C# ConcurrentQueue实现

 我们从C# Queue 和Stack的实现知道Queue是用数组来实现的，数组的元素不断的通过Array.Copy从一个数组移动到另一个数组，ConcurrentQueue我们需要关心2点：1线程安全是怎么实现的，2队列又是怎么实现的?我们来看看其实现code：

public interface IProducerConsumerCollection<T> : IEnumerable<T>, ICollection
{
    void CopyTo(T[] array, int index);
    bool TryAdd(T item);
    bool TryTake(out T item);
    T[] ToArray();
}

 public class ConcurrentQueue<T> : IProducerConsumerCollection<T>, IReadOnlyCollection<T>
{
    [NonSerialized]
    private volatile Segment m_head;

    [NonSerialized]
    private volatile Segment m_tail;

    private T[] m_serializationArray; // Used for custom serialization.

    private const int SEGMENT_SIZE = 32;

    //number of snapshot takers, GetEnumerator(), ToList() and ToArray() operations take snapshot.
    [NonSerialized]
    internal volatile int m_numSnapshotTakers = 0;
    
    public ConcurrentQueue()
    {
        m_head = m_tail = new Segment(0, this);
    }
    
    public ConcurrentQueue(IEnumerable<T> collection)
    {
        if (collection == null)
        {
            throw new ArgumentNullException("collection");
        }

        InitializeFromCollection(collection);
    }
    
    private void InitializeFromCollection(IEnumerable<T> collection)
    {
        Segment localTail = new Segment(0, this);//use this local variable to avoid the extra volatile read/write. this is safe because it is only called from ctor
        m_head = localTail; 

        int index = 0;
        foreach (T element in collection)
        {
            Contract.Assert(index >= 0 && index < SEGMENT_SIZE);
            localTail.UnsafeAdd(element);
            index++;

            if (index >= SEGMENT_SIZE)
            {
                localTail = localTail.UnsafeGrow();
                index = 0;
            }
        }

        m_tail = localTail;
    }
    
    public void Enqueue(T item)
    {
        SpinWait spin = new SpinWait();
        while (true)
        {
            Segment tail = m_tail;
            if (tail.TryAppend(item))
                return;
            spin.SpinOnce();
        }
    }
    
    public bool TryDequeue(out T result)
    {
        while (!IsEmpty)
        {
            Segment head = m_head;
            if (head.TryRemove(out result))
                return true;
            //since method IsEmpty spins, we don't need to spin in the while loop
        }
        result = default(T);
        return false;
    }
    
    private class Segment
   {
        internal volatile T[] m_array;
        internal volatile VolatileBool[] m_state;
        private volatile Segment m_next;
        internal readonly long m_index;
        private volatile int m_low;
        private volatile int m_high;
        private volatile ConcurrentQueue<T> m_source;
      
        internal Segment(long index, ConcurrentQueue<T> source)
        {
            m_array = new T[SEGMENT_SIZE];
            m_state = new VolatileBool[SEGMENT_SIZE]; //all initialized to false
            m_high = -1;
            Contract.Assert(index >= 0);
            m_index = index;
            m_source = source;
        }
        
        internal void UnsafeAdd(T value)
        {
            Contract.Assert(m_high < SEGMENT_SIZE - 1);
            m_high++;
            m_array[m_high] = value;
            m_state[m_high].m_value = true;
        }
        
       internal Segment UnsafeGrow()
        {
            Contract.Assert(m_high >= SEGMENT_SIZE - 1);
            Segment newSegment = new Segment(m_index + 1, m_source); //m_index is Int64, we don't need to worry about overflow
            m_next = newSegment;
            return newSegment;
        }
        
      internal void Grow()
        {
            //no CAS is needed, since there is no contention (other threads are blocked, busy waiting)
            Segment newSegment = new Segment(m_index + 1, m_source);  //m_index is Int64, we don't need to worry about overflow
            m_next = newSegment;
            Contract.Assert(m_source.m_tail == this);
            m_source.m_tail = m_next;
        }    
        
       internal bool TryAppend(T value)
       {             
            if (m_high >= SEGMENT_SIZE - 1)
            {
                return false;
            }
            try
            { }
            finally
            {
                newhigh = Interlocked.Increment(ref m_high);
                if (newhigh <= SEGMENT_SIZE - 1)
                {
                    m_array[newhigh] = value;
                    m_state[newhigh].m_value = true;
                }

                if (newhigh == SEGMENT_SIZE - 1)
                {
                    Grow();
                }
            }
        
            return newhigh <= SEGMENT_SIZE - 1;
        }
        
      internal bool TryRemove(out T result)
        {
            SpinWait spin = new SpinWait();
            int lowLocal = Low, highLocal = High;
            while (lowLocal <= highLocal)
            {
                if (Interlocked.CompareExchange(ref m_low, lowLocal + 1, lowLocal) == lowLocal)
                {
                  
                    SpinWait spinLocal = new SpinWait();
                    while (!m_state[lowLocal].m_value)
                    {
                        spinLocal.SpinOnce();
                    }
                    result = m_array[lowLocal];

                    if (m_source.m_numSnapshotTakers <= 0)
                    {
                        m_array[lowLocal] = default(T); //release the reference to the object. 
                    }

                 
                    if (lowLocal + 1 >= SEGMENT_SIZE)
                    {
                       
                        spinLocal = new SpinWait();
                        while (m_next == null)
                        {
                            spinLocal.SpinOnce();
                        }
                        Contract.Assert(m_source.m_head == this);
                        m_source.m_head = m_next;
                    }
                    return true;
                }
                else
                {
                   
                    spin.SpinOnce();
                    lowLocal = Low; highLocal = High;
                }
            }//end of while
            result = default(T);
            return false;
        }        
 }    
}

首先ConcurrentQueue构造函数没有 int capacity参数了，里面的线程安全是用SpinWait自旋来实现的，当我想往队列ConcurrentQueue添加一个元素的时候，如果添加失败，那程序自旋等待一下，再次添加元素，直到添加成功。里面用到了一个Segment自定义的类型，Segment的m_array是一个含有32个元素的数组，m_state和m_array一一对应，主要是用来标记m_array里面的元素是否有效。m_next是用来连接到下一个Segment的，m_high与添加元素密切相关，m_low与移除元素有关。先看TryAppend方法，优先将当前的newhigh原子加1【newhigh = Interlocked.Increment(ref m_high);】，这样假如有多个线程同时添加元素，每一个线程拿到的newhigh 值不用，那么它们操作m_array的下标就不同了，所以彼此之间不影响，到现在添加的线程安全就明白了。那么队列又是如何实现的了？我们来看看Grow()方法，当Segment的32个元素都被使用了，那么这个时候添加元素需要扩容，扩容的方式是重新实例一个Segment，旧的Segment的m_next属性指向新的得Segment，这样就组成了一个Segment链表（Segment核心是数组），它们的index从0开始，第一个Segment的index为0，第2个Segment的index为1....。TryRemove的实现类似，m_low其实是Segment的m_array下标，程序自旋一次，查找是否有元素，如果有元素必须检查元素是否有效（!m_state[lowLocal].m_value，因为在天加元素的时候是先增加newhigh变量，然后在设置m_state[newhigh].m_value = true有效），所以移除元素的时候必选验证元素是否有效。然后释放元素m_array[lowLocal] = default(T);，如果第一个Segment的元素全部移除了【if (lowLocal + 1 >= SEGMENT_SIZE)】，那么我们就应该开始移除第2个Segment元素了，需要检查是否有第2个Segment，如果没有 就自旋等待吧，然后m_head指向第下一个Segment【m_source.m_head = m_next;】线程安全依靠SpinWait 的自旋和原子操作Interlocked.Increment和Interlocked.CompareExchange来实现的。