I'm trying to get a grasp on asynchronous programming in C#/.NET. I read an article (link) on Brown University's website for the cs168 course that defines asynchronous programming as interleaving tasks within the same thread. It says, "Now we can introduce the asynchronous model... In this model, the tasks are interleaved with one another, but in a single thread of control", and shows interleaving very clearly in a figure. But I can't seem to get two tasks to interleave within the same thread in .NET. Is there a way to do that?
I wrote some simple apps to try to test this theory, but I'm not sure if I'm doing it correctly. The main program outputs to the screen every so often, using Thread.Sleep() to simulate work. The asynchronous task does the same. If multiple threads are used, the output is interleaved. But I'm trying to test on a single thread.
I have a WPF app that runs everything on the UI thread, but the task and main program always output sequentially. I create and start the task like this:
var taskFactory = new TaskFactory(TaskScheduler.FromCurrentSynchronizationContext());
var task = taskFactory.StartNew(workDelegate);
I have a console app that starts the task delegate using Task.Run(workDelegate);, but that runs them on different thread pool threads. I'm not sure how to make them both run on the same thread. If I try the same approach I used in WPF I get a runtime InvalidOperationException, "The current SynchronizationContext may not be used as a TaskScheduler".
Multiple tasks won't automatically be interleaved on a single thread. To do that, you have to specify the points in task code where the thread is allowed to cut over to another task. You can do this via a mechanism like await Task.Yield. If you're running on a single thread, the thread will not be able to allow other work to progress unless it explicitly yields.
When you use your TaskScheduler to start every task, the message pump in WPF schedules each task to run on the UI thread, and they will run sequentially.
I have a console app that starts the task delegate using Task.Run(workDelegate);, but that runs them on different thread pool threads. I'm not sure how to make them both run on the same thread.
You would need to install a custom SynchronizationContext into a thread which allowed you to post work to that thread.
You cannot run two concurrent Tasks in the same thread-pool thread. Each thread-pool thread can run one Task at a time. If you want to do two things in one thread, your options what I see now:
1. Combine the two things into one Task
2. Create two tasks and one would depend on the other one. SO in the end they would run sequentially after each other. By default it's not guaranteed that they would run in the same thread though, but you should not rely on that anyway.
It's not clear to me what you want to do. According to the books the UI and the thread of your WPF should do any heavy lifting number crunching work. It should take care of the UI and organize the worker threads/tasks. You would start operations in the background using async.
Related
I know the differences between a thread and a task., but I cannot understand if creating threads inside tasks is the same as creating only threads.
It depends on how you use the multithreaded capabilities and the asynchronous programming semantics of the language.
Simple facts first. Assume you have an initial, simple, single-threaded, and near empty application (that just reads a line of input with Console.ReadLine for simplicity sake). If you create a new Thread, then you've created it from within another thread, the main thread. Therefore, creating a thread from within a thread is a perfectly valid operation, and the starting point of any multithreaded application.
Now, a Task is not a thread per se, but it gets executed in one when you do Task.Run which is selected from a .NET managed thread pool. As such, if you create a new thread from within a task, you're essentially creating a thread from within a thread (same as above, no harm done). The caveat here is, that you don't have control of the thread or its lifetime, that is, you can't kill it, suspend it, resume it, etc., because you don't have a handle to that thread. If you want some unit of work done, and you don't care which thread does it, just that's it not the current one, then Task.Run is basically the way to go. With that said, you can always start a new thread from within a task, actually, you can even start a task from within a task, and here is some official documentation on unwrapping nested tasks.
Also, you can await inside a task, and create a new thread inside an async method if you want. However, the usability pattern for async and await is that you use them for I/O bound operations, these are operations that require little CPU time but can take long because they need to wait for something, such as network requests, and disk access. For responsive UI implementations, this technique is often used to prevent blocking of the UI by another operation.
As for being pointless or not, it's a use case scenario. I've faced situations where that could have been the solution, but found that redesigning my program logic so that if I need to use a thread from within a task, then what I do is to have two tasks instead of one task plus the inner thread, gave me a cleaner, and more readable code structure, but that it's just personal flair.
As a final note, here are some links to official documentation and another post regarding multithreaded programming in C#:
Async in Depth
Task based asynchronous programming
Chaining Tasks using Continuation Tasks
Start multiple async Tasks and process them as they complete
Should one use Task.Run within another Task
It depends how you use tasks and what your reason is for wanting another thread.
Task.Run
If you use Task.Run, the work will "run on the ThreadPool". It will be done on a different thread than the one you call it from. This is useful in a desktop application where you have a long-running processor-intensive operation that you just need to get off the UI thread.
The difference is that you don't have a handle to the thread, so you can't control that thread in any way (suspend, resume, kill, reuse, etc.). Essentially, you use Task.Run when you don't care which thread the work happens on, as long as it's not the current one.
So if you use Task.Run to start a task, there's nothing stopping you from starting a new thread within, if you know why you're doing it. You could pass the thread handle between tasks if you specifically want to reuse it for a specific purpose.
Async methods
Methods that use async and await are used for operations that use very little processing time, but have I/O operations - operations that require waiting. For example, network requests, read/writing local storage, etc. Using async and await means that the thread is free to do other things while you wait for a response. The benefits depend on the type of application:
Desktop app: The UI thread will be free to respond to user input while you wait for a response. I'm sure you've seen some programs that totally freeze while waiting for a response from something. This is what asynchronous programming helps you avoid.
Web app: The current thread will be freed up to do any other work required. This can include serving other incoming requests. The result is that your application can handle a bigger load than it could if you didn't use async and await.
There is nothing stopping you from starting a thread inside an async method too. You might want to move some processor-intensive work to another thread. But in that case you could use Task.Run too. So it all depends on why you want another thread.
It would be pointless in most cases of everyday programming.
There are situations where you would create threads.
I would like to find out why the following needs to be blocked in order to get the console to write:
Task.Factory.StartNew(() => Console.WriteLine("KO"), TaskCreationOptions.LongRunning);
and this does not:
new Thread(() => Console.WriteLine("KO")).Start();
According to C# 5.0 in a nutshell, TaskCreationOptions.LongRunning is supposed to make the task NOT use pooled threads (which are background threads), meaning it should be using a foreground thread just like a regular thread, except with a regular thread, one does not need to Console.Readline or Wait() but with a Task, doesn't matter whether it's long running or not, I always have to block the main thread in some way.
So what good is LongRunning or OnComplete() or GetAwaiter() or GetResult() or any other function which is supposed to render a result If I always have to block the main thread myself to actually get the result?
You're relying on undefined behaviour. Don't do that.
You don't need to wait for a task to have it work - it's just the only way to be sure that it actually completed in some way. I assume you're just using a console application with nothing but the code above - by the time the thread actually gets to the Console.WriteLine part, the main thread is dead, and with it all the background threads. new Thread creates a foreground thread by default, which prevents the application as a whole from exiting, despite the fact that the "main" thread was terminated.
The idea behind tasks (and any kind of asynchronous operations, really) is that they allow you to make concurrent requests, and build chains of asynchronous operations (making them behave synchronously, which you usually want). But you still need points of synchronization to actually make a workable application - if your application exits before the tasks are done, too bad :)
You can see this if you just do a Console.ReadLine instead of waiting for the task to finish explicitly - it still runs in the background, independently of the main thread of execution, but now you give it enough time to complete. In most applications, you do asynchronous operations asynchronously to the main thread - for example, a result of a button click might be an asynchronous HTTP request that doesn't block the UI, but if the UI is closed, the request is still terminated.
Can a task still run if an application is in an APPHANG state? On the UI thread, the application is hanging (this is by design as I have forced it for testing how to rectify the issue). I understand that it is not at all good practice to have an application in an apphang state but for testing purposes I'd like to know if other tasks can still run seeing how they are not on the main UI thread.
Or does every task go into a locked state?
Can a task still run if an application is in an APPHANG state?
That depends entirely on the specific task. Some tasks will require the use of the UI thread, based on how they were defined, and some won't.
Perhaps the task is scheduled using a TaskScheduler or SynchronizationContext that intends to marshal the execution of code to the UI thread. Perhaps it will only be completed when some other code running on the UI thread triggers an event. Perhaps it is a continuation of some other task that is in some way dependent on the UI thread. Perhaps it is running code that, through some form of alternate mechanism is dependent on running code in the UI thread. There are literally an infinite number of possible ways, through any number of levels of indirection, for a task to not complete if the UI thread is blocked.
And of course if the UI thread is blocking until the task completes at the same time, then you get a deadlock.
Or does every task go into a locked state?
That's certainly not the case. It's certainly possible to write tasks that will run to completion without regard for what the UI thread is doing.
Okay , let me try to put it in sentences ...
Lets consider an example,
where I create an async method and call it with await keyword,
As far as my knowledge tells me,
The main thread will be released
In a separate thread, async method will start executing
Once it is executed, The pointer will resume from last position It left in main thread.
Question 1 : Will it come back to main thread or it will be a new thread ?
Question 2: Does it make any difference if the async method is CPU bound or network bound ? If yes, what ?
The important question
Question 3 : Assuming that is was a CPU bound method, What did I achieve? I mean - main thread was released, but at the same time, another thread was used from thread pool. what's the point ?
async does not start a new thread. Neither does await. I recommend you read my async intro post and follow up with the resources at the bottom.
async is not about parallel programming; it's about asynchronous programming. If you need parallel programming, then use the Task Parallel Library (e.g., PLINQ, Parallel, or - in very complex cases - raw Tasks).
For example, you could have an async method that does I/O-bound operations. There's no need for another thread in this scenario, and none will be created.
If you do have a CPU-bound method, then you can use Task.Run to create an awaitable Task that executes that method on a thread pool thread. For example, you could do something like await Task.Run(() => Parallel...); to treat some parallel processing as an asynchronous operation.
Execution of the caller and async method will be entirely on the current thread. async methods don't create a new thread and using async/await does not actually create additional threads. Instead, thread completions/callbacks are used with a synchronization context and suspending/giving control (think Node.js style programming). However, when control is issued to or returns to the await statement, it may end up being on a different completion thread (this depends on your application and some other factors).
Yes, it will run slower if it is CPU or Network bound. Thus the await will take longer.
The benefit is not in terms of threads believe it or not... Asynchronous programming does not necessarily mean multiple threads. The benefit is that you can continue doing other work that doesn't require the async result, before waiting for the async result... An example is a web server HTTP listener thread pool. If you have a pool of size 20 then your limit is 20 concurrent requests... If all of these requests spend 90% of their time waiting on database work, you could async/await the database work and the time during which you await the database result callback will be freed... The thread will return to the HTTP listener thread pool and another user can access your site while the original one waits for the DB work to be done, upping your total limit.
It's really about freeing up threads that wait on externally-bound and slow operations to do other things while those operations execute... Taking advantage of built-in thread pools.
Don't forget that the async part could be some long-running job, e.g. running a giant database query over the network, downloading a file from the internet, etc.
what happens if i put a System.Threading.Thread.Sleep() into a Task that is started with TaskFactory.StartNew()?
Does it put the executing Thread itself to sleep or will this Thread execute another Task while the first one Sleeps?
I`m asking this because I´m currently working on a Server which must sometimes wait to receive data from its clients, and if I put the Task asleep, which is handling the client-connection, I would like to know if the Thread that executes this specific Task is then able to Handle another connection while the first one waits for data.
Thread.Sleep always make the currently executed thread sleep, whether that thread is executing a task or not.
There's no concept of a task sleeping - only a thread. Of course the thread pool will allow multiple threads to execute, so you'll probably just end up with more threads than you really need.
It sounds like you may want to wait for the data asynchronously, continuing with the rest of the work for the task when that data is available.
C# 5's async methods will make all of this a lot simpler.