使用操作

Question

我正在使用Task类进行多线程计算。

例如：

   List<Task> taskList = new List<Task>();
   for(int i=0;i<10;i++){
        var task = new Task(() => Console.WriteLine("Hello from taskA."));
        taskList.Add(task);
        task.Start();
    }

有没有办法让只有3个任务最多运行，其余的要等待？

Answer 1

My blog post显示了如何使用“任务”和“操作”执行此操作，并提供了一个示例项目，您可以下载并运行该项目以查看两者的实际操作。

使用操作

如果使用Actions，则可以使用内置的.Net Parallel.Invoke函数。在这里，我们将其限制为最多并行运行3个线程。

var listOfActions = new List<Action>();
for (int i = 0; i < 10; i++)
{
    // Note that we create the Action here, but do not start it.
    listOfActions.Add(() => DoSomething());
}

var options = new ParallelOptions {MaxDegreeOfParallelism = 3};
Parallel.Invoke(options, listOfActions.ToArray());

使用任务

由于您在此处使用任务，因此没有内置功能。但是，您可以使用我在博客上提供的那个。

    /// <summary>
    /// Starts the given tasks and waits for them to complete. This will run, at most, the specified number of tasks in parallel.
    /// <para>NOTE: If one of the given tasks has already been started, an exception will be thrown.</para>
    /// </summary>
    /// <param name="tasksToRun">The tasks to run.</param>
    /// <param name="maxTasksToRunInParallel">The maximum number of tasks to run in parallel.</param>
    /// <param name="cancellationToken">The cancellation token.</param>
    public static void StartAndWaitAllThrottled(IEnumerable<Task> tasksToRun, int maxTasksToRunInParallel, CancellationToken cancellationToken = new CancellationToken())
    {
        StartAndWaitAllThrottled(tasksToRun, maxTasksToRunInParallel, -1, cancellationToken);
    }

    /// <summary>
    /// Starts the given tasks and waits for them to complete. This will run, at most, the specified number of tasks in parallel.
    /// <para>NOTE: If one of the given tasks has already been started, an exception will be thrown.</para>
    /// </summary>
    /// <param name="tasksToRun">The tasks to run.</param>
    /// <param name="maxTasksToRunInParallel">The maximum number of tasks to run in parallel.</param>
    /// <param name="timeoutInMilliseconds">The maximum milliseconds we should allow the max tasks to run in parallel before allowing another task to start. Specify -1 to wait indefinitely.</param>
    /// <param name="cancellationToken">The cancellation token.</param>
    public static void StartAndWaitAllThrottled(IEnumerable<Task> tasksToRun, int maxTasksToRunInParallel, int timeoutInMilliseconds, CancellationToken cancellationToken = new CancellationToken())
    {
        // Convert to a list of tasks so that we don&#39;t enumerate over it multiple times needlessly.
        var tasks = tasksToRun.ToList();

        using (var throttler = new SemaphoreSlim(maxTasksToRunInParallel))
        {
            var postTaskTasks = new List<Task>();

            // Have each task notify the throttler when it completes so that it decrements the number of tasks currently running.
            tasks.ForEach(t => postTaskTasks.Add(t.ContinueWith(tsk => throttler.Release())));

            // Start running each task.
            foreach (var task in tasks)
            {
                // Increment the number of tasks currently running and wait if too many are running.
                throttler.Wait(timeoutInMilliseconds, cancellationToken);

                cancellationToken.ThrowIfCancellationRequested();
                task.Start();
            }

            // Wait for all of the provided tasks to complete.
            // We wait on the list of "post" tasks instead of the original tasks, otherwise there is a potential race condition where the throttler&#39;s using block is exited before some Tasks have had their "post" action completed, which references the throttler, resulting in an exception due to accessing a disposed object.
            Task.WaitAll(postTaskTasks.ToArray(), cancellationToken);
        }
    }

然后创建任务列表并调用函数让它们运行，一次最多同时执行3次，你可以这样做：

var listOfTasks = new List<Task>();
for (int i = 0; i < 10; i++)
{
    var count = i;
    // Note that we create the Task here, but do not start it.
    listOfTasks.Add(new Task(() => Something()));
}
Tasks.StartAndWaitAllThrottled(listOfTasks, 3);

Answer 2

更改MaxDegreeOfParallelism属性。

example

Answer 3

我建议您查看帖子here。

要解释一下，您创建了相关任务的集合，然后使用ParallelOptions.MaxDegreeOfParallelism来控制同时执行的执行次数。

Answer 4

您可以将此作为通用

使用

public TaskThrottle(int initialCount, int maxTasksToRunInParallel)
{
    _semaphore = new SemaphoreSlim(initialCount, maxTasksToRunInParallel);
}

public void TaskThrottler<T>(IEnumerable<Task<T>> tasks, int timeoutInMilliseconds, CancellationToken cancellationToken = default(CancellationToken)) where T : class
{
        // Get Tasks as List
        var taskList = tasks as IList<Task<T>> ?? tasks.ToList();
        var semaphoreTasks = new List<Task<int>>();

        // When the first task completed, flag as done/release
        taskList.ForEach(x =>
        {
            semaphoreTasks.Add(x.ContinueWith(y => _semaphore.Release(), cancellationToken));
        });

        semaphoreTasks.ForEach(async x =>
        {
            // It will not pass this until one free slot available or timeout occure  
            if(timeoutInMilliseconds > 0)
                await _semaphore.WaitAsync(timeoutInMilliseconds, cancellationToken);
            else
                await _semaphore.WaitAsync(cancellationToken);

            // Throws a OperationCanceledException if this token has had cancellation requested
            cancellationToken.ThrowIfCancellationRequested();

            // Start the task 
            x.Start();
        });

        Task.WaitAll(semaphoreTasks.ToArray(), cancellationToken);
}

我可以限制同时运行的System.Threading.Tasks.Task对象的数量吗？

4 个答案:

使用操作

使用任务