Question

对于大型数组（10 ^ 7个元素）上的信号处理，我使用与环形缓冲区连接的不同线程。可悲的是，将数据复制到缓冲区或从缓冲区复制数据只需要太多时间。当前的实现基于boost::lockfree::spsc_queue。

因此，我正在寻找一种解决方案，以通过在向量和向量之间使用unique_ptr在向量和线程之间交换向量的所有权（请参见附图：swapping pointer between threads and the queue）

移动智能指针不符合我的需求，因为因此，我需要在运行时不断为新的向量元素分配内存。该开销大于复制数据。

我在设计上缺少缺陷吗？

是否有线程安全甚至无锁的环形缓冲区实现允许对push和pop进行交换操作？

编辑：我修改了一个锁环缓冲区以交换unique_ptr。性能提升巨大。虽然这并不是一个优雅的解决方案。有什么建议吗？

// https://github.com/embeddedartistry/embedded-resources/blob/master/examples/cpp/circular_buffer.cpp

#include <memory>
#include <mutex>

template <typename T, int SIZE>
class RingbufferPointer {
typedef std::unique_ptr<T> TPointer;
public:
    explicit RingbufferPointer() {
        // create objects
        for (int i=0; i<SIZE; i++) {
            buf_[i] = std::make_unique<T>();
        }
    }

    bool push(TPointer &item) {
        std::lock_guard<std::mutex> lock(mutex_);
        if (full())
            return false;

        std::swap(buf_[head_], item);

        if (full_)
            tail_ = (tail_ + 1) % max_size_;

        head_ = (head_ + 1) % max_size_;
        full_ = head_ == tail_;

        return true;
    }

    bool pop(TPointer &item) {
        std::lock_guard<std::mutex> lock(mutex_);
        if (empty())
            return false;

        std::swap(buf_[tail_], item);

        full_ = false;
        tail_ = (tail_ + 1) % max_size_;

        return true;
    }

    void reset() {
        std::lock_guard<std::mutex> lock(mutex_);
        head_ = tail_;
        full_ = false;
    }

    bool empty() const {
        return (!full_ && (head_ == tail_));
    }

    bool full() const {
        return full_;
    }

    int capacity() const {
        return max_size_;
    }

    int size() const {
        int size = max_size_;

        if(!full_) {
            if(head_ >= tail_)
                size = head_ - tail_;
            else
                size = max_size_ + head_ - tail_;
        }

        return size;
    }

private:
    TPointer buf_[SIZE];

    std::mutex mutex_;
    int head_ = 0;
    int tail_ = 0;
    const int max_size_ = SIZE;
    bool full_ = 0;
};

Answer 1

移动智能指针不符合我的需求，因为因此我需要在运行时不断为新的矢量元素分配内存。

如果您预先分配了足够的存储空间并实现了自己的内存管理（例如简单的独立存储，又称为池），则不一定正确。

如果这样做，就不会有交换的麻烦，您可以使用任何支持元素交换并保持与线程安全相同的 ring-buffer 来保持现有体系结构之前。您可以仅选择using boost::pool的选项，而不用自己实现。

Answer 2

如果我正确理解您的任务-您需要2个容器：

线程安全和无锁池，用于释放元素-用于不分配/ 每次都释放它。推送和弹出无需等待。
线程安全且无锁的单写/单读 FIFO 队列，推送和弹出无需等待。

您可以执行以下操作：

开始时，您分配N元素并将其推送到池中。
生产者从池中弹出免费项目（而不是分配内存）
准备商品数据
将其推送到 FIFO 队列
如果池中没有免费物品-消费者发出等待信号

FIFO 队列中的消费者弹出项目
处理项目数据
将项目推回池中（而不是释放它的内存）
如果队列为空-来自生产者的等待信号

FIFO 队列可以通过以下方式实现：

class CyclicBufer
{
    struct alignas(8) Position 
    {
        ULONG _begin, _data_size;
    };

    std::atomic<Position> _pos;
    void** _items;

    ULONG _buf_size;

public:

    // Requires: only one thread is allowed to push data to the CyclicBufer
    bool push(void* item, bool* bWasEmpty = 0);

    // Requires: only one thread is allowed to pop data to the CyclicBufer
    bool pop(void** pitem, bool* bNotEmpty = 0);

    ~CyclicBufer()
    {
        if (_items)
        {
            delete [] _items;
        }
    }

    CyclicBufer() : _items(0), _buf_size(0)
    {
        _pos._My_val._begin = 0, _pos._My_val._data_size = 0;
    }

    bool create(ULONG buf_size)
    {
        if (_items = new(std::nothrow) void*[buf_size])
        {
            _buf_size = buf_size;
            return true;
        }

        return false;
    }

    bool is_empty()
    {
        Position current_pos = _pos.load(std::memory_order_relaxed);

        return !current_pos._data_size;
    }
};

bool CyclicBufer::push(void* item, bool* bWasEmpty /*= 0*/)
{
    Position current_pos = _pos.load(std::memory_order_relaxed);

    if (current_pos._data_size >= _buf_size) return false;

    // (_pos._begin + _pos._data_size) % _buf_size never changed in pop
    _items[(current_pos._begin + current_pos._data_size) % _buf_size] = item;

    for (;;)
    {
        Position new_pos = {
            current_pos._begin, current_pos._data_size + 1
        };

        if (_pos.compare_exchange_weak(current_pos, new_pos, std::memory_order_release))
        {
            if (bWasEmpty) *bWasEmpty = current_pos._data_size == 0;
            return true;
        }
    }
}

bool CyclicBufer::pop(void** pitem, bool* bNotEmpty /*= 0*/)
{
    Position current_pos = _pos.load(std::memory_order_acquire);

    if (!current_pos._data_size) return false;

    // current_pos._begin never changed in push
    void* item = _items[current_pos._begin];

    for (;;)
    {
        Position new_pos = {
            (current_pos._begin + 1) % _buf_size, current_pos._data_size - 1
        };

        if (_pos.compare_exchange_weak(current_pos, new_pos, std::memory_order_relaxed))
        {
            if (bNotEmpty) *bNotEmpty = new_pos._data_size != 0;
            *pitem = item;
            return true;
        }
    }
}

对于Windows上的线程安全和无锁池implementation，可以使用InterlockedPushEntrySList和InterlockedPopEntrySList，但当然可以自己实现此api：

struct list_entry {
    list_entry *Next;
};

#if defined(_M_X64) || defined(_M_ARM64)
#define MACHINE_64
#endif

struct alignas(sizeof(PVOID)*2) list_head 
{  
    union {
        struct {
            INT_PTR DepthAndSequence;
            union {
                list_entry* NextEntry;
                INT_PTR iNextEntry;
            };
        };
        __int64 value; // for 32-bit only
    };

    void init()
    {
        iNextEntry = 0, DepthAndSequence = 0;
    }

    bool push(list_entry* entry)
    {
        list_head current = { { DepthAndSequence, NextEntry } }, new_head;

        for (;;)
        {
            entry->Next = current.NextEntry;
            new_head.NextEntry = entry;
            new_head.DepthAndSequence = current.DepthAndSequence + 0x10001;

#ifdef MACHINE_64
            if (_INTRIN_RELEASE(_InterlockedCompareExchange128)(
                &DepthAndSequence, 
                new_head.iNextEntry, new_head.DepthAndSequence, 
                &current.DepthAndSequence))
            {
                // return is list was empty before push
                return !current.NextEntry;
            }
#else
            new_head.value = _INTRIN_RELEASE(_InterlockedCompareExchange64)(
                &value, new_head.value, current.value);

            if (new_head.value == current.value)
            {
                // return is list was empty before push
                return !current.NextEntry;
            }

            current.value = new_head.value;
#endif
        }
    }

    list_entry* pop()
    {
        list_head current = { { DepthAndSequence, NextEntry } }, new_head;

        for (;;)
        {
            list_entry* entry = current.NextEntry;

            if (!entry)
            {
                return 0;
            }

            // entry must be valid memory
            new_head.NextEntry = entry->Next;
            new_head.DepthAndSequence = current.DepthAndSequence - 1;

#ifdef MACHINE_64
            if (_INTRIN_ACQUIRE(_InterlockedCompareExchange128)(&DepthAndSequence, 
                new_head.iNextEntry, new_head.DepthAndSequence, 
                &current.DepthAndSequence))
            {
                return entry;
            }
#else
            new_head.value = _INTRIN_ACQUIRE(_InterlockedCompareExchange64)(
                &value, new_head.value, current.value);

            if (new_head.value == current.value)
            {
                return entry;
            }

            current.value = new_head.value;
#endif
        }
    }
};

#pragma warning(disable : 4324)

template <class _Ty>
class FreeItems : list_head
{
    void* _items;

    union Chunk {
        list_entry entry;
        char buf[sizeof(_Ty)];
    };

public:

    ~FreeItems()
    {
        if (_items)
        {
            delete [] _items;
        }
    }

    FreeItems() : _items(0)
    {
        init();
    }

    bool create(ULONG count)
    {
        if (Chunk* items = new(std::nothrow) Chunk[count])
        {
            _items = items;

            union {
                list_entry* entry;
                Chunk* item;
            };

            item = items;

            do 
            {
                list_head::push(entry);

            } while (item++, --count);

            return true;
        }

        return false;
    }

    _Ty* pop()
    {
        return (_Ty*)list_head::pop();
    }

    bool push(_Ty* item)
    {
        return list_head::push((list_entry*)item);
    }
};

使用这2个容器，演示/测试代码看起来像（用于Windows的代码，但主要-我们如何使用池和队列）

struct BigData 
{
    ULONG _id;
};

struct CPData : CyclicBufer, FreeItems<BigData>
{
    HANDLE _hDataEvent, _hFreeEvent, _hConsumerStop, _hProducerStop;

    ULONG _waitReadId, _writeId, _setFreeCount, _setDataCount;

    std::_Atomic_integral_t _dwRefCount;
    bool _bStop;

    static ULONG WINAPI sProducer(void* This)
    {
        reinterpret_cast<CPData*>(This)->Producer();
        reinterpret_cast<CPData*>(This)->Release();
        return __LINE__;
    }

    void Producer()
    {
        HANDLE Handles[] = { _hProducerStop, _hFreeEvent  };

        for (;;)
        {
            BigData* item;

            while (!_bStop && (item = FreeItems::pop()))
            {
                // init data item
                item->_id = _writeId++;

                bool bWasEmpty;

                if (!CyclicBufer::push(item, &bWasEmpty)) __debugbreak();

                if (bWasEmpty)
                {
                    _setDataCount++;
                    SetEvent(_hDataEvent);
                }
            }

            switch (WaitForMultipleObjects(2, Handles, FALSE, INFINITE))
            {
            case WAIT_OBJECT_0:
                SetEvent(_hConsumerStop);
                return;
            case WAIT_OBJECT_0 + 1:
                break;
            default:
                __debugbreak();
            }
        }
    }

    static ULONG WINAPI sConsumer(void* This)
    {
        reinterpret_cast<CPData*>(This)->Consumer();
        reinterpret_cast<CPData*>(This)->Release();
        return __LINE__;
    }

    void Consumer()
    {
        HANDLE Handles[] = { _hDataEvent, _hConsumerStop };

        for (;;)
        {
            switch (WaitForMultipleObjects(2, Handles, FALSE, INFINITE))
            {
            case WAIT_OBJECT_0:
                break;
            case WAIT_OBJECT_0 + 1:
                return;
            default:
                __debugbreak();
            }

            bool bNotEmpty;

            do 
            {
                BigData* item;

                if (!CyclicBufer::pop((void**)&item, &bNotEmpty)) __debugbreak();

                // check FIFO order
                if (item->_id != _waitReadId) __debugbreak();

                _waitReadId++;

                // process item

                // free item to the pool
                if (FreeItems::push(item))
                {
                    // stack was empty
                    _setFreeCount++;
                    SetEvent(_hFreeEvent);
                }

            } while (bNotEmpty);
        }
    }

    ~CPData()
    {
        if (_hConsumerStop) CloseHandle(_hConsumerStop);
        if (_hProducerStop) CloseHandle(_hProducerStop);
        if (_hFreeEvent) CloseHandle(_hFreeEvent);
        if (_hDataEvent) CloseHandle(_hDataEvent);

        if (_waitReadId != _writeId || !CyclicBufer::is_empty()) __debugbreak();

        DbgPrint("%s(%u %u %u)\n", __FUNCTION__, _writeId, _setFreeCount, _setDataCount);
    }

public:

    CPData()
    {
        _hFreeEvent = 0, _hDataEvent = 0, _hProducerStop = 0, _hConsumerStop = 0;
        _waitReadId = 0, _writeId = 0, _dwRefCount = 1;
        _setFreeCount = 0, _setDataCount = 0, _bStop = false;
    }

    void AddRef()
    {
        _MT_INCR(_dwRefCount);
    }

    void Release()
    {
        if (!_MT_DECR(_dwRefCount))
        {
            delete this;
        }
    }

    ULONG Create(ULONG n)
    {
        if (!CyclicBufer::create(n) || !FreeItems::create(n))
        {
            return ERROR_NO_SYSTEM_RESOURCES;
        }

        return (_hDataEvent = CreateEvent(0, FALSE, FALSE, 0)) &&
            (_hFreeEvent = CreateEvent(0, FALSE, FALSE, 0)) &&
            (_hProducerStop = CreateEvent(0, TRUE, FALSE, 0)) &&
            (_hConsumerStop = CreateEvent(0, TRUE, FALSE, 0)) ? 0 : GetLastError();
    }

    ULONG StartThread(bool bConsumer)
    {
        AddRef();

        if (HANDLE hThread = CreateThread(0, 0, bConsumer ? sConsumer : sProducer, this, 0, 0))
        {
            CloseHandle(hThread);
            return 0;
        }

        Release();

        return GetLastError();
    }

    ULONG Stop()
    {
        ULONG err = SetEvent(_hProducerStop) ? 0 : GetLastError();
        _bStop = true;
        return err;
    }
};

void BufTest()
{
    if (CPData* p = new CPData)
    {
        if (!p->Create(16))
        {
            if (!p->StartThread(false))
            {
                p->StartThread(true);
            }

            MessageBoxW(0, 0, L"Wait Stop", MB_ICONINFORMATION);
            p->Stop();
        }
        p->Release();
    }
    MessageBoxW(0,0,0,1);
}

Answer 3

尽管boost::lockfree::spsc_queue缺乏移动支持，您仍然可以这样做。

将向量移入和移出队列的示例：

struct Element {
    std::vector<int> data_;

    Element(std::vector<int>& data)
        : data_(std::move(data))
    {}

    Element(Element const&) = delete;
    Element operator=(Element const&) = delete;

    operator std::vector<int>&&() {
        return std::move(data_);
    }
};

int main() {
    boost::lockfree::spsc_queue<Element, boost::lockfree::capacity<2>> q;

    std::vector<int> a(1);
    assert(!a.empty());
    q.push(&a, &a + 1); // Move the vector into the queue.
    assert(a.empty());

    std::vector<int> b = q.front(); // Move the vector from queue.
    assert(!b.empty());
    q.pop();
}

Answer 4

我使用的一种技术是...

ng-options

不交换或复制单个元素。快速高效。

迈克

适用于大型阵列的无拷贝线程安全环形缓冲区

4 个答案: