async await performance?

Question

(Just a theoretical question - for non-gui apps)

Assuming I have this code with many awaits:

public async Task<T> ConsumeAsync()
    {
          await A();
          await b();
          await c();
          await d();
          //..
    }

Where each task can take a very short period of time ,

Question (again , theoretical)

There could be a situation where the overall time dealing with all those "releasing back threads" and "fetching threads back" ( red & green here :)

Is taking more time than a single thread which could done all the work with a small amount of delay ,

I mean , I wanted to be the most productive , but instead , since all those switches back and forth - I actually lost productivity.

Can such scenario occur ?

Answer 1

A Task object represent the deferred result of a pending operation. You don't have to use tasks and async/await if you don't have any pending operations. Otherwise, I believe async/await code is generally more efficient than its bare TPL ContinueWith analogue.

Let's do some timing:

using System;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApplication
{
    class Program
    {
        // async/await version
        static async Task<int> Test1Async(Task<int> task)
        {
            return await task;
        }

        // TPL version
        static Task<int> Test2Async(Task<int> task)
        {
            return task.ContinueWith(
                t => t.Result,
                CancellationToken.None,
                TaskContinuationOptions.ExecuteSynchronously,
                TaskScheduler.Default);
        }

        static void Tester(string name, Func<Task<int>, Task<int>> func)
        {
            var sw = new System.Diagnostics.Stopwatch();
            sw.Start();
            for (int i = 0; i < 10000000; i++)
            {
                func(Task.FromResult(0)).Wait();
            }
            sw.Stop();
            Console.WriteLine("{0}: {1}ms", name, sw.ElapsedMilliseconds);
        }

        static void Main(string[] args)
        {
            Tester("Test1Async", Test1Async);
            Tester("Test2Async", Test2Async);
        }
    }
}

The output:

Test1Async: 1582ms
Test2Async: 4975ms

So, by default, await continuations are handled more efficiently than ContinueWith continuations. Let's optimize this code slightly:

// async/await version
static async Task<int> Test1Async(Task<int> task)
{
    if (task.IsCompleted)
        return task.Result;
    return await task;
}

// TPL version
static Task<int> Test2Async(Task<int> task)
{
    if (task.IsCompleted)
        return Task.FromResult(task.Result);

    return task.ContinueWith(
        t => t.Result,
        CancellationToken.None,
        TaskContinuationOptions.ExecuteSynchronously,
        TaskScheduler.Default);
}

The output:

Test1Async: 1557ms
Test2Async: 429ms

Now the non-async version wins. In case with the async version, I believe this optimization has already been done internally by the async/await infrastructure.

Anyway, so far we've dealt only with completed tasks (Task.FromResult). Let's introduce the actual asynchrony (naturally, we'll do less iterations this time):

static Task<int> DoAsync()
{
    var tcs = new TaskCompletionSource<int>();
    ThreadPool.QueueUserWorkItem(_ => tcs.SetResult(0));
    return tcs.Task;
}

static void Tester(string name, Func<Task<int>, Task<int>> func)
{
    ThreadPool.SetMinThreads(200, 200);
    var sw = new System.Diagnostics.Stopwatch();
    sw.Start();
    for (int i = 0; i < 1000000; i++)
    {
        func(DoAsync()).Wait();
    }
    sw.Stop();
    Console.WriteLine("{0}: {1}ms", name, sw.ElapsedMilliseconds);
}

The output:

Test1Async: 4207ms
Test2Async: 4734ms

Now the difference is very marginal, although the async version still performs slightly better. Yet I think such gain is really neglectable, comparable to the actual cost of the asynchronous operation or to the cost of restoring the captured context for when SynchronizationContext.Current != null.

The bottom line is, if you deal with asynchronous tasks, go for async/await if you have a choice, not for performance reason but for ease of use, readability and maintainability.