/*
- * Copyright 2013 Facebook, Inc.
+ * Copyright 2014 Facebook, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
#include <atomic>
#include <assert.h>
#include <boost/noncopyable.hpp>
-#include <errno.h>
#include <limits>
-#include <linux/futex.h>
#include <string.h>
-#include <sys/syscall.h>
#include <type_traits>
#include <unistd.h>
/// 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,
/// when the MPMCQueue's capacity is smaller than the number of enqueuers
/// or dequeuers).
///
-/// NOEXCEPT INTERACTION: Ticket-based queues separate the assignment
-/// of In benchmarks (contained in tao/queues/ConcurrentQueueTests)
+/// 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
/// queue because it uses futex() to block and unblock waiting threads,
/// rather than spinning with sched_yield.
///
-/// 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 boost::has_nothrow_constructor,
-/// which is common for folly types. In particular, if you can declare
-/// FOLLY_ASSUME_FBVECTOR_COMPATIBLE then your type can be put in
-/// MPMCQueue.
+/// 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 boost::has_nothrow_constructor, 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>
class MPMCQueue : boost::noncopyable {
/// 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 readIfNotFull will succeed.
+ /// write a readIfNotEmpty will succeed.
template <typename ...Args>
bool writeIfNotFull(Args&&... args) noexcept {
uint64_t ticket;
/// 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
- Atom<int> FOLLY_ALIGN_TO_AVOID_FALSE_SHARING pushSpinCutoff_;
+ Atom<uint32_t> FOLLY_ALIGN_TO_AVOID_FALSE_SHARING pushSpinCutoff_;
/// The adaptive spin cutoff when the queue is empty on dequeue
- Atom<int> FOLLY_ALIGN_TO_AVOID_FALSE_SHARING popSpinCutoff_;
+ Atom<uint32_t> FOLLY_ALIGN_TO_AVOID_FALSE_SHARING popSpinCutoff_;
/// Alignment doesn't prevent false sharing at the end of the struct,
/// so fill out the last cache line
- char padding_[detail::CacheLocality::kFalseSharingRange - sizeof(Atom<int>)];
+ char padding_[detail::CacheLocality::kFalseSharingRange -
+ sizeof(Atom<uint32_t>)];
/// We assign tickets in increasing order, but we don't want to
/// before blocking and will adjust spinCutoff based on the results,
/// otherwise it will spin for at most spinCutoff spins.
void waitForTurn(const uint32_t turn,
- Atom<int>& spinCutoff,
+ Atom<uint32_t>& spinCutoff,
const bool updateSpinCutoff) noexcept {
- int prevThresh = spinCutoff.load(std::memory_order_relaxed);
- const int effectiveSpinCutoff =
+ uint32_t prevThresh = spinCutoff.load(std::memory_order_relaxed);
+ const uint32_t effectiveSpinCutoff =
updateSpinCutoff || prevThresh == 0 ? kMaxSpins : prevThresh;
- int tries;
+ uint32_t tries;
const uint32_t sturn = turn << kTurnShift;
for (tries = 0; ; ++tries) {
uint32_t state = state_.load(std::memory_order_acquire);
if (updateSpinCutoff || prevThresh == 0) {
// if we hit kMaxSpins then spinning was pointless, so the right
// spinCutoff is kMinSpins
- int target;
+ uint32_t target;
if (tries >= kMaxSpins) {
target = kMinSpins;
} else {
// to account for variations, we allow ourself to spin 2*N when
// we think that N is actually required in order to succeed
- target = std::min(int{kMaxSpins}, std::max(int{kMinSpins}, tries * 2));
+ target = std::min<uint32_t>(kMaxSpins,
+ std::max<uint32_t>(kMinSpins, tries * 2));
}
if (prevThresh == 0) {
// bootstrap
- spinCutoff = target;
+ spinCutoff.store(target);
} else {
// try once, keep moving if CAS fails. Exponential moving average
// with alpha of 7/8
+ // Be careful that the quantity we add to prevThresh is signed.
spinCutoff.compare_exchange_weak(
- prevThresh, prevThresh + (target - prevThresh) / 8);
+ prevThresh, prevThresh + int(target - prevThresh) / 8);
}
}
}
typename = typename std::enable_if<
std::is_nothrow_constructible<T,Args...>::value>::type>
void enqueue(const uint32_t turn,
- Atom<int>& spinCutoff,
+ Atom<uint32_t>& spinCutoff,
const bool updateSpinCutoff,
Args&&... args) noexcept {
sequencer_.waitForTurn(turn * 2, spinCutoff, updateSpinCutoff);
boost::has_nothrow_constructor<T>::value) ||
std::is_nothrow_constructible<T,T&&>::value>::type>
void enqueue(const uint32_t turn,
- Atom<int>& spinCutoff,
+ Atom<uint32_t>& spinCutoff,
const bool updateSpinCutoff,
T&& goner) noexcept {
if (std::is_nothrow_constructible<T,T&&>::value) {
}
void dequeue(uint32_t turn,
- Atom<int>& spinCutoff,
+ Atom<uint32_t>& spinCutoff,
const bool updateSpinCutoff,
T& elem) noexcept {
if (folly::IsRelocatable<T>::value) {
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