使用 async/await 会创建一个新线程吗？

Question

I am new to TPL and I am wondering: How does the asynchronous programming support that is new to C# 5.0 (via the new async and await keywords) relate to the creation of threads?

Specifically, does the use of async/await create a new thread each time that they are used? And if there many nested methods that use async/await , is a new thread created for each of those methods?

Answer 1

In short NO

From Asynchronous Programming with Async and Await : Threads

The async and await keywords don't cause additional threads to be created. Async methods don't require multithreading because an async method doesn't run on its own thread. The method runs on the current synchronization context and uses time on the thread only when the method is active. You can use Task.Run to move CPU-bound work to a background thread, but a background thread doesn't help with a process that's just waiting for results to become available.

Answer 2

According to MSDN : async keyword

An async method runs synchronously until it reaches its first await expression, at which point the method is suspended until the awaited task is complete. In the meantime, control returns to the caller of the method, as the example in the next section shows.

Here is a sample code to check it :

class Program

{
    static void Main(string[] args)
    {
        Program p = new Program();
        p.Run();
    }

    private void Print(string txt)
    {
        string dateStr = DateTime.Now.ToString("HH:mm:ss.fff");
        Console.WriteLine($"{dateStr} Thread #{Thread.CurrentThread.ManagedThreadId}\t{txt}");
    }

    private void Run()
    {
        Print("Program Start");
        Experiment().Wait();
        Print("Program End. Press any key to quit");
        Console.Read();
    }

    private async Task Experiment()
    {
        Print("Experiment code is synchronous before await");
        await Task.Delay(500);
        Print("Experiment code is asynchronous after first await");
    }
}

And the result :

We see the code of Experiment() method after await executes on another Thread .

But if I replace the Task.Delay by my own code (method SomethingElse) :

   class Program
{
    static void Main(string[] args)
    {
        Program p = new Program();
        p.Run();
    }

    private void Print(string txt)
    {
        string dateStr = DateTime.Now.ToString("HH:mm:ss.fff");
        Console.WriteLine($"{dateStr} Thread #{Thread.CurrentThread.ManagedThreadId}\t{txt}");
    }

    private void Run()
    {
        Print("Program Start");
        Experiment().Wait();
        Print("Program End. Press any key to quit");
        Console.Read();
    }

    private async Task Experiment()
    {
        Print("Experiment code is synchronous before await");
        await SomethingElse();
        Print("Experiment code is asynchronous after first await");
    }

    private Task SomethingElse()
    {
        Print("Experiment code is asynchronous after first await");
        Thread.Sleep(500);
        return (Task.CompletedTask);
    }
}

I notice the thread remains the same !

In conclusion, I'll say async/await code could use another thread, but only if the thread is created by another code, not by async/await.

In this case, I think Task.Delay created the thread, so I can conclude async/await does not create a new Thread like said by @Adriaan Stander.

Answer 3

Sorry for being late to the party.

I am new to TPL and I am wondering: How does the asynchronous programming support that is new to C# 5.0 (via the new async and await keywords) relate to the creation of threads?

async/await is not introduced for thread creation, but to utilize the current thread optimally.

Your app might read files, wait for response from another server or even do a computation with high memory access (Simply any IO task). These tasks are not CPU intensive (Any task that will not use 100% of your thread).

Think about the case when you are processing 1000 non CPU intensive tasks . In this case, process of creating 1000s of OS level thread might eat up more CPU and Memory than doing actual work on a single thread (4mb per thread in Windows, 4MB * 1000 = 4GB ). At the same time if you run all the tasks sequentially, you might have to wait until the IO tasks gets finished. Which end up in long time to complete the task, while keeping the CPU idle.

Since we require parallelism to complete multiple tasks quickly, at the same time all parallel tasks are not CPU hungry, but creating threads is inefficient.

The compiler will break the execution at any method call to an async method (which gets called with an await) and immediately execute the code outside of the current code branch, once an await is reached, the execution will go inside the previous async . This will be repeated again and again until all the async calls are completed and their awaiters are satisfied.

If any of the async method have heavy CPU load without a call to an async method, then yes, your system will become unresponsive and all the remaining async methods will not get called until the current task is finished.

Answer 4

So I've been reading up on the threading model, and Async / Await can certainly lead to new threads being used (not necessarily created - the pool creates them at application start). It's up to the scheduler to determine if a new thread is needed. And as I see it, a call to an awaitable function may have internal details that increase the chances of the scheduler utilizing another thread; simply because more work means more opportunities / reasons for the scheduler to divvy out work.

WinRT async operations automatically happen on the thread pool. And typically you will be calling FROM the thread pool, except for UI thread work .. Xaml/Input/Events.

Async operations started on Xaml/UI threads have their results delivered back to the [calling] UI thread. But asynchronous operation results started from a thread pool thread are delivered wherever the completion happens, which may not be the same thread you were on before. The reason behind this is that code written for the thread pool is likely to be written to be thread safe and it is also for efficiency, Windows doesn't have to negotiate that thread switch.

So again, in answer to the OP, new threads are not necessarily created but your application can and will use multiple threads to complete asynchronous work.

I know this seems to contradict some of the literature regarding async / await, but that's because although the async / await construct is not by itself multithreaded. Awaitables are the, or one of the mechanisms by which the scheduler can divide work and construct calls across threads.

This is at the limit of my knowledge right now regarding async and threading, so I might not have it exactly right, but I do think it's important to see the relationship between awaitables and threading.

Answer 5

What really helped me get my head around this.

Think of await keyword as a return from your method

And everything after it - is a "callback".

That "callback" is executed later. Probably even on the same thread in most cases. Can be a different thread in some cases, but it's still ONE THREAD .

PS If your async method has many nested async methods deeper down the chain - well, it's the same. It's just many "callers" and "callbacks" all the way down. But they all execute on ONE thread. And somewhere deep inside those nested calls, at the end of the chain, there's a truly async-API (i/o writing to disk, or a delay, or a network request or something else "truly" async in nature) that will do the async work via i/o drivers and system interrupts, and call your callbacks once finished.

PPS If the async method you're calling explicitly calls Task.Run or Task.Yield then it's a different story.

Answer 6

Using Async/Await doesn't necessarily cause a new thread to be created. But the use of Async/Await can lead to a new thread to be created because the awaitable function may internally spawn a new thread. And it often does, making the statement 'No, it doesn't spawn threads' almost useless in practice. For example, the following code spawns new threads.

VisualProcessor.Ctor()
{
    ...
    BuildAsync();
}

async void BuildAsync()
{
    ...
    TextureArray dudeTextures = await TextureArray.FromFilesAsync(…);
}

public static async Task<TextureArray> FromFilesAsync(...)
{    
    Debug.WriteLine("TextureArray.FromFilesAsync() T1 : Thread Id = " + GetCurrentThreadId());
    List<StorageFile> files = new List<StorageFile>();
    foreach (string path in paths)
    {
        if (path != null)
            files.Add(await Package.Current.InstalledLocation.GetFileAsync(path)); // << new threads
        else
            files.Add(null);
    }
    Debug.WriteLine("TextureArray.FromFilesAsync() T2 : Thread Id = " + GetCurrentThreadId());
    ...
}

Answer 7

In case of Java Spring Framework, a method annotated with @Async runs in a separate thread. Quoting from official guide ( https://spring.io/guides/gs/async-method ) -

The findUser method is flagged with Spring's @Async annotation, indicating that it should run on a separate thread. The method's return type is CompletableFuture instead of User, a requirement for any asynchronous service.

Of course in the backend it uses a Thread Pool and a Queue (where async tasks wait for a thread to be back in the pool).