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How to: Use SpinLock for Low-Level Synchronization

The following example demonstrates how to use a SpinLock.

In this example, the critical section performs a minimal amount of work, which makes it a good candidate for a SpinLock. Increasing the work a small amount increases the performance of the SpinLock compared to a standard lock. However, there is a point at which a SpinLock becomes more expensive than a standard lock. You can use the concurrency profiling functionality in the profiling tools to see which type of lock provides better performance in your program. For more information, see Concurrency Visualizer.

Imports System
Imports System.Threading
Imports System.Threading.Tasks
Class SpinLockDemo2

    Const N As Integer = 100000
    Shared _queue = New Queue(Of Data)()
    Shared _lock = New Object()
    Shared _spinlock = New SpinLock()

    Class Data
        Public Name As String 
        Public Number As Double 
    End Class 
    Shared Sub Main()

        ' First use a standard lock for comparison purposes.
        UseLock()
        _queue.Clear()
        UseSpinLock()

        Console.WriteLine("Press a key")
        Console.ReadKey()

    End Sub 

    Private Shared Sub UpdateWithSpinLock(ByVal d As Data, ByVal i As Integer)

        Dim lockTaken As Boolean = False 
        Try
            _spinlock.Enter(lockTaken)
            _queue.Enqueue(d)
        Finally 

            If lockTaken Then
                _spinlock.Exit(False)
            End If 
        End Try 
    End Sub 

    Private Shared Sub UseSpinLock()


        Dim sw = Stopwatch.StartNew()

        Parallel.Invoke(
               Sub()
                   For i As Integer = 0 To N - 1
                       UpdateWithSpinLock(New Data() With {.Name = i.ToString(), .Number = i}, i)
                   Next 
               End Sub,
                Sub()
                    For i As Integer = 0 To N - 1
                        UpdateWithSpinLock(New Data() With {.Name = i.ToString(), .Number = i}, i)
                    Next 
                End Sub
            )
        sw.Stop()
        Console.WriteLine("elapsed ms with spinlock: {0}", sw.ElapsedMilliseconds)
    End Sub 

    Shared Sub UpdateWithLock(ByVal d As Data, ByVal i As Integer)

        SyncLock (_lock)
            _queue.Enqueue(d)
        End SyncLock 
    End Sub 

    Private Shared Sub UseLock()

        Dim sw = Stopwatch.StartNew()

        Parallel.Invoke(
                Sub()
                    For i As Integer = 0 To N - 1
                        UpdateWithLock(New Data() With {.Name = i.ToString(), .Number = i}, i)
                    Next 
                End Sub,
               Sub()
                   For i As Integer = 0 To N - 1
                       UpdateWithLock(New Data() With {.Name = i.ToString(), .Number = i}, i)
                   Next 
               End Sub
                )
        sw.Stop()
        Console.WriteLine("elapsed ms with lock: {0}", sw.ElapsedMilliseconds)
    End Sub 
End Class

SpinLock might be useful when a lock on a shared resource is not going to be held for very long. In such cases, on multi-core computers it can be efficient for the blocked thread to spin for a few cycles until the lock is released. By spinning, the thread does not become blocked, which is a CPU-intensive process. SpinLock will stop spinning under certain conditions to prevent starvation of logical processors or priority inversion on systems with Hyper-Threading.

This example uses the System.Collections.Generic.Queue(Of T) class, which requires user synchronization for multi-threaded access. In applications that target the .NET Framework version 4, another option is to use the System.Collections.Concurrent.ConcurrentQueue(Of T), which does not require any user locks.

Note the use of false (False in Visual Basic) in the call to Exit. This provides the best performance. Specify true (True)on IA64 architectures to use the memory fence, which flushes the write buffers to ensure that the lock is now available for other threads to exit.

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