folly::futures::Barrier Class Reference

#include <Barrier.h>

Classes
struct	ControlBlock
struct	ControlBlockAndPromise

Public Member Functions
	Barrier (uint32_t n)
	~Barrier ()
folly::Future< bool >	wait ()

Private Types
typedef folly::Promise< bool >	BoolPromise

Private Member Functions
ControlBlock *	allocateControlBlock ()
void	freeControlBlock (ControlBlock *b)

Static Private Member Functions
static BoolPromise *	promises (ControlBlock *cb)
static size_t	controlBlockSize (size_t n)

Private Attributes
uint32_t	size_
std::atomic< ControlBlock * >	controlBlock_

Static Private Attributes
static constexpr uint64_t	kReaderShift = 32
static constexpr uint64_t	kReader = uint64_t(1) << kReaderShift
static constexpr uint64_t	kValueMask = kReader - 1

Detailed Description

Definition at line 45 of file Barrier.h.

Member Typedef Documentation

typedef folly::Promise<bool> folly::futures::Barrier::BoolPromise

private

Definition at line 53 of file Barrier.h.

Constructor & Destructor Documentation

folly::futures::Barrier::Barrier ( uint32_t  n )

explicit

Definition at line 23 of file Barrier.cpp.

    : size_(n), controlBlock_(allocateControlBlock()) {}

folly::futures::Barrier::~Barrier

(

)

Definition at line 26 of file Barrier.cpp.

References controlBlock_, freeControlBlock(), i, kReaderShift, kValueMask, promises(), uint32_t, and val.

                  {
  auto block = controlBlock_.load(std::memory_order_relaxed);
  auto prev = block->valueAndReaderCount.load(std::memory_order_relaxed);
  DCHECK_EQ(prev >> kReaderShift, 0u);
  auto val = prev & kValueMask;
  auto p = promises(block);

  for (uint32_t i = 0; i < val; ++i) {
    p[i].setException(
        folly::make_exception_wrapper<std::runtime_error>("Barrier destroyed"));
  }

  freeControlBlock(controlBlock_);
}

Member Function Documentation

auto folly::futures::Barrier::allocateControlBlock ( )

private

Definition at line 41 of file Barrier.cpp.

References controlBlockSize(), i, promises(), size_, and uint32_t.

Referenced by controlBlockSize(), and wait().

                                                    {
  auto storage = malloc(controlBlockSize(size_));
  if (!storage) {
    throw_exception<std::bad_alloc>();
  }
  auto block = ::new (storage) ControlBlock();

  auto p = promises(block);
  uint32_t i = 0;
  try {
    for (i = 0; i < size_; ++i) {
      new (p + i) BoolPromise();
    }
  } catch (...) {
    for (; i != 0; --i) {
      p[i - 1].~BoolPromise();
    }
    throw;
  }

  return block;
}

static size_t folly::futures::Barrier::controlBlockSize ( size_t  n )

inlinestaticprivate

Definition at line 86 of file Barrier.h.

References allocateControlBlock(), b, freeControlBlock(), and promises().

Referenced by allocateControlBlock().

                                           {
    return offsetof(ControlBlockAndPromise, promises) + n * sizeof(BoolPromise);
  }

void folly::futures::Barrier::freeControlBlock ( ControlBlock *  b )

private

Definition at line 64 of file Barrier.cpp.

References bm::free(), i, promises(), size_, and uint32_t.

Referenced by controlBlockSize(), wait(), and ~Barrier().

                                                  {
  auto p = promises(block);
  for (uint32_t i = size_; i != 0; --i) {
    p[i - 1].~BoolPromise();
  }
  free(block);
}

static BoolPromise* folly::futures::Barrier::promises ( ControlBlock *  cb )

inlinestaticprivate

Definition at line 82 of file Barrier.h.

Referenced by allocateControlBlock(), controlBlockSize(), freeControlBlock(), wait(), and ~Barrier().

                                                 {
    return reinterpret_cast<ControlBlockAndPromise*>(cb)->promises;
  }

folly::Future< bool > folly::futures::Barrier::wait

(

)

Definition at line 72 of file Barrier.cpp.

References allocateControlBlock(), controlBlock_, freeControlBlock(), i, kReader, kValueMask, promises(), size_, uint32_t, and uint64_t.

Referenced by folly::futures::test::TEST(), TEST_F(), and TYPED_TEST().

                                {
  // Load the current control block first. As we know there is at least
  // one thread in the current epoch (us), this means that the value is
  // < size_, so controlBlock_ can't change until we bump the value below.
  auto block = controlBlock_.load(std::memory_order_acquire);
  auto p = promises(block);

  // Bump the value and record ourselves as reader.
  // This ensures that block stays allocated, as the reader count is > 0.
  auto prev = block->valueAndReaderCount.fetch_add(
      kReader + 1, std::memory_order_acquire);

  auto prevValue = static_cast<uint32_t>(prev & kValueMask);
  DCHECK_LT(prevValue, size_);
  auto future = p[prevValue].getFuture();

  if (prevValue + 1 == size_) {
    // Need to reset the barrier before fulfilling any futures. This is
    // when the epoch is flipped to the next.
    controlBlock_.store(allocateControlBlock(), std::memory_order_release);

    p[0].setValue(true);
    for (uint32_t i = 1; i < size_; ++i) {
      p[i].setValue(false);
    }
  }

  // Free the control block if we're the last reader at max value.
  prev =
      block->valueAndReaderCount.fetch_sub(kReader, std::memory_order_acq_rel);
  if (prev == (kReader | uint64_t(size_))) {
    freeControlBlock(block);
  }

  return future;
}

Member Data Documentation

std::atomic<ControlBlock*> folly::futures::Barrier::controlBlock_

private

Definition at line 94 of file Barrier.h.

Referenced by wait(), and ~Barrier().

constexpr uint64_t folly::futures::Barrier::kReader = uint64_t(1) << kReaderShift

staticprivate

Definition at line 56 of file Barrier.h.

Referenced by wait().

constexpr uint64_t folly::futures::Barrier::kReaderShift = 32

staticprivate

Definition at line 55 of file Barrier.h.

Referenced by ~Barrier().

constexpr uint64_t folly::futures::Barrier::kValueMask = kReader - 1

staticprivate

Definition at line 57 of file Barrier.h.

Referenced by wait(), and ~Barrier().

uint32_t folly::futures::Barrier::size_

private

Definition at line 93 of file Barrier.h.

Referenced by allocateControlBlock(), freeControlBlock(), and wait().

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The documentation for this class was generated from the following files:

• proxygen/folly/folly/futures/Barrier.h
• proxygen/folly/folly/futures/Barrier.cpp