Here's an excellent article I would recommend you reading to better understand asynchronous processing in ASP.NET (which is what asynchronous controllers basically represent).
Let's first consider a standard synchronous action:
public ActionResult Index()
{
// some processing
return View();
}
When a request is made to this action a thread is drawn from the thread pool and the body of this action is executed on this thread. So if the processing inside this action is slow you are blocking this thread for the entire processing, so this thread cannot be reused to process other requests. At the end of the request execution, the thread is returned to the thread pool.
Now let's take an example of the asynchronous pattern:
public void IndexAsync()
{
// perform some processing
}
public ActionResult IndexCompleted(object result)
{
return View();
}
When a request is sent to the Index action, a thread is drawn from the thread pool and the body of the IndexAsync method is executed. Once the body of this method finishes executing, the thread is returned to the thread pool. Then, using the standard AsyncManager.OutstandingOperations, once you signal the completion of the async operation, another thread is drawn from the thread pool and the body of the IndexCompleted action is executed on it and the result rendered to the client.
So what we can see in this pattern is that a single client HTTP request could be executed by two different threads.
Now the interesting part happens inside the IndexAsync method. If you have a blocking operation inside it, you are totally wasting the whole purpose of the asynchronous controllers because you are blocking the worker thread (remember that the body of this action is executed on a thread drawn from the thread pool).
So when can we take real advantage of asynchronous controllers you might ask?
IMHO we can gain most when we have I/O intensive operations (such as database and network calls to remote services). If you have a CPU intensive operation, asynchronous actions won't bring you much benefit.
So why can we gain benefit from I/O intensive operations? Because we could use I/O Completion Ports. IOCP are extremely powerful because you do not consume any threads or resources on the server during the execution of the entire operation.
How do they work?
Suppose that we want to download the contents of a remote web page using the WebClient.DownloadStringAsync method. You call this method which will register an IOCP within the operating system and return immediately. During the processing of the entire request, no threads are consumed on your server. Everything happens on the remote server. This could take lots of time but you don't care as you are not jeopardizing your worker threads. Once a response is received the IOCP is signaled, a thread is drawn from the thread pool and the callback is executed on this thread. But as you can see, during the entire process, we have not monopolized any threads.
The same stands true with methods such as FileStream.BeginRead, SqlCommand.BeginExecute, ...
What about parallelizing multiple database calls? Suppose that you had a synchronous controller action in which you performed 4 blocking database calls in sequence. It's easy to calculate that if each database call takes 200ms, your controller action will take roughly 800ms to execute.
If you don't need to run those calls sequentially, would parallelizing them improve performance?
That's the big question, which is not easy to answer. Maybe yes, maybe no. It will entirely depend on how you implement those database calls. If you use async controllers and I/O Completion Ports as discussed previously you will boost the performance of this controller action and of other actions as well, as you won't be monopolizing worker threads.
On the other hand if you implement them poorly (with a blocking database call performed on a thread from the thread pool), you will basically lower the total time of execution of this action to roughly 200ms but you would have consumed 4 worker threads so you might have degraded the performance of other requests which might become starving because of missing threads in the pool to process them.
So it is very difficult and if you don't feel ready to perform extensive tests on your application, do not implement asynchronous controllers, as chances are that you will do more damage than benefit. Implement them only if you have a reason to do so: for example you have identified that standard synchronous controller actions are a bottleneck to your application (after performing extensive load tests and measurements of course).
Now let's consider your example:
public ViewResult Index() {
Task.Factory.StartNew(() => {
//Do an advanced looging here which takes a while
});
return View();
}
When a request is received for the Index action a thread is drawn from the thread pool to execute its body, but its body only schedules a new task using TPL. So the action execution ends and the thread is returned to the thread pool. Except that, TPL uses threads from the thread pool to perform their processing. So even if the original thread was returned to the thread pool, you have drawn another thread from this pool to execute the body of the task. So you have jeopardized 2 threads from your precious pool.
Now let's consider the following:
public ViewResult Index() {
new Thread(() => {
//Do an advanced looging here which takes a while
}).Start();
return View();
}
In this case we are manually spawning a thread. In this case the execution of the body of the Index action might take slightly longer (because spawning a new thread is more expensive than drawing one from an existing pool). But the execution of the advanced logging operation will be done on a thread which is not part of the pool. So we are not jeopardizing threads from the pool which remain free for serving another requests.
