使用Interlocked在多线程下进行原子操作，无锁无阻塞的实现线程运行状态判断

Posted 2020-10-22 又见阿郎

巧妙地使用Interlocked的各个方法，再无锁无阻塞的情况下判断出所有线程的运行完成状态。

昨晚耐着性子看完了clr via c#的第29章<<基元线程同步构造>>，尽管这本书不是第一次看了，但是之前看的都是一带而过，没有深入理解，甚至可以说是不理解，实习了之后发现自己的知识原来这么表面，很多的实现都不能做出来，这很大程度上打击了我，而且，春招也快来了，更需要打扎实基础。引起我注意的是jeffrey在第29章说的:使用Interlocked，代码很短，绝不阻塞任何线程，二期使用线程池线程来实现自动伸缩。下载了源码，然后分析了下书中的示例，code如下:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace vlr_via_cs
{
    internal static class AsyncCoordinatorDemo
    {
        public static void Go()
        {
            const Int32 timeout = 50000;   // Change to desired timeout
            MultiWebRequests act = new MultiWebRequests(timeout);
            Console.WriteLine("All operations initiated (Timeout={0}). Hit <Enter> to cancel.",
               (timeout == Timeout.Infinite) ? "Infinite" : (timeout.ToString() + "ms"));
            Console.ReadLine();
            act.Cancel();

            Console.WriteLine();
            Console.WriteLine("Hit enter to terminate.");
            Console.ReadLine();
        }

        private sealed class MultiWebRequests
        {
            // This helper class coordinates all the asynchronous operations
            private AsyncCoordinator m_ac = new AsyncCoordinator();

            // Set of Web servers we want to query & their responses (Exception or Int32)
            private Dictionary<String, Object> m_servers = new Dictionary<String, Object> {
                { "http://cjjjs.com/", null },
                { "http://cnblogs.com/", null },
                { "http://www.jobbole.com/", null }
            };

            public MultiWebRequests(Int32 timeout = Timeout.Infinite)
            {
                // Asynchronously initiate all the requests all at once
                var httpClient = new HttpClient();
                foreach (var server in m_servers.Keys)
                {
                    m_ac.AboutToBegin(1); //确保先做三次加法, 若是有Sleep，在调用完这个函数后，执行
                    httpClient.GetByteArrayAsync(server).ContinueWith(task => ComputeResult(server, task));
                }

                // Tell AsyncCoordinator that all operations have been initiated and to call
                // AllDone when all operations complete, Cancel is called, or the timeout occurs
                m_ac.AllBegun(AllDone, timeout);
            }

            private void ComputeResult(String server, Task<Byte[]> task)
            {
                Object result;
                if (task.Exception != null)
                {
                    result = task.Exception.InnerException;
                }
                else
                {
                    // Process I/O completion here on thread pool thread(s)
                    // Put your own compute-intensive algorithm here...
                    result = task.Result.Length;   // This example just returns the length
                }

                // Save result (exception/sum) and indicate that 1 operation completed
                m_servers[server] = result;
                m_ac.JustEnded();
            }

            // Calling this method indicates that the results don\'t matter anymore
            public void Cancel() { m_ac.Cancel(); }

            // This method is called after all Web servers respond, 
            // Cancel is called, or the timeout occurs
            private void AllDone(CoordinationStatus status)
            {
                switch (status)
                {
                    case CoordinationStatus.Cancel:
                        Console.WriteLine("Operation canceled.");
                        break;

                    case CoordinationStatus.Timeout:
                        Console.WriteLine("Operation timed-out.");
                        break;

                    case CoordinationStatus.AllDone:
                        Console.WriteLine("Operation completed; results below:");
                        foreach (var server in m_servers)
                        {
                            Console.Write("{0} ", server.Key);
                            Object result = server.Value;
                            if (result is Exception)
                            {
                                Console.WriteLine("failed due to {0}.", result.GetType().Name);
                            }
                            else
                            {
                                Console.WriteLine("returned {0:N0} bytes.", result);
                            }
                        }
                        break;
                }
            }
        }

        private enum CoordinationStatus
        {
            AllDone,
            Timeout,
            Cancel
        };

        private sealed class AsyncCoordinator
        {
            private Int32 m_opCount = 1;        // Decremented when AllBegun calls JustEnded
            private Int32 m_statusReported = 0; // 0=false, 1=true
            private Action<CoordinationStatus> m_callback;
            private Timer m_timer;

            // This method MUST be called BEFORE initiating an operation
            public void AboutToBegin(Int32 opsToAdd = 1)
            {
                Interlocked.Add(ref m_opCount, opsToAdd);
            }

            // This method MUST be called AFTER an operations result has been processed
            public void JustEnded()
            {
                if (Interlocked.Decrement(ref m_opCount) == 0)
                    ReportStatus(CoordinationStatus.AllDone);
            }

            // This method MUST be called AFTER initiating ALL operations
            public void AllBegun(Action<CoordinationStatus> callback, Int32 timeout = Timeout.Infinite)
            {
                m_callback = callback;
                if (timeout != Timeout.Infinite)
                {
                    // 在指定的时间点(dueTime) 调用回调函数，随后在指定的时间间隔(period)调用回调函数
                    m_timer = new Timer(TimeExpired, null, timeout, Timeout.Infinite);
                }
                JustEnded();
            }

            // 处理过时的线程
            private void TimeExpired(Object o) {
                ReportStatus(CoordinationStatus.Timeout);
            }

            public void Cancel()
            {
                if (m_callback == null)
                    throw new InvalidOperationException("Cancel cannot be called before AllBegun");
                ReportStatus(CoordinationStatus.Cancel);
            }

            private void ReportStatus(CoordinationStatus status)
            {
                if (m_timer != null)
                {  // If timer is still in play, kill it
                    Timer timer = Interlocked.Exchange(ref m_timer, null);
                    if (timer != null) timer.Dispose();
                }

                // If status has never been reported, report it; else ignore it
                if (Interlocked.Exchange(ref m_statusReported, 1) == 0)
                    m_callback(status);
            }
        }
    }


    class Program
    {
        static void Main(string[] args)
        {
            AsyncCoordinatorDemo.Go();

            Console.Read();
        }
    }
}

的确是无锁的操作，Interlocked方法是用户模式下的原子操作，针对的是CPU，不是线程内存，而且它是自旋等待的，耗费的是CPU资源。分析了下AsyncCoordinator类，主要就是利用Interlocked的Add方法，实时计数线程的数量，随后待一个线程运行的最后又调用Interlocked的Decrement方法自减。如果你留心的话，你会发现，目前绝大多数的并发判断中都用到了Interlocked的这些方法，尤其是interlocked的anything模式下的compareexchange方法，在这里提一嘴，除了compareexchange和exchange方法的返回值是返回ref类型原先的值之外，其余的方法都是返回改变之后的值。最后我们可以通过AllBegun方法来判断是不是所有的线程都执行完了，随后将状态变量m_statusReported设置为1，防止在进行状态判断。

这个类很好，之前写并发的时候，老是烦恼怎么判断并发是否已经完事了，又不想用到阻塞，这个类很好，当然应用到具体项目中可能还需要改，但是基本的模型还是这个，不变的。

有点感慨：好东西需要我们自己去发掘，之前查生产者消费者模型的时候，java代码一大堆，愣是没有看到几个C#，就算有也是简易，尽管可以把java的改变为C#的，但有点感慨C#的技术栈和资源少