为什么这么慢，.net任务嵌套子任务

Question

I create an asp.net webapi test application, here is my test code:

When I use the result = index.ToString() + this.getresult().Result; , the response time is 2s. If I use Thread.Sleep(100); result = index.ToString(); Thread.Sleep(100); result = index.ToString(); , it only needs 200ms.

public class HomeController : Controller
{
    public string Test()
    {
        var listName = new List<int>();

        for (int i = 0; i < 100; i++)
        {
            listName.Add(i);
        }

        var response = Task.WhenAll(listName.Select(sendrequest)).Result;

        return string.Join(",", response);
    }

    public async Task<string> sendrequest(int index)
    {
        return await Task.Factory.StartNew(() =>
        {
            string result = string.Empty;

            try
            {
                Thread.Sleep(100); result = index.ToString();
                //result = index.ToString() + this.getresult().Result;
            }
            catch (Exception ex)
            {
                System.IO.File.AppendAllText("D:\\WebService\\FelixTest\\log.txt",ex.ToString());
            }

            return result;

        }).ConfigureAwait(false);
    }

    public async Task<string> getresult()
    {
        await Task.Delay(100);

        return "OK";
    }
}

Answer 1

I think you are suffering from ThreadPool thread starvation. The problem lies in the line you have commented-out

//result = index.ToString() + this.getresult().Result;

By accessing the Task.Result property, you are causing a Wait on the Task returned from the getresult() method. The Wait operation blocks the current thread from executing, which means that the next Task must be scheduled on another thread and so on. As you are starting 100 tasks all of which will block, the ThreadPool will attempt to start new managed threads to satisfy the demand, and this is a time-consuming process.

I would reorganise your code so that everything is asynchronous except for the one top-level Test method. In this example I'm not sure why you need Task.Factory.StartNew() as the tasks are already run (at least partially) in parallel by virtue of the fact that they use the async-await pattern. Perhaps your real-life example requires the tasks to be scheduled on a background thread? If you don't need to use Factory.StartNew() I'd suggest rewriting sendrequest like this:-

public async Task<string> sendrequest(int index)
{
    string result = string.Empty;
    try
    {
        result = index.ToString() + await this.getresult();
    }
    catch (Exception ex)
    {
        System.IO.File.AppendAllText("D:\\WebService\\FelixTest\\log.txt",ex.ToString());
    }

    return result;
}

If you do need to use Factory.StartNew() you can still achieve the same thing, but the delegate you pass to StartNew should be marked async .

Answer 2

According to your description the difference is of 1800 milliseconds. You have a delay of 100 milliseconds in getresult . If we subtract that, the difference is of 1700 milliseconds. A string concatenation with "OK" does not explain that. As a result, you probably use a loop and the delays of 100 milliseconds in getresult add up to a significant wait.