首先输入线程如何发出信号到其他并发线程相同方法的末尾？

Question

首先输入线程如何发出信号到其他并发线程相同方法的末尾？

我有名为say polldpram（）的方法。它必须通过网络进行旅行，以获取一些慢速硬件并刷新对象私人数据。如果其他线程同时调用了相同的方法，则不得进行旅行，而是等待第一个即将到来的线程完成工作并简单地退出，因为数据是新鲜的（例如10-30 ms，没有什么不同） 。它易于在方法中易于检测，第二，第三个等线程未首先输入。我使用互锁计数器来检测并发。

问题：我做出了一个不当的选择，可以通过观察计数器（Interlocked.Read）来检测第一个线程的出口，以观察计数器减少的价值降低的价值小于n> 1螺纹入口处检测到的。选择是不好的，因为第一个线程可以在离开后几乎立即重新进入该方法。因此，n> 1个线将永远不会在计数器中检测到浸入。

所以问题：即使第一个线程可以立即再次输入该方法，如何正确检测到第一个输入线程已退出该方法？

谢谢

PS代码

        private void pollMotorsData()
    {
        // Execute single poll with "foreground" handshaking 
        DateTime start = DateTime.Now;
        byte retryCount = 0;
        // Pick old data atomically to detect change
        uint motorsDataTimeStampPrev = this.MotorsDataTimeStamp;
        bool changeDetected = false;
        // The design goal of DPRAM is to ease the bottleneck
        // Here is a sensor if bottleneck is actually that tight
        long parallelThreads = Interlocked.Increment(ref this.motorsPollThreadCount);
        try
        {
            // For first thread entering the counter will be 1
            if (parallelThreads <= 1)
            {
                do
                {
                    // Handshake signal to DPRAM write process on controller side that host PC is reading
                    this.controller.deltaTauTcpClient.Pmac_SetBit(OFFSET_0x006A_BIT15_FOREGROUND_READ, 15, true);
                    try
                    {
                        bool canReadMotors = false;
                        byte[] canReadFrozenDataFlag = new byte[2];
                        do
                        {
                            this.controller.deltaTauTcpClient.Pmac_GetMem(OFFSET_0x006E_BIT15_FOREGROUND_DONE, canReadFrozenDataFlag);
                            canReadMotors = (canReadFrozenDataFlag[1] & 0x80) == 0x80;
                            if (canReadMotors) break;
                            retryCount++;
                            Thread.Sleep(1);
                        } while (retryCount < 10);
                        if (!canReadMotors)
                        {
                            throw new DeltaTauControllerException(this.controller, "Timeout waiting on DPRAM Foreground Handshaking Bit");
                        }
                        // The lock is meaningless in contructor as it is certainly single threaded
                        // but for practice sake the access to data should always be serialized
                        lock (motorsDataLock)
                        {
                            // Obtain fresh content of DPRAM
                            this.controller.deltaTauTcpClient.Pmac_GetMem(OFFSET_0x006A_394BYTES_8MOTORS_DATA, this.motorsData);
                            this.motorsDataBorn = DateTime.Now;
                        }
                    }
                    finally
                    {
                        // Handshake signal to DPRAM write process on controller side that host PC has finished reading
                        this.controller.deltaTauTcpClient.Pmac_SetBit(OFFSET_0x006A_BIT15_FOREGROUND_READ, 15, false);
                    }
                    // Check live change in a separate atom
                    changeDetected = this.MotorsDataTimeStamp != motorsDataTimeStampPrev;
                } while ((!changeDetected) && ((DateTime.Now - start).TotalMilliseconds < 255));
                // Assert that result is live
                if (!changeDetected)
                {
                    throw new DeltaTauControllerException(this.controller, "DPRAM Background Data timestamp is not updated. DPRAM forground handshaking failed.");
                }
            }
            else
            {
                // OK. Bottleneck ! The concurrent polls have collided 
                // Give the controller a breathe by waiting for other thread do the job
                // Avoid aggressive polling of stale data, which is not able to be written, locked by reader
                // Just wait for other thread do whole polling job and return with no action
                // because the data is milliseconds fresh
                do
                {
                    // Amount of parallel threads must eventually decrease
                    // But no thread will leave and decrease the counter until job is done
                    if (Interlocked.Read(ref this.motorsPollThreadCount) < parallelThreads)
                    {
                        // Return is possible because decreased value of concurrentThreads means that
                        // this very time other thread has finished the poll 1 millisecond ago at most
                        return;
                    }
                    Thread.Sleep(1);
                    retryCount++;
                } while ((DateTime.Now - start).TotalMilliseconds < 255);
                throw new DeltaTauControllerException(this.controller, "Timeout 255ms waiting on concurrent thread to complete DPRAM polling");
            }
        }
        finally
        {
            // Signal to other threads that work is done
            Interlocked.Decrement(ref this.motorsPollThreadCount);
            // Trace the timing and bottleneck situations
            TimeSpan duration = DateTime.Now - start;
            if (duration.TotalMilliseconds > 50 || parallelThreads > 1 || retryCount > 0)
            {
                Trace.WriteLine(string.Format("Controller {0}, DPRAM poll {1:0} ms, threads {2}, retries {3}",
                    this.controller.number,
                    duration.TotalMilliseconds,
                    parallelThreads,
                    retryCount));
            }
        }
    }

Answer 1

您可以做很多不同的方法。如某人已经提到的那样，您可以使用关键部分，但是如果另一个线程阻止，这不会给您“仅退出”的行为。为此，您需要某种标志。您可以使用挥发性的布尔，并锁定该布尔的访问权限，也可以使用单个计数的信号量。最后，您可以使用Mutex。使用同步对象的好处是您可以执行waitforsingleobject并将超时设置为0。然后您可以检查等待是否成功（如果是第一个线程已退出）（在这种情况下，第一个线程为仍在运行）。

Answer 2

同步该方法，内部方法检查了最后完成网络访问时间的记录，以确定是否需要再次完成。

Answer 3

您可以使用“锁”关键字支持的C＃监视类。

基本上，您的方法可以包裹在锁（lockobj）{callmethod（）}中

假设所有线程都处于相同的过程中，这将为您提供保护。

如果您需要跨进程锁定，则需要使用MUTEX。

至于您的程序，我将考虑将静态时间戳和缓存的价值放入您的方法中。因此，当方法进入时，如果时间戳在我的可接受范围内，请返回缓存值，否则只需执行获取即可。结合锁定机制，这应该做您需要的事情。

当然，这假定在C＃监视器上使用和阻止的时间不会影响您的应用程序的性能。

更新：我已经更新了您的代码，以向您展示我对使用缓存和时间戳的含义。我以为您的“ MotorsData”变量是从电动机投票中返回的东西，因此我没有变量。但是，如果我“被误解了，只需添加一个变量，该变量从代码返回后存储数据。请注意，我没有为您检查任何错误检查，因此您需要处理异常。

    static DateTime lastMotorPoll;
    const TimeSpan CACHE_PERIOD = new TimeSpan(0, 0, 0, 0, 250);
    private object cachedCheckMotorsDataLock = new object();

    private void CachedCheckMotorsData()
    {
        lock (cachedCheckMotorsDataLock)  //Could refactor this to perform a try enter which returns quickly if required
        {
            //If the last time the data was polled is older than the cache period, poll
            if (lastMotorPoll.Add(CACHE_PERIOD) < DateTime.Now)
            {
                pollMotorsData();
                lastMotorPoll = DateTime.Now;
            }
            else //Data is fresh so don't poll
            {
                return;
            }
        }       
    }

    private void pollMotorsData()
    {
        // Execute single poll with "foreground" handshaking 
        DateTime start = DateTime.Now;
        byte retryCount = 0;
        // Pick old data atomically to detect change
        uint motorsDataTimeStampPrev = this.MotorsDataTimeStamp;
        bool changeDetected = false;
        try
        {
            do
            {
                // Handshake signal to DPRAM write process on controller side that host PC is reading
                this.controller.deltaTauTcpClient.Pmac_SetBit(OFFSET_0x006A_BIT15_FOREGROUND_READ, 15, true);
                try
                {
                    bool canReadMotors = false;
                    byte[] canReadFrozenDataFlag = new byte[2];
                    do
                    {
                        this.controller.deltaTauTcpClient.Pmac_GetMem(OFFSET_0x006E_BIT15_FOREGROUND_DONE, canReadFrozenDataFlag);
                        canReadMotors = (canReadFrozenDataFlag[1] & 0x80) == 0x80;
                        if (canReadMotors) break;
                        retryCount++;
                        Thread.Sleep(1);
                    } while (retryCount < 10);
                    if (!canReadMotors)
                    {
                        throw new DeltaTauControllerException(this.controller, "Timeout waiting on DPRAM Foreground Handshaking Bit");
                    }
                    // Obtain fresh content of DPRAM
                    this.controller.deltaTauTcpClient.Pmac_GetMem(OFFSET_0x006A_394BYTES_8MOTORS_DATA, this.motorsData);
                    this.motorsDataBorn = DateTime.Now;
                }
                finally
                {
                    // Handshake signal to DPRAM write process on controller side that host PC has finished reading
                    this.controller.deltaTauTcpClient.Pmac_SetBit(OFFSET_0x006A_BIT15_FOREGROUND_READ, 15, false);
                }
                // Check live change in a separate atom
                changeDetected = this.MotorsDataTimeStamp != motorsDataTimeStampPrev;
            } while ((!changeDetected) && ((DateTime.Now - start).TotalMilliseconds < 255));

            // Assert that result is live
            if (!changeDetected)
            {
                throw new DeltaTauControllerException(this.controller, "DPRAM Background Data timestamp is not updated. DPRAM forground handshaking failed.");
            }
        }
    }