state);
+ if (ticket < offset) {
+ // There was an expansion after the corresponding push ticket
+ // was issued.
+ updateFromClosed(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) {
+ this->stride_ = this->computeStride(cap);
+ this->slots_ = new Slot[cap + 2 * this->kSlotPadding];
+ 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)) {
+ continue;
+ }
+ uint64_t offset = getOffset(state);
+ if (ticket < offset) {
+ // There was an expansion with offset greater than this ticket
+ updateFromClosed(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)) {
+ continue;
+ }
+ int64_t n = ticket - numPops;
+ if (n >= static_cast<ssize_t>(this->capacity_)) {
+ return false;
+ }
+ if ((n >= static_cast<ssize_t>(cap))) {
+ if (tryExpand(state, cap)) {
+ // This or another thread started an expansion. Start over
+ // with a new state.
+ continue;
+ } else {
+ // Can't expand.
+ return false;
+ }
+ }
+ uint64_t offset = getOffset(state);
+ if (ticket < offset) {
+ // There was an expansion with offset greater than this ticket
+ updateFromClosed(state, ticket, offset, slots, cap, stride);
+ }
+ 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)) {
+ continue;
+ }
+ uint64_t offset = getOffset(state);
+ if (ticket < offset) {
+ // There was an expansion after the corresponding push ticket
+ // was issued.
+ updateFromClosed(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)) {
+ continue;
+ }
+ if (ticket >= numPushes) {
+ return false;
+ }
+ if (this->popTicket_.compare_exchange_strong(ticket, ticket + 1)) {
+ // Adjust ticket
+ uint64_t offset = getOffset(state);
+ if (ticket < offset) {
+ // There was an expansion after the corresponding push
+ // ticket was issued.
+ updateFromClosed(state, ticket, offset, slots, cap, stride);
+ }
+ // Adjust ticket
+ 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)) {}
+ offset = getOffset(state);
+ if (ticket < offset) {
+ // There was an expansion after this ticket was issued.
+ updateFromClosed(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));
+ }
+
+ /// Update local variables of a lagging operation using the
+ /// most recent closed array with offset <= ticket
+ void updateFromClosed(
+ const uint64_t state, const uint64_t ticket,
+ uint64_t& offset, Slot*& slots, size_t& cap, int& stride
+ ) noexcept {
+ 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;;
+ }
+ }
+ // A closed array with offset <= ticket should have been found
+ assert(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>> : boost::noncopyable {
+
+// 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");
- friend class detail::MPMCPipelineStageImpl<T>;
public:
typedef T value_type;
- explicit MPMCQueue(size_t queueCapacity)
+ using Slot = detail::SingleElementQueue<T,Atom>;
+
+ explicit MPMCQueueBase(size_t queueCapacity)
: capacity_(queueCapacity)
, pushTicket_(0)
, popTicket_(0)
, pushSpinCutoff_(0)
, popSpinCutoff_(0)
{
- if (queueCapacity == 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
);
-
- // would sigfpe if capacity is 0
- stride_ = computeStride(queueCapacity);
- slots_ = new detail::SingleElementQueue<T,Atom>[queueCapacity +
- 2 * kSlotPadding];
+ }
// ideally this would be a static assert, but g++ doesn't allow it
assert(alignof(MPMCQueue<T,Atom>)
/// A default-constructed queue is useful because a usable (non-zero
/// capacity) queue can be moved onto it or swapped with it
- MPMCQueue() noexcept
+ MPMCQueueBase() noexcept
: capacity_(0)
, slots_(nullptr)
, stride_(0)
+ , dstate_(0)
+ , dcapacity_(0)
, pushTicket_(0)
, popTicket_(0)
, pushSpinCutoff_(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).
- MPMCQueue(MPMCQueue<T,Atom>&& rhs) noexcept
+ 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))
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);
/// 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).
- MPMCQueue<T,Atom> const& operator= (MPMCQueue<T,Atom>&& rhs) {
+ MPMCQueueBase<Derived<T,Atom,Dynamic>> const& operator=
+ (MPMCQueueBase<Derived<T,Atom,Dynamic>>&& rhs) {
if (this != &rhs) {
- this->~MPMCQueue();
- new (this) MPMCQueue(std::move(rhs));
+ this->~MPMCQueueBase();
+ new (this) MPMCQueueBase(std::move(rhs));
}
return *this;
}
/// MPMCQueue can only be safely destroyed when there are no
/// pending enqueuers or dequeuers (this is not checked).
- ~MPMCQueue() {
+ ~MPMCQueueBase() {
delete[] slots_;
}
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
/// to a T constructor.
template <typename ...Args>
void blockingWrite(Args&&... args) noexcept {
- enqueueWithTicket(pushTicket_++, std::forward<Args>(args)...);
+ enqueueWithTicketBase(pushTicket_++, slots_, capacity_, stride_,
+ std::forward<Args>(args)...);
}
/// If an item can be enqueued with no blocking, does so and returns
template <typename ...Args>
bool write(Args&&... args) noexcept {
uint64_t ticket;
- if (tryObtainReadyPushTicket(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
- enqueueWithTicket(ticket, std::forward<Args>(args)...);
+ enqueueWithTicketBase(ticket, slots, cap, stride,
+ std::forward<Args>(args)...);
return true;
} else {
return false;
bool tryWriteUntil(const std::chrono::time_point<Clock>& when,
Args&&... args) noexcept {
uint64_t ticket;
- if (tryObtainPromisedPushTicketUntil(ticket, 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.
- enqueueWithTicket(ticket, std::forward<Args>(args)...);
+ 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;
template <typename ...Args>
bool writeIfNotFull(Args&&... args) noexcept {
uint64_t ticket;
- if (tryObtainPromisedPushTicket(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
- enqueueWithTicket(ticket, std::forward<Args>(args)...);
+ 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 {
- dequeueWithTicket(popTicket_++, elem);
+ 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 {
+ 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;
- if (tryObtainReadyPopTicket(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
- dequeueWithTicket(ticket, elem);
+ dequeueWithTicketBase(ticket, slots, cap, stride, elem);
return true;
} else {
return false;
/// prefer read.
bool readIfNotEmpty(T& elem) noexcept {
uint64_t ticket;
- if (tryObtainPromisedPopTicket(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
- dequeueWithTicket(ticket, elem);
+ dequeueWithTicketBase(ticket, slots, cap, stride, elem);
return true;
} else {
return false;
}
}
- private:
+ protected:
enum {
/// Once every kAdaptationFreq we will spin longer, to try to estimate
/// the proper spin backoff
/// allocations, we pad it with this many SingleElementQueue-s at
/// each end
kSlotPadding = (detail::CacheLocality::kFalseSharingRange - 1)
- / sizeof(detail::SingleElementQueue<T,Atom>) + 1
+ / sizeof(Slot) + 1
};
/// The maximum number of items in the queue at once
size_t FOLLY_ALIGN_TO_AVOID_FALSE_SHARING capacity_;
- /// 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
- detail::SingleElementQueue<T,Atom>* slots_;
+ /// 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 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_;
+ /// 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
Atom<uint64_t> FOLLY_ALIGN_TO_AVOID_FALSE_SHARING pushTicket_;
char padding_[detail::CacheLocality::kFalseSharingRange -
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
/// Returns the index into slots_ that should be used when enqueuing or
/// dequeuing with the specified ticket
- size_t idx(uint64_t ticket) noexcept {
- return ((ticket * stride_) % capacity_) + kSlotPadding;
+ 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) noexcept {
- return ticket / capacity_;
+ uint32_t turn(uint64_t ticket, size_t cap) noexcept {
+ return 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& rv) noexcept {
- auto ticket = pushTicket_.load(std::memory_order_acquire); // A
+ 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)].mayEnqueue(turn(ticket))) {
+ 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
// 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)) {
- rv = ticket;
return true;
}
}
/// ticket is filled on success AND failure.
template <class Clock>
bool tryObtainPromisedPushTicketUntil(
- uint64_t& ticket, const std::chrono::time_point<Clock>& when) noexcept {
+ uint64_t& ticket, Slot*& slots, size_t& cap, int& stride,
+ const std::chrono::time_point<Clock>& when
+ ) noexcept {
bool deadlineReached = false;
while (!deadlineReached) {
- if (tryObtainPromisedPushTicket(ticket)) {
+ 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)].tryWaitForEnqueueTurnUntil(
- turn(ticket), pushSpinCutoff_, (ticket % kAdaptationFreq) == 0, when);
+ deadlineReached = !slots[idx(ticket, cap, stride)]
+ .tryWaitForEnqueueTurnUntil(turn(ticket, cap), pushSpinCutoff_,
+ (ticket % kAdaptationFreq) == 0, when);
}
return false;
}
/// 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& rv) noexcept {
+ 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) {
auto numPops = popTicket_.load(std::memory_order_acquire); // B
// n will be negative if pops are pending
int64_t n = numPushes - numPops;
- rv = numPushes;
+ ticket = numPushes;
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
/// 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& rv) noexcept {
- auto ticket = popTicket_.load(std::memory_order_acquire);
+ 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)].mayDequeue(turn(ticket))) {
+ if (!slots[idx(ticket, cap, stride)]
+ .mayDequeue(turn(ticket, cap))) {
auto prev = ticket;
ticket = popTicket_.load(std::memory_order_acquire);
if (prev == ticket) {
}
} else {
if (popTicket_.compare_exchange_strong(ticket, ticket + 1)) {
- rv = ticket;
return true;
}
}
/// 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& rv) noexcept {
+ bool tryObtainPromisedPopTicket(
+ uint64_t& ticket, Slot*& slots, size_t& cap, int& stride
+ ) noexcept {
auto numPops = popTicket_.load(std::memory_order_acquire); // A
while (true) {
auto numPushes = pushTicket_.load(std::memory_order_acquire); // B
return false;
}
if (popTicket_.compare_exchange_strong(numPops, numPops + 1)) {
- rv = numPops;
+ ticket = numPops;
+ slots = slots_;
+ cap = capacity_;
+ stride = stride_;
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 {
- slots_[idx(ticket)].enqueue(turn(ticket),
- pushSpinCutoff_,
- (ticket % kAdaptationFreq) == 0,
- std::forward<Args>(args)...);
+ enqueueWithTicketBase(ticket, slots_, capacity_, stride_,
+ std::forward<Args>(args)...);
}
// Given a ticket, dequeues the corresponding element
- void dequeueWithTicket(uint64_t ticket, T& elem) noexcept {
- slots_[idx(ticket)].dequeue(turn(ticket),
- popSpinCutoff_,
- (ticket % kAdaptationFreq) == 0,
- elem);
+ void dequeueWithTicketBase(
+ uint64_t ticket, Slot* slots, size_t cap, int stride, T& elem
+ ) noexcept {
+ slots[idx(ticket, cap, stride)]
+ .dequeue(turn(ticket, cap),
+ popSpinCutoff_,
+ (ticket % kAdaptationFreq) == 0,
+ elem);
}
};
-
-namespace detail {
-
/// 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
projects / folly.git / blobdiff