当多个任务或线程并行运行时,难以避免的对某些有限的资源进行并发的访问。可以考虑使用信号量来进行这方面的控制(System.Threading.Semaphore)是表示一个Windows内核的信号量对象。如果预计等待的时间较短,可以考虑使用SemaphoreSlim,它则带来的开销更小。
.NetFrameWork中的信号量通过跟踪进入和离开的任务或线程来协调对资源的访问。信号量需要知道资源的最大数量,当一个任务进入时,资源计数器会被减1,当计数器为0时,如果有任务访问资源,它会被阻塞,直到有任务离开为止。
示例程序: 10个任务并行访问3个资源
using System;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Diagnostics;
namespace Sample5_8_semaphoreslim
{
class Program
{
private static int _TaskNum = 10;
private static Task[] _Tasks;
private const int MAX_RESOURCE = 3;
private const int RUN_LOOP = 10;
private static SemaphoreSlim m_Semaphore;
private static void Work1(int TaskID)
{
int i = 0;
var sw = Stopwatch.StartNew();
var rnd = new Random();
while (i < RUN_LOOP)
{
Thread.Sleep(rnd.Next(200, 500));
Console.WriteLine("TASK " + TaskID + " REQUESTing {");
m_Semaphore.Wait();
try
{
Console.WriteLine("TASK " + TaskID + " WOrking ... ..." + i);
sw.Restart();
Thread.Sleep(rnd.Next(200, 500));
}
finally
{
Console.WriteLine("TASK " + TaskID + " REQUESTing }");
m_Semaphore.Release();
i++;
}
}
}
static void Main(string[] args)
{
_Tasks = new Task[_TaskNum];
m_Semaphore = new SemaphoreSlim(MAX_RESOURCE);
int i = 0;
for (i = 0; i < _TaskNum; i++)
{
_Tasks[i] = Task.Factory.StartNew((num) =>
{
var taskid = (int)num;
Work1(taskid);
}, i);
}
var finalTask = Task.Factory.ContinueWhenAll(_Tasks, (tasks) =>
{
Task.WaitAll(_Tasks);
Console.WriteLine("==========================================================");
Console.WriteLine("All Phase is completed");
Console.WriteLine("==========================================================");
});
try
{
finalTask.Wait();
}
catch (AggregateException aex)
{
Console.WriteLine("Task failed And Canceled" + aex.ToString());
}
finally
{
m_Semaphore.Dispose();
}
Console.ReadLine();
}
}
}
信号量当然不可能永久的阻塞在那里。信号量也提供了超时处理机制。方法是在Wait函数中传入一个超时等待时间 - Wait(int TIMEOUT)。当Wait返回值为false时表明它超时了。如果传入了 -1,则表示无限期的等待。
程序示例:注意其中的m_Semaphore.Release();已经被注释掉了,任务会等待1秒钟然后超时。
using System;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Diagnostics;
namespace Sample5_8_semaphoreslim
{
class Program
{
private static int _TaskNum = 10;
private static Task[] _Tasks;
private const int MAX_RESOURCE = 3;
private const int RUN_LOOP = 10;
private static SemaphoreSlim m_Semaphore;
private static void Work1(int TaskID)
{
int i = 0;
var sw = Stopwatch.StartNew();
var rnd = new Random();
while (i < RUN_LOOP)
{
Thread.Sleep(rnd.Next(200, 500));
Console.WriteLine("TASK " + TaskID + " REQUESTing {");
if (!m_Semaphore.Wait(1000))
{
Console.WriteLine("TASK " + TaskID + " TIMEOUT!!!");
return;
}
try
{
Console.WriteLine("TASK " + TaskID + " WOrking ... ..." + i);
sw.Restart();
Thread.Sleep(rnd.Next(2000, 5000));
}
finally
{
Console.WriteLine("TASK " + TaskID + " REQUESTing }");
//m_Semaphore.Release();
i++;
}
}
}
static void Main(string[] args)
{
_Tasks = new Task[_TaskNum];
m_Semaphore = new SemaphoreSlim(MAX_RESOURCE);
int i = 0;
for (i = 0; i < _TaskNum; i++)
{
_Tasks[i] = Task.Factory.StartNew((num) =>
{
var taskid = (int)num;
Work1(taskid);
}, i);
}
var finalTask = Task.Factory.ContinueWhenAll(_Tasks, (tasks) =>
{
Task.WaitAll(_Tasks);
Console.WriteLine("==========================================================");
Console.WriteLine("All Phase is completed");
Console.WriteLine("==========================================================");
});
try
{
finalTask.Wait();
}
catch (AggregateException aex)
{
Console.WriteLine("Task failed And Canceled" + aex.ToString());
}
finally
{
m_Semaphore.Dispose();
}
Console.ReadLine();
}
}
}
如果需要有跨进程或AppDomain的同步时,可以考虑使用Semaphore。Semaphore是取得的Windows 内核的信号量,所以在整个系统中是有效的。
它主要的接口时 Release和WaitOne,使用的方式和SemaphoreSlim是一致的。
C# 并行编程 之 限制资源的并发访问 使用SemaphoreSlim
原文地址:http://blog.csdn.net/wangzhiyu1980/article/details/45886805