Cómo: Usar SpinWait para implementar una operación de espera de dos fases
En el siguiente ejemplo se muestra cómo utilizar un objeto System.Threading.SpinWait para implementar una operación de espera de dos fases. En la primera fase, el objeto de sincronización, Latch, gira durante unos ciclos mientras comprueba si el bloqueo está disponible. En la segunda fase, si el bloqueo está disponible, el método Wait vuelve sin utilizar System.Threading.ManualResetEvent para realizar su espera; de lo contrario, Wait realiza la espera.
Ejemplo
En este ejemplo, se muestra una implementación muy básica de una primitiva de sincronización de Latch. Puede utilizar esta estructura de datos mientras se prevé que los tiempos de espera sean muy cortos. El único fin de este ejemplo es usarlo para realizar una demostración. Si necesita una funcionalidad de tipo bloqueo temporal en el programa, considere la posibilidad de utilizar 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();
}
}
}
El bloqueo temporal utiliza el objeto SpinWait para girar en su posición solo hasta que la llamada siguiente a SpinOnce haga que SpinWait genere el intervalo de tiempo del subproceso. A partir de ese momento, el bloqueo temporal produce su propio cambio de contexto llamando a WaitOne(Int32, Boolean) en ManualResetEvent y pasando el resto del valor del tiempo de espera.
El resultado del registro muestra con qué frecuencia Latch pudo aumentar el rendimiento adquiriendo el bloqueo sin utilizar ManualResetEvent.