/*
- * Copyright 2014 Facebook, Inc.
+ * Copyright 2015 Facebook, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
/// 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 {
explicit MPMCQueue(size_t queueCapacity)
: capacity_(queueCapacity)
- , slots_(new detail::SingleElementQueue<T,Atom>[queueCapacity +
- 2 * kSlotPadding])
- , stride_(computeStride(queueCapacity))
, pushTicket_(0)
, popTicket_(0)
, pushSpinCutoff_(0)
, popSpinCutoff_(0)
{
+ if (queueCapacity == 0)
+ throw std::invalid_argument(
+ "MPMCQueue with explicit capacity 0 is impossible"
+ );
+
+ // 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>)
>= detail::CacheLocality::kFalseSharingRange);
/// 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;
projects / folly.git / blobdiff