Simple WebAPI app with Console client that shows that async methods do release threads to the thread pool.

Async WebAPI

Simple WebAPI app with Console client that shows that async methods do release threads to the thread pool.

Example Output

Synchronous time for 8 connections: 00:00:01.0194025
Synchronous time for 9 connections: 00:00:02.0362007
Asynchronous time for 8 connections: 00:00:01.0413737
Asynchronous time for 9 connections: 00:00:01.0238674

Global.asax.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Web;
using System.Web.Http;
using System.Web.Mvc;
using System.Web.Optimization;
using System.Web.Routing;

namespace MvcApplication2
{
    // Note: For instructions on enabling IIS6 or IIS7 classic mode, 
    // visit http://go.microsoft.com/?LinkId=9394801

    public class WebApiApplication : System.Web.HttpApplication
    {
        protected void Application_Start()
        {
            // Begin Changes
            int workerThreads, ioThreads;
            ThreadPool.GetMaxThreads(out workerThreads, out ioThreads);
            ThreadPool.SetMaxThreads(Environment.ProcessorCount, ioThreads);
            // End Changes

            AreaRegistration.RegisterAllAreas();

            WebApiConfig.Register(GlobalConfiguration.Configuration);
            FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
            RouteConfig.RegisterRoutes(RouteTable.Routes);
            BundleConfig.RegisterBundles(BundleTable.Bundles);
        }
    }
}

Program.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Text;
using System.Threading.Tasks;

class Program
{
    static void Main(string[] args)
    {
        try
        {
            MainAsync().Wait();
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex);
        }

        Console.ReadKey();
    }

    static async Task MainAsync()
    {
        ServicePointManager.DefaultConnectionLimit = int.MaxValue;

        int connections;
        var syncUrl = "http://localhost:35697/api/values/";
        var asyncUrl = "http://localhost:35697/api/values/13";

        connections = Environment.ProcessorCount;
        Console.WriteLine(" Synchronous time for " + connections + " connections: " +
            await RunTest(syncUrl, connections));

        connections = Environment.ProcessorCount + 1;
        Console.WriteLine(" Synchronous time for " + connections + " connections: " +
            await RunTest(syncUrl, connections));

        connections = Environment.ProcessorCount;
        Console.WriteLine("Asynchronous time for " + connections + " connections: " +
            await RunTest(asyncUrl, connections));

        connections = Environment.ProcessorCount + 1;
        Console.WriteLine("Asynchronous time for " + connections + " connections: " +
            await RunTest(asyncUrl, connections));
    }

    static async Task<TimeSpan> RunTest(string url, int concurrentConnections)
    {
        var sw = new Stopwatch();
        var client = new HttpClient();

        await client.GetStringAsync(url); // warmup
        sw.Start();
        await Task.WhenAll(Enumerable.Range(0, concurrentConnections).Select(i => client.GetStringAsync(url)));
        sw.Stop();

        return sw.Elapsed;
    }
}

ValuesController.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;
using System.Web.Http;

namespace MvcApplication2.Controllers
{
    public class ValuesController : ApiController
    {
        // Synchronous
        public IEnumerable<string> Get()
        {
            Thread.Sleep(1000);
            return new string[] { "value1", "value2" };
        }

        // Asynchronous
        public async Task<IEnumerable<string>> Get(int id)
        {
            await Task.Delay(1000);
            return new string[] { "value1", "value2" };
        }
    }
}