Question

如果您需要并行运行多个异步I / O任务，但需要确保同时运行的I / O进程不多，那该怎么办？ I / O处理前后的任务不应该有这样的限制。

这是一个场景 - 让我们说有1000个任务;每个人都接受一个文本字符串作为输入参数;转换该文本（预I / O处理）然后将转换后的文本写入文件。目标是使预处理逻辑利用100％的CPU /内核和任务的I / O部分以最大10度并行性运行（同时打开最多10个用于一次写入文件）。

您能提供一些示例代码，了解如何使用C＃/ .NET 4.5进行操作吗？

http://blogs.msdn.com/b/csharpfaq/archive/2012/01/23/using-async-for-file-access-alan-berman.aspx

Answer 1

我认为使用TPL Dataflow是一个好主意：您创建具有无限并行性的前后处理块，具有有限并行性的文件写入块并将它们链接在一起。类似的东西：

var unboundedParallelismOptions =
    new ExecutionDataflowBlockOptions
    {
        MaxDegreeOfParallelism = DataflowBlockOptions.Unbounded
    };

var preProcessBlock = new TransformBlock<string, string>(
    s => PreProcess(s), unboundedParallelismOptions);

var writeToFileBlock = new TransformBlock<string, string>(
    async s =>
            {
                await WriteToFile(s);
                return s;
            },
    new ExecutionDataflowBlockOptions { MaxDegreeOfParallelism = 10 });

var postProcessBlock = new ActionBlock<string>(
    s => PostProcess(s), unboundedParallelismOptions);

var propagateCompletionOptions =
    new DataflowLinkOptions { PropagateCompletion = true };

preProcessBlock.LinkTo(writeToFileBlock, propagateCompletionOptions);
writeToFileBlock.LinkTo(postProcessBlock, propagateCompletionOptions);

// use something like await preProcessBlock.SendAsync("text") here

preProcessBlock.Complete();
await postProcessBlock.Completion;

WriteToFile()看起来像这样：

private static async Task WriteToFile(string s)
{
    using (var writer = new StreamWriter(GetFileName()))
        await writer.WriteAsync(s);
}

Answer 2

听起来你似乎想要考虑使用Djikstra信号量来控制对任务开始的访问。

然而，这听起来像一个典型的排队/固定数量的消费者问题，这可能是一种更合适的结构方式。

Answer 3

我会创建一个扩展方法，其中可以设置最大并行度。 SemaphoreSlim将成为这里的救世主。

    /// <summary>
    /// Concurrently Executes async actions for each item of <see cref="IEnumerable<typeparamref name="T"/>
    /// </summary>
    /// <typeparam name="T">Type of IEnumerable</typeparam>
    /// <param name="enumerable">instance of <see cref="IEnumerable<typeparamref name="T"/>"/></param>
    /// <param name="action">an async <see cref="Action" /> to execute</param>
    /// <param name="maxDegreeOfParallelism">Optional, An integer that represents the maximum degree of parallelism,
    /// Must be grater than 0</param>
    /// <returns>A Task representing an async operation</returns>
    /// <exception cref="ArgumentOutOfRangeException">If the maxActionsToRunInParallel is less than 1</exception>
    public static async Task ForEachAsyncConcurrent<T>(
        this IEnumerable<T> enumerable,
        Func<T, Task> action,
        int? maxDegreeOfParallelism = null)
    {
        if (maxDegreeOfParallelism.HasValue)
        {
            using (var semaphoreSlim = new SemaphoreSlim(
                maxDegreeOfParallelism.Value, maxDegreeOfParallelism.Value))
            {
                var tasksWithThrottler = new List<Task>();

                foreach (var item in enumerable)
                {
                    // Increment the number of currently running tasks and wait if they are more than limit.
                    await semaphoreSlim.WaitAsync();

                    tasksWithThrottler.Add(Task.Run(async () =>
                    {
                        await action(item).ContinueWith(res =>
                        {
                            // action is completed, so decrement the number of currently running tasks
                            semaphoreSlim.Release();
                        });
                    }));
                }

                // Wait for all tasks to complete.
                await Task.WhenAll(tasksWithThrottler.ToArray());
            }
        }
        else
        {
            await Task.WhenAll(enumerable.Select(item => action(item)));
        }
    }

样本用法：

await enumerable.ForEachAsyncConcurrent(
    async item =>
    {
        await SomeAsyncMethod(item);
    },
    5);

如何正确地并行运行多个异步任务？

3 个答案: