According to the classification it's MPMC, array-based, fails on overflow, does not require GC, w/o priorities, causal FIFO, blocking producers and consumers queue. The algorithm is pretty simple and fast. It's not lockfree in the official meaning, just implemented by means of atomic RMW operations w/o mutexes.
The cost of enqueue/dequeue is 1 CAS per operation. No amortization, just 1 CAS. No dynamic memory allocation/management during operation. Producers and consumers are separated from each other (as in the two-lock queue), i.e. do not touch the same data while queue is not empty.
On my dual-core laptop enqueue/dequeue takes 75 cycles on average in a synthetic multi-threaded benchmark.
Source code test suite are attached below (the file contains limited implementation of std::atomic, ready to run on Windows, MSVC, x86-32) .
template<typename T>
class mpmc_bounded_queue
{
public:
mpmc_bounded_queue(size_t buffer_size)
: buffer_(new cell_t [buffer_size])
, buffer_mask_(buffer_size - 1)
{
assert((buffer_size >= 2) &&
((buffer_size & (buffer_size - 1)) == 0));
for (size_t i = 0; i != buffer_size; i += 1)
buffer_[i].sequence_.store(i, std::memory_order_relaxed);
enqueue_pos_.store(0, std::memory_order_relaxed);
dequeue_pos_.store(0, std::memory_order_relaxed);
}
~mpmc_bounded_queue()
{
delete [] buffer_;
}
bool enqueue(T const& data)
{
cell_t* cell;
size_t pos = enqueue_pos_.load(std::memory_order_relaxed);
for (;;)
{
cell = &buffer_[pos & buffer_mask_];
size_t seq =
cell->sequence_.load(std::memory_order_acquire);
intptr_t dif = (intptr_t)seq - (intptr_t)pos;
if (dif == 0)
{
if (enqueue_pos_.compare_exchange_weak
(pos, pos + 1, std::memory_order_relaxed))
break;
}
else if (dif < 0)
return false;
else
pos = enqueue_pos_.load(std::memory_order_relaxed);
}
cell->data_ = data;
cell->sequence_.store(pos + 1, std::memory_order_release);
return true;
}
bool dequeue(T& data)
{
cell_t* cell;
size_t pos = dequeue_pos_.load(std::memory_order_relaxed);
for (;;)
{
cell = &buffer_[pos & buffer_mask_];
size_t seq =
cell->sequence_.load(std::memory_order_acquire);
intptr_t dif = (intptr_t)seq - (intptr_t)(pos + 1);
if (dif == 0)
{
if (dequeue_pos_.compare_exchange_weak
(pos, pos + 1, std::memory_order_relaxed))
break;
}
else if (dif < 0)
return false;
else
pos = dequeue_pos_.load(std::memory_order_relaxed);
}
data = cell->data_;
cell->sequence_.store
(pos + buffer_mask_ + 1, std::memory_order_release);
return true;
}
private:
struct cell_t
{
std::atomic<size_t> sequence_;
T data_;
};
static size_t const cacheline_size = 64;
typedef char cacheline_pad_t [cacheline_size];
cacheline_pad_t pad0_;
cell_t* const buffer_;
size_t const buffer_mask_;
cacheline_pad_t pad1_;
std::atomic<size_t> enqueue_pos_;
cacheline_pad_t pad2_;
std::atomic<size_t> dequeue_pos_;
cacheline_pad_t pad3_;
mpmc_bounded_queue(mpmc_bounded_queue const&);
void operator = (mpmc_bounded_queue const&);
};