Continuation on multiple concurrent threads in async method after async IO API call

Question

If I have this

static void Main(string[] args)
{
    var tasks = new List<Task>();
    for (int i = 0; i < 10; i++)
    {
        tasks.Add(AsyncWithBlockingIO(i));
    }

    Task.WaitAll(tasks.ToArray());
}

private static async Task AsyncWithBlockingIO(int fileNum)
{
    var result = await File.ReadAllTextAsync($"File{fileNum}").ConfigureAwait(false);
    //will the below run concurrently on multiple threads?
    CpuNoIOWork(result);
}

Will CpuNoIOWork() run concurrently on multiple threads (using the thread pool) as the IO calls complete or will it only use 1 thread at a time?

You can add Debug.WriteLine(Thread.CurrentThread.ManagedThreadId); to CpuNoIOWork() to prove that the continuations are indeed all run concurrently. — Matthew Watson
– Matthew Watson, Commented Apr 15, 2021 at 13:54
You may want to take a look at this: Why File.ReadAllLinesAsync() blocks the UI thread? Not all Async-advertised methods are genuinely asynchronous. — Theodor Zoulias
– Theodor Zoulias, Commented Apr 15, 2021 at 16:47
@TheodorZoulias, thanks! not using the file APIs in the real code, but useful to know. — axk
– axk, Commented Apr 15, 2021 at 17:43

Stephen Cleary · Accepted Answer · 2021-04-15 13:53:11Z

7

Will CpuNoIOWork() run concurrently on multiple threads (using the thread pool) as the IO calls complete or will it only use 1 thread at a time?

It will run concurrently, on thread pool threads.

However, depending on the internals of what you're awaiting, it might continue on an I/O thread pool thread, which is not desirable. If you have CPU-bound work, I recommend wrapping that in Task.Run, which will ensure the work will be run on a worker thread pool thread:

var result = await File.ReadAllTextAsync($"File{fileNum}").ConfigureAwait(false);
await Task.Run(() => CpuNoIOWork(result));

answered Apr 15, 2021 at 13:53

Stephen Cleary

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5 Comments

JohanP Over a year ago

Instead of await Task.Run in this specific case, would await Task.Yield() force the continuation to run on a threadpool thread?

Stephen Cleary Over a year ago

@JohanP: Yes, but I prefer Task.Run so that the code is more explicit.

Theodor Zoulias Over a year ago

@JohanP the Task.Yield is a poor man's substitute of the non-existing API TaskScheduler.SwitchTo. The behavior of the Task.Yield depends on the ambient SynchronizationContext.Current (or the ambient TaskScheduler.Current if the context is null), so it's not a reliable (platform agnostic) way to switch to the ThreadPool, if that's your intention.

JohanP Over a year ago

@TheodorZoulias Would you say in .net core that it is a reliable way to get to threadpool thread in this specific instance?

Theodor Zoulias Over a year ago

@JohanP no, I wouldn't. It's not the right tool for the job IMHO. If I really wanted to avoid the Task.Run for some reason, I would use a custom awaiter like noseratio's TaskSchedulerAwaiter, or a simplified version of it.

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