Task.Factory.StartNew vs Task.Factory.FromAsync

Let's suppose we have a I/O bound method (such as a method making DB calls). This method can be run both in synchronously and asynchronously. That is,

1. Sync:

   IOMethod()
   

2. Async:

   BeginIOMethod()
   EndIOMethod()
   

Then when we execute the method in different ways as shown below, what's the performance difference in terms of the resource utilization?

1. var task = Task.Factory.StartNew(() => { IOMethod(); });
   task.Wait();
   

2. var task = Task.Factory.FromAsync(BeginIOMethod, EndIOMethod, ... );
   task.Wait();
   

Solution 1:

var task = Task.Factory.StartNew(() => { IOMethod(); });
task.Wait();

This will block a thread pool thread while IOMethod() is executing and also block your current thread because of the Wait(). Total blocked threads: 2.

var task = Task.Factory.FromAsync(BeginIOMethod, EndIOMethod, ... );
task.Wait();

This will (most likely) perform the operation asynchronously without using a thread, but it will block the current thread because of the Wait(). Total blocked threads: 1.

IOMethod();

This will block the current thread while IOMethod() is executing. Total blocked threads: 1.

If you need to block the current thread, or if blocking it is okay for you, then you should use this, because trying to use TPL won't actually give you anything.

var task = Task.Factory.FromAsync(BeginIOMethod, EndIOMethod, ... );
await task;

This will perform the operation asynchronously without using a thread, and it will also wait for the operation to complete asynchronously, thanks to await. Total blocked threads: 0.

This is what you should use if you want to take advantage of asynchrony and you can use C# 5.0.

var task = Task.Factory.FromAsync(BeginIOMethod, EndIOMethod, ... );
task.ContinueWith(() => /* rest of the method here */);

This will perform the operation asynchronously without using a thread, and it will also wait for the operation to complete asynchronously, thanks to ContinueWith(). Total blocked threads: 0.

This is what you should use if you want to take advantage of asynchrony and you can't use C# 5.0.

Solution 2:

(1) will (likely) cause the .NET thread pool to process your Task.

(2) will use whatever mechanism your BeginIOMethod / EndIOMethod pair natively uses to handle the asynchronous part, which may or may not involve the .NET thread pool.

For example, if your BeginIOMethod is sending a TCP message across the internet, and at a later time the recipient is going to send you a TCP message in response (received by EndIOMethod), then the asynchronous nature of the operation is not being provided by the .NET thread pool. The TCP library being used is providing the asynchronous part.

This can be accomplished by using the TaskCompletionSource class. Task.Factory.FromAsync can create a TaskCompletionSource<T>, return its Task<T>, then use EndIOMethod as a trigger to place the Result into the Task<T> that was returned form Task.Factory.FromAsync at the time of calling.

What's the performance difference in terms of the resource utilization?

The difference between (1) and (2) is primarily just whether the .NET thread pool is going to have its workload added to or not. In general, the correct thing to do is to choose Task.Factory.FromAsync if you only have a Begin... / End... pair and Task.Factory.StartNew otherwise.

If you're using C# 5.0, then you should be using the non-blocking await task; instead of task.Wait();. (See svick's answer.)