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SpinWait

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System.Threading.SpinWait is a lightweight synchronization type that you can use in low-level scenarios to avoid the expensive context switches and kernel transitions that are required for kernel events. On multicore computers, when a resource is not expected to be held for long periods of time, it can be more efficient for a waiting thread to spin in user mode for a few dozen or a few hundred cycles, and then retry to acquire the resource. If the resource is available after spinning, then you have saved several thousand cycles. If the resource is still not available, then you have spent only a few cycles and can still enter a kernel-based wait. This spinning-then-waiting combination is sometimes referred to as a two-phase wait operation.

SpinWait is designed to be used in conjunction with the .NET types that wrap kernel events such as ManualResetEvent. SpinWait can also be used by itself for basic spinning functionality in just one program.

SpinWait is more than just an empty loop. It is carefully implemented to provide correct spinning behavior for the general case, and will itself initiate context switches if it spins long enough (roughly the length of time required for a kernel transition). For example, on single-core computers, SpinWait yields the time slice of the thread immediately because spinning blocks forward progress on all threads. SpinWait also yields even on multi-core machines to prevent the waiting thread from blocking higher-priority threads or the garbage collector. Therefore, if you are using a SpinWait in a two-phase wait operation, we recommend that you invoke the kernel wait before the SpinWait itself initiates a context switch. SpinWait provides the NextSpinWillYield property, which you can check before every call to SpinOnce. When the property returns true, initiate your own Wait operation. For an example, see How to: Use SpinWait to Implement a Two-Phase Wait Operation.

If you are not performing a two-phase wait operation but are just spinning until some condition is true, you can enable SpinWait to perform its context switches so that it is a good citizen in the Windows operating system environment. The following basic example shows a SpinWait in a lock-free stack. If you require a high-performance, thread-safe stack, consider using System.Collections.Concurrent.ConcurrentStack<T>.

public class LockFreeStack<T>
{
    private volatile Node m_head;

    private class Node { public Node Next; public T Value; }

    public void Push(T item)
    {
        var spin = new SpinWait();
        Node node = new Node { Value = item }, head;
        while (true)
        {
            head = m_head;
            node.Next = head;
            if (Interlocked.CompareExchange(ref m_head, node, head) == head) break;
            spin.SpinOnce();
        }
    }

    public bool TryPop(out T result)
    {
        result = default(T);
        var spin = new SpinWait();

        Node head;
        while (true)
        {
            head = m_head;
            if (head == null) return false;
            if (Interlocked.CompareExchange(ref m_head, head.Next, head) == head)
            {
                result = head.Value;
                return true;
            }
            spin.SpinOnce();
        }
    }
}

Imports System.Threading

Module SpinWaitDemo


    Public Class LockFreeStack(Of T)
        Private m_head As Node

        Private Class Node
            Public [Next] As Node
            Public Value As T
        End Class

        Public Sub Push(ByVal item As T)
            Dim spin As New SpinWait()
            Dim head As Node, node As New Node With {.Value = item}

            While True
                Thread.MemoryBarrier()
                head = m_head
                node.Next = head
                If Interlocked.CompareExchange(m_head, node, head) Is head Then Exit While
                spin.SpinOnce()
            End While
        End Sub

        Public Function TryPop(ByRef result As T) As Boolean
            result = CType(Nothing, T)
            Dim spin As New SpinWait()

            Dim head As Node
            While True
                Thread.MemoryBarrier()
                head = m_head
                If head Is Nothing Then Return False
                If Interlocked.CompareExchange(m_head, head.Next, head) Is head Then
                    result = head.Value
                    Return True
                End If
                spin.SpinOnce()
            End While
        End Function
    End Class


End Module

See also