Does the use of async/await create a new thread?

问题:

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?

回答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.

回答2:

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.

回答3:

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());
    ...
}