C# 并行编程 之 Barrier的使用 - 通过屏障同步并发任务

基本信息

Barrier 是 .Net 提供的一直并发的机制，它允许多个任务同步他们不同阶段的并发工作。

这里的关键点是【多个任务】和【不同阶段】。
假设有4个相同的任务(Task)，每个任务都有4个阶段(Phase)，当他们并发工作时，只有当所有任务的相同步骤都完成时，所有任务才可以开始下一个步骤。

如图所示：

这里的 Barrier 就是 .NetFramework 提供的一种机制。它像一个篱笆(屏障)，把所有任务的阶段隔离开来，当前阶段不完成，不会开始下一个阶段。

代码示例：

using System;using System.Threading;using System.Threading.Tasks;namespace Sample5_1_barrier{    class Program    {        private static void Phase0Doing(int TaskID)        {            Console.WriteLine("Task : #{0}   =====  Phase 0", TaskID);        }        private static void Phase1Doing(int TaskID)        {            Console.WriteLine("Task : #{0}   *****  Phase 1", TaskID);        }        private static void Phase2Doing(int TaskID)        {            Console.WriteLine("Task : #{0}   ^^^^^  Phase 2", TaskID);        }        private static void Phase3Doing(int TaskID)        {            Console.WriteLine("Task : #{0}   $$$$$  Phase 3", TaskID);        }        private static int _TaskNum = 4;        private static Task[] _Tasks;        private static Barrier _Barrier;        static void Main(string[] args)        {            _Tasks = new Task[_TaskNum];            _Barrier = new Barrier(_TaskNum, (barrier) =>            {                Console.WriteLine("-------------------------- Current Phase:{0} --------------------------",                                   _Barrier.CurrentPhaseNumber);            });            for (int i = 0; i < _TaskNum; i++)            {                _Tasks[i] = Task.Factory.StartNew((num) =>                {                    var taskid = (int)num;                    Phase0Doing(taskid);                    _Barrier.SignalAndWait();                    Phase1Doing(taskid);                    _Barrier.SignalAndWait();                    Phase2Doing(taskid);                    _Barrier.SignalAndWait();                    Phase3Doing(taskid);                    _Barrier.SignalAndWait();                }, i);            }            var finalTask = Task.Factory.ContinueWhenAll(_Tasks, (tasks) =>                {                    Task.WaitAll(_Tasks);                    Console.WriteLine("========================================");                    Console.WriteLine("All Phase is completed");                    _Barrier.Dispose();                });            finalTask.Wait();            Console.ReadLine();        }    }}

测试结果：

使用屏障时的异常处理

如果进入屏障后，工作的代码出现了异常，这个异常会被包装在BarrierPostPhaseException中，而且所有任务都能够捕捉到这个异常。原始的异常可以通过NarrierPostPhaseException 对象的InnerException进行访问。

代码示例： 注意其中抛出异常的位置，和捕获异常的位置。

using System;using System.Threading;using System.Threading.Tasks;namespace Sample5_1_barrier{    class Program    {        private static void Phase0Doing(int TaskID)        {            Console.WriteLine("Task : #{0}   =====  Phase 0", TaskID);        }        private static void Phase1Doing(int TaskID)        {            Console.WriteLine("Task : #{0}   *****  Phase 1", TaskID);        }        private static void Phase2Doing(int TaskID)        {            Console.WriteLine("Task : #{0}   ^^^^^  Phase 2", TaskID);        }        private static void Phase3Doing(int TaskID)        {            Console.WriteLine("Task : #{0}   $$$$$  Phase 3", TaskID);        }        private static int _TaskNum = 4;        private static Task[] _Tasks;        private static Barrier _Barrier;        static void Main(string[] args)        {            _Tasks = new Task[_TaskNum];            _Barrier = new Barrier(_TaskNum, (barrier) =>            {                Console.WriteLine("-------------------------- Current Phase:{0} --------------------------",                                   _Barrier.CurrentPhaseNumber);                if (_Barrier.CurrentPhaseNumber == 1)                    throw new InvalidOperationException("Phase 2 need to be TERMINTED!!!!!");            });            for (int i = 0; i < _TaskNum; i++)            {                _Tasks[i] = Task.Factory.StartNew((num) =>                {                    var taskid = (int)num;                    Phase0Doing(taskid);                    _Barrier.SignalAndWait();                    Phase1Doing(taskid);                    try                    {                        _Barrier.SignalAndWait();                    }                    catch (BarrierPostPhaseException bpp_ex)                    {                        Console.WriteLine("Got an Exception in Phase1: " + bpp_ex.InnerException);                    }                    Phase2Doing(taskid);                    _Barrier.SignalAndWait();                    Phase3Doing(taskid);                    _Barrier.SignalAndWait();                }, i);            }            var finalTask = Task.Factory.ContinueWhenAll(_Tasks, (tasks) =>                {                    Task.WaitAll(_Tasks);                    Console.WriteLine("========================================");                    Console.WriteLine("All Phase is completed");                    _Barrier.Dispose();                });            finalTask.Wait();            Console.ReadLine();        }    }}

Barrier 关于超时的处理

这里会使用Barrier.SignalAndWait(TIMEOUT))，来对超时进行判断。示例代码中Phase 2的Task 3会等待10秒，超过了超时时间 2秒，在Barrier中会检查到Task的Phase2超时了并返回错误。

示例代码：

using System;using System.Threading;using System.Threading.Tasks;namespace Sample5_1_barrier{    class Program    {        private static void Phase0Doing(int TaskID)        {            Console.WriteLine("Task : #{0}   =====  Phase 0", TaskID);        }        private static void Phase1Doing(int TaskID)        {            Console.WriteLine("Task : #{0}   *****  Phase 1", TaskID);        }        private static void Phase2Doing(int TaskID)        {            int i = 0;            Console.WriteLine("Task : #{0}   ^^^^^  Phase 2", TaskID);            if (TaskID == 3)                while (i < 10)                {                    System.Threading.Thread.Sleep(1000);                    i++;                }        }        private static void Phase3Doing(int TaskID)        {            Console.WriteLine("Task : #{0}   $$$$$  Phase 3", TaskID);        }        private static int _TaskNum = 4;        private static Task[] _Tasks;        private static Barrier _Barrier;        private static int TIMEOUT = 2000;        static void Main(string[] args)        {            var cts = new System.Threading.CancellationTokenSource();            var ct = cts.Token;            _Tasks = new Task[_TaskNum];            _Barrier = new Barrier(_TaskNum, (barrier) =>            {                Console.WriteLine("-------------------------- Current Phase:{0} --------------------------",                                   _Barrier.CurrentPhaseNumber);                //if (_Barrier.CurrentPhaseNumber == 1)                //    throw new InvalidOperationException("Phase 2 need to be TERMINTED!!!!!");            });            for (int i = 0; i < _TaskNum; i++)            {                _Tasks[i] = Task.Factory.StartNew((num) =>                {                    var taskid = (int)num;                    Phase0Doing(taskid);                    if (!_Barrier.SignalAndWait(TIMEOUT))                    {                        Console.WriteLine("``````````````  This Phase {0} TIMEOUT  ``````````````", _Barrier.CurrentPhaseNumber);                        throw new OperationCanceledException("Phase 0 canceled: ", ct);                    }                    Phase1Doing(taskid);                    if (!_Barrier.SignalAndWait(TIMEOUT))                    {                        Console.WriteLine("``````````````  This Phase {0} TIMEOUT  ``````````````", _Barrier.CurrentPhaseNumber);                        throw new OperationCanceledException("Phase 1 canceled: ", ct);                    }                    Phase2Doing(taskid);                    if (!_Barrier.SignalAndWait(TIMEOUT))                    {                        Console.WriteLine("``````````````  This Phase {0} TIMEOUT  ``````````````", _Barrier.CurrentPhaseNumber);                        throw new OperationCanceledException("Phase 2 canceled: ", ct);                    }                    Phase3Doing(taskid);                    if (!_Barrier.SignalAndWait(TIMEOUT))                    {                        Console.WriteLine("``````````````  This Phase {0} TIMEOUT  ``````````````", _Barrier.CurrentPhaseNumber);                        throw new OperationCanceledException("Phase 3 canceled: ", ct);                    }                }, i, ct);            }            var finalTask = Task.Factory.ContinueWhenAll(_Tasks, (tasks) =>                {                    Task.WaitAll(_Tasks);                    Console.WriteLine("========================================");                    Console.WriteLine("All Phase is completed");                }, ct);            try            {                finalTask.Wait();            }            catch (AggregateException aex)            {                Console.WriteLine("Task failed And Canceled" + aex.ToString());            }            finally            {                _Barrier.Dispose();            }            Console.ReadLine();        }    }}