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How to: Use SpinWait to Implement a Two-Phase Wait Operation

The following example shows how to use a System.Threading.SpinWait object to implement a two-phase wait operation. In the first phase, the synchronization object, a Latch, spins for a few cycles while it checks whether the lock has become available. In the second phase, if the lock becomes available, then the Wait method returns without using the System.Threading.ManualResetEvent to perform its wait; otherwise, Wait performs the wait.

Example

This example shows a very basic implementation of a Latch synchronization primitive. You can use this data structure when wait times are expected to be very short. This example is for demonstration purposes only. If you require latch-type functionality in your program, consider using System.Threading.ManualResetEventSlim.

#Const LOGGING = 1

Imports System
Imports System.Collections.Generic
Imports System.Diagnostics
Imports System.Linq
Imports System.Text
Imports System.Threading
Imports System.Threading.Tasks

Namespace CDS_Spinwait

    Class Latch

        ' 0 = unset, 1 = set
        Private m_state As Integer = 0

        Private m_ev = New ManualResetEvent(False)

#If LOGGING Then
        ' For fast logging with minimal impact on latch behavior.
        ' Spin counts greater than 20 might be encountered depending on machine config.
        Dim spinCountLog As Integer()

        Private totalKernelWaits As Integer = 0
        Public Sub New()
            ReDim spinCountLog(19)
        End Sub

        Public Sub PrintLog()

            For i As Integer = 0 To spinCountLog.Length - 1
                Console.WriteLine("Wait succeeded with spin count of {0} on {1} attempts", i, spinCountLog(i))
            Next
            Console.WriteLine("Wait used the kernel event on {0} attempts.", totalKernelWaits)
            Console.WriteLine("Logging complete")
        End Sub
#End If

        Public Sub SetLatch()

            ' Trace.WriteLine("Setlatch")
            Interlocked.Exchange(m_state, 1)
            m_ev.Set()
        End Sub


        Public Sub Wait()

            Trace.WriteLine("Wait timeout infinite")
            Wait(Timeout.Infinite)
        End Sub


        Public Function Wait(ByVal timeout As Integer) As Boolean

            ' Allocated on the stack.
            Dim spinner = New SpinWait()
            Dim watch As Stopwatch



            While (m_state = 0)

                ' Lazily allocate and start stopwatch to track timeout.
                watch = Stopwatch.StartNew()

                ' Spin only until the SpinWait is ready
                ' to initiate its own context switch.
                If (spinner.NextSpinWillYield = False) Then

                    spinner.SpinOnce()

                    ' Rather than let SpinWait do a context switch now,
                    '  we initiate the kernel Wait operation, because
                    ' we plan on doing this anyway.
                Else
#If LOGGING Then
                    Interlocked.Increment(totalKernelWaits)
#End If
                    ' Account for elapsed time.
                    Dim realTimeout As Long = timeout - watch.ElapsedMilliseconds

                    Debug.Assert(realTimeout <= Integer.MaxValue)
                    ' Do the wait.
                    If (realTimeout <= 0) Then

                        Trace.WriteLine("wait timed out.")
                        Return False
                    ElseIf m_ev.WaitOne(realTimeout) = False Then
                        Return False
                    End If

                End If

            End While

            ' Take the latch.

            Interlocked.Exchange(m_state, 0)


#If LOGGING Then
            Interlocked.Increment(spinCountLog(spinner.Count))
#End If


            Return True
        End Function
    End Class

    Class Program
        Shared latch = New Latch()
        Shared count As Integer = 2
        Shared cts = New CancellationTokenSource()

        Shared Sub TestMethod()

            While (cts.IsCancellationRequested = False And count < Integer.MaxValue - 1)

                ' Obtain the latch.
                If (latch.Wait(50)) Then
                    ' Do the work. Here we vary the workload a slight amount
                    ' to help cause varying spin counts in latch.
                    Dim d As Double = 0
                    If (count Mod 2 <> 0) Then
                        d = Math.Sqrt(count)
                    End If
                    Interlocked.Increment(count)

                    ' Release the latch.
                    latch.SetLatch()
                End If
            End While
        End Sub
        Shared Sub Main()
            ' Demonstrate latch with a simple scenario:
            ' two threads updating a shared integer and
            ' accessing a shared StringBuilder. Both operations
            ' are relatively fast, which enables the latch to
            ' demonstrate successful waits by spinning only. 
            latch.SetLatch()

            ' UI thread. Press 'c' to cancel the loop.
            Task.Factory.StartNew(Sub()
                                      Console.WriteLine("Wait a few seconds, then press 'c' to see results.")
                                      If (Console.ReadKey().KeyChar = "c"c) Then
                                          cts.Cancel()
                                      End If
                                  End Sub)

            Parallel.Invoke(
                Sub() TestMethod(),
               Sub() TestMethod(),
                Sub() TestMethod()
                )

#If LOGGING Then
            latch.PrintLog()
#End If
            Console.WriteLine(vbCrLf & "To exit, press the Enter key.")
            Console.ReadLine()
        End Sub
    End Class
End Namespace
#define LOGGING

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace CDS_Spinwait
{
    class Latch
    {
        // 0 = unset, 1 = set
        private volatile int m_state = 0;

        private ManualResetEvent m_ev = new ManualResetEvent(false);

#if LOGGING
        // For fast logging with minimal impact on latch behavior.
        // Spin counts greater than 20 might be encountered depending on machine config.
        private int[] spinCountLog = new int[20];
        private volatile int totalKernelWaits = 0;

        public void PrintLog()
        {

            for (int i = 0; i < spinCountLog.Length; i++)
            {
                Console.WriteLine("Wait succeeded with spin count of {0} on {1} attempts", i, spinCountLog[i]);
            }
            Console.WriteLine("Wait used the kernel event on {0} attempts.", totalKernelWaits);
            Console.WriteLine("Logging complete");
        }
#endif

        public void Set()
        {
            // Trace.WriteLine("Set");
            m_state = 1;
            m_ev.Set();
        }

        public void Wait()
        {
            Trace.WriteLine("Wait timeout infinite");
            Wait(Timeout.Infinite);
        }

        public bool Wait(int timeout)
        {
            // Allocated on the stack.
            SpinWait spinner = new SpinWait();
            Stopwatch watch;



            while (m_state == 0)
            {

                // Lazily allocate and start stopwatch to track timeout.
                watch = Stopwatch.StartNew();

                // Spin only until the SpinWait is ready
                // to initiate its own context switch.
                if (!spinner.NextSpinWillYield)
                {
                    spinner.SpinOnce();

                }
                // Rather than let SpinWait do a context switch now,
                //  we initiate the kernel Wait operation, because
                // we plan on doing this anyway.
                else
                {
                    totalKernelWaits++;
                    // Account for elapsed time.
                    int realTimeout = timeout - (int)watch.ElapsedMilliseconds;

                    // Do the wait.
                    if (realTimeout <= 0 || !m_ev.WaitOne(realTimeout))
                    {
                        Trace.WriteLine("wait timed out.");
                        return false;
                    }
                }
            }

            // Take the latch.
            m_state = 0;
            //   totalWaits++;

#if LOGGING
            spinCountLog[spinner.Count]++;
#endif


            return true;
        }
    }

    class Program
    {
        static Latch latch = new Latch();
        static int count = 2;
        static CancellationTokenSource cts = new CancellationTokenSource();

        static void TestMethod()
        {
            while (!cts.IsCancellationRequested)
            {
                // Obtain the latch.
                if (latch.Wait(50))
                {
                    // Do the work. Here we vary the workload a slight amount
                    // to help cause varying spin counts in latch.
                    double d = 0;
                    if (count % 2 != 0)
                    {
                        d = Math.Sqrt(count);
                    }
                    count++;

                    // Release the latch.
                    latch.Set();
                }
            }
        }
        static void Main(string[] args)
        {
            // Demonstrate latch with a simple scenario:
            // two threads updating a shared integer and
            // accessing a shared StringBuilder. Both operations
            // are relatively fast, which enables the latch to
            // demonstrate successful waits by spinning only. 

            latch.Set();


            // UI thread. Press 'c' to cancel the loop.
            Task.Factory.StartNew(() =>
            {
                Console.WriteLine("Press 'c' to cancel.");
                if (Console.ReadKey().KeyChar == 'c')
                {
                    cts.Cancel();

                }
            });

            Parallel.Invoke(

                () => TestMethod(),
                () => TestMethod(),
                () => TestMethod()
                );

#if LOGGING
            latch.PrintLog();
#endif
            Console.WriteLine("\r\nPress the Enter Key.");
            Console.ReadLine();
        }
    }
}

The latch uses the SpinWait object to spin in place only until the next call to SpinOnce causes the SpinWait to yield the time slice of the thread. At that point, the latch causes its own context switch by calling WaitOne(Int32, Boolean) on the ManualResetEvent and passing in the remainder of the time-out value.

The logging output shows how often the Latch was able to increase performance by acquiring the lock without using the ManualResetEvent.

See Also

Concepts

SpinWait

Other Resources

Threading Objects and Features