What would be an appropriate way to re-write my SlowMethodAsync async method, which executes a long running task, that can be awaited, but without using Task.Run?
I can do it with Task.Run as following:
public async Task SlowMethodAsync()
{
await Task.Run(() => SlowMethod());
}
public void SlowMethod()
{
//heavy math calculation process takes place here
}
The code, as it shown above, will spawn a new thread from a thread-pool. If it can be done differently, will it run on the invocation thread, and block it any way, as the SlowMethod content is a solid chunk of math processing without any sort of yielding and time-slice surrendering.
Just to clarify that I need my method to stay asynchronous, as it unblocks my UI thread. Just looking for another possible way to do that in a different way to how it's currently done, while keeping async method signature.
async methods are meant for asynchronous operations. They enable not blocking threads for non-CPU-bound work. If your SlowMethod has any IO-bound operations which are currently executed synchronously (e.g. Stream.Read or Socket.Send) you can exchange these for async ones and await them in your async method.
In your case (math processing) the code's probably mostly CPU-bound, so other than offloading the work to a ThreadPool thread (using Task.Run) there's no reason to use async as all. Keep SlowMethod as it is and it will run on the calling thread.
Regarding your update: You definitely want to use Task.Run (or one of the Task.Factory.StartNew overloads) to offload your work to a different thread.
Actually, for your specific case, you should use
await Task.Factory.StartNew(() => SlowMethod(), TaskCreationOptions.LongRunning)
which will allow the scheduler to run your synchronous task in the appropriate place (probably in a new thread) so it doesn't gum up the ThreadPool (which isn't really designed for CPU heavy workloads).
Wouldn't it be better just to call SlowMethod synchronously? What are you gaining by awaiting it?
Related
Can calling await in the same line as the async method be slower than calling normal method?
From what I know asynchronous methods are good for I/O operations like getting data from the database. But what if there is nothing to do between calling the async method and awaiting it I need to do it in the same line.
In this post Await and Async in the same line they discussed that the benefit comes from freeing thread.
I have some unit tests for testing my services and methods using async methods as I described are always taking longer than their non-async equivalents. I assume it's because creating work in a separate thread and then awaiting it has some price.
So what I want to ask is, if using async in this case has truly some benefits.
public async Task AssignHighestRank(User user)
{
user.Rank = await _rankRepository.GetHighestRank();
_userRepository.Update(user);
await _userRepository.SaveChanges();
}
async implementation uses additional CPU cycles, so in this sense an async method would be slightly slower than its equivalent that is not asynchronous. However, using such method together with other async methods may improve performance.
For example, consider a situation when you need to make multiple changes at once:
public async Task RecordBattleResultAsync(User winner, User loser) {
await Task.WhenAll(
AssignHighestRankAsync(winner)
, AssignLowestRankAsync(loser)
).ConfigureAwait(false);
}
This method would exploit the fact that both your methods are async for a potential speed-up.
Based on this investigation async method works slowly with consistently operations then regular one does the same (if we do not regard that async methods does not hold execution thread unlike regular one) :
Should I worry about "This async method lacks 'await' operators and will run synchronously" warning
due to huge amount of compiler's works under the hood. But using operation Task.WhenAll (creation task that is completed when all task are completed too - main thread is not blocked) and Task.WaitAll (almost the same save for main thread is blocked) with independence data task might increase speed-up of method execution (methods but not whole application in case Task.WaitAll) because of parallel task execution .
I have an async method that get data from a database:
private async Task getDataAsync()
{
await getDataFromDatabaseAsync();
}
I have a contructor that uses this method, in this way:
public MyViewModel()
{
Task.WhenAll(getDataAsync());
//getDataAsync().ConfigureAwait(false);
//next line of code
}
The only way that I get to avoid the applications isn't blocked is using one the two options, both seems to work fine. I can use WhenAll or I can use ConfigureAwait(false).
Another options it is calling the method getDataASync() inside a task, but I guess it is a worse option becase it takes more resources.
So I would like to know which is the differences between WhenAll and ConfigureAwait.
When I use WhenAll in this way, the line of code "next line of code" is run after the async method is finisihed or it will run before finish?
Thanks.
There is a common misconception that when you make a method async, it will be actually executed asynchronously (i.e. on a separate thread). That is not the case: async and await are a means to synchronize already asnychronous code. If you have nothing that is executed on a separate thread, your async code will run fully synchronously to the end.
Since getDataAsync is executed on the same thread as the MyViewModel constructor, you can run into a deadlock, since the thread waits for itself. When you use ConfigureAwait, you can avoid this situation.
Whether you actually should do that is a different question. What you are actually doing with ConfigureAwait is to start the task and to allow the await to continue on a separate context (which can be on a separate thread). Apart from the fact that you are not even using await here, continuation in a different context can become a problem when you want to execute UI operations after the await.
If you want to be sure that you can wait for getDataAsync in your constructor, you can use Task.Run to force execution in the thread pool:
var getDataTask = Task.Run((Func<Task>)getDataAsync);
//Do something else
getDataTask.Wait();
Everything you execute between Task.Run and the Wait() can run during the execution of getDataAsync. Whether parallelism is actually worth it here depends on what else you do until the Wait().
What you are doing in the MyViewModel constructor is to finally synchronize all asynchronous operations and make the execution of the constructor synchronous. If you want to run that operation asynchronously, you would need to start another task to do it. So you should be really sure that beyond that point async is not required anymore. If it is, run the initialization on another async method, await getDataAsync() there and synchronize somewhere up the call chain.
I've been trying to figure out why the UI was blocking from a ViewModel method, and realized that this part of the code:
await Task.WhenAll(getOutput1(), getOutput2());
was the problem. I managed to unblock the UI by using:
await Task.WhenAll(Task.Run(() => getOutput1()), Task.Run(() => getOutput2()));
getOutput1() and getOutput2() are both async with Task return types in the ViewModel, and the code is called from the View.
What's the difference with calling Task.WhenAll when I call Task.Run() and just directly supplying the task?
What's the difference with calling Task.WhenAll when I call Task.Run() and just directly supplying the task?
Calling the methods directly will invoke them on the UI thread. Calling them from within Task.Run will invoke them on a thread pool thread.
Conclusion: getOutput1 and/or getOutput2 are not actually asynchronous. (It is entirely possible for a method to return Task - and thus appear asynchronous - but in reality just block synchronously).
If the methods are pure async operations then you should not use Task.Run while calling them from the UI thread and it will work properly.
However if the methods are also involved in a long running CPU bound work, the UI thread won't be blocked while the async I/O operation is executing but will be while the CPU bound work is.
In this scenario you need the use Task.Run while you are calling the methods even though the methods already look like a async ones, because the methods are actually a combination of synchronous and asynchronous operations and you don't want to block the UI while the synchronous work is done.
Another option instead of using Task.Run from the ViewModel is to use ConfigureAwait(false) while awaiting the I/O async operations in your methods, doing that will inform to the rest of the method after the await part that it does not need the original context it had (which is probably the UI one) and that the rest of the method can also be executed in another ThreadPool thread instead.
Please refer this question for more info about async and await methods that combine I/O and CPU bound works.
The async-await features make it elegant to write non-blocking code. But, while non blocking, the work performed within an async function can still be non-trivial.
When writing async code, I find it natural to write code that follows the pattern 'all the way down the rabbit hole', so to speak, where all methods within the calling tree are marked async and the APIs used are async; but even while non blocking, the executed code can take up a fair amount of the contextual thread's time.
How and when do you decide to run an async-able method concurrently on top of asynchronously? Should one err on having the new Task created higher or lower in the call tree? Are there any best practices for this type of 'optimization'?
I've been using async in production for a couple of years. There are a few core "best practices" that I recommend:
Don't block on async code. Use async "all the way down". (Corollary: prefer async Task to async void unless you have to use async void).
Use ConfigureAwait(false) wherever possible in your "library" methods.
You've already figured out the "async all the way down" part, and you're at the point that ConfigureAwait(false) becomes useful.
Say you have an async method A that calls another async method B. A updates the UI with the results of B, but B doesn't depend on the UI. So we have:
async Task A()
{
var result = await B();
myUIElement.Text = result;
}
async Task<string> B()
{
var rawString = await SomeOtherStuff();
var result = DoProcessingOnRawString(rawString);
return result;
}
In this example, I would call B a "library" method since it doesn't really need to run in the UI context. Right now, B does run in the UI thread, so DoProcessingOnRawString is causing responsiveness issues.
So, add a ConfigureAwait(false) to every await in B:
async Task<string> B()
{
var rawString = await SomeOtherStuff().ConfigureAwait(false);
var result = DoProcessingOnRawString(rawString);
return result;
}
Now, when B resumes after awaiting SomeOtherStuff (assuming it did actually have to await), it will resume on a thread pool thread instead of the UI context. When B completes, even though it's running on the thread pool, A will resume on the UI context.
You can't add ConfigureAwait(false) to A because A depends on the UI context.
You also have the option of explicitly queueing tasks to the thread pool (await Task.Run(..)), and you should do this if you have particular CPU-intensive functionality. But if your performance is suffering from "thousands of paper cuts", you can use ConfigureAwait(false) to offload a lot of the async "housekeeping" onto the thread pool.
You may find my intro post helpful (it goes into more of the "why's"), and the async FAQ also has lots of great references.
Async-await does not actually use threads in the current .NET process-space. it is designed for "blocking" IO and network operations, like database calls, web requests, some file IO.
I cannot perceive what advantage there would be in C# to what you call the rabbit-hole technique. Doing so only obscures the code and unnecessarily couples your potentially high-cpu code to your IO code.
To answer your question directly, I would only use async-await for the aforementioned IO/network scenarios, right at the point where you are doing the blocking operations, and for anything that was CPU bound I would use threading techniques to make the best use of the available CPU cores. No need to mix the two concerns.
I've been considering the new async stuff in C# 5, and one particular question came up.
I understand that the await keyword is a neat compiler trick/syntactic sugar to implement continuation passing, where the remainder of the method is broken up into Task objects and queued-up to be run in order, but where control is returned to the calling method.
My problem is that I've heard that currently this is all on a single thread. Does this mean that this async stuff is really just a way of turning continuation code into Task objects and then calling Application.DoEvents() after each task completes before starting the next one?
Or am I missing something? (This part of the question is rhetorical - I'm fully aware I'm missing something :) )
It is concurrent, in the sense that many outstanding asychronous operations may be in progress at any time. It may or may not be multithreaded.
By default, await will schedule the continuation back to the "current execution context". The "current execution context" is defined as SynchronizationContext.Current if it is non-null, or TaskScheduler.Current if there's no SynchronizationContext.
You can override this default behavior by calling ConfigureAwait and passing false for the continueOnCapturedContext parameter. In that case, the continuation will not be scheduled back to that execution context. This usually means it will be run on a threadpool thread.
Unless you're writing library code, the default behavior is exactly what's desired. WinForms, WPF, and Silverlight (i.e., all the UI frameworks) supply a SynchronizationContext, so the continuation executes on the UI thread (and can safely access UI objects). ASP.NET also supplies a SynchronizationContext that ensures the continuation executes in the correct request context.
Other threads (including threadpool threads, Thread, and BackgroundWorker) do not supply a SynchronizationContext. So Console apps and Win32 services by default do not have a SynchronizationContext at all. In this situation, continuations execute on threadpool threads. This is why Console app demos using await/async include a call to Console.ReadLine/ReadKey or do a blocking Wait on a Task.
If you find yourself needing a SynchronizationContext, you can use AsyncContext from my Nito.AsyncEx library; it basically just provides an async-compatible "main loop" with a SynchronizationContext. I find it useful for Console apps and unit tests (VS2012 now has built-in support for async Task unit tests).
For more information about SynchronizationContext, see my Feb MSDN article.
At no time is DoEvents or an equivalent called; rather, control flow returns all the way out, and the continuation (the rest of the function) is scheduled to be run later. This is a much cleaner solution because it doesn't cause reentrancy issues like you would have if DoEvents was used.
The whole idea behind async/await is that it performs continuation passing nicely, and doesn't allocate a new thread for the operation. The continuation may occur on a new thread, it may continue on the same thread.
The real "meat" (the asynchronous) part of async/await is normally done separately and the communication to the caller is done through TaskCompletionSource. As written here http://blogs.msdn.com/b/pfxteam/archive/2009/06/02/9685804.aspx
The TaskCompletionSource type serves two related purposes, both alluded to by its name: it is a source for creating a task, and the source for that task’s completion. In essence, a TaskCompletionSource acts as the producer for a Task and its completion.
and the example is quite clear:
public static Task<T> RunAsync<T>(Func<T> function)
{
if (function == null) throw new ArgumentNullException(“function”);
var tcs = new TaskCompletionSource<T>();
ThreadPool.QueueUserWorkItem(_ =>
{
try
{
T result = function();
tcs.SetResult(result);
}
catch(Exception exc) { tcs.SetException(exc); }
});
return tcs.Task;
}
Through the TaskCompletionSource you have access to a Task object that you can await, but it isn't through the async/await keywords that you created the multithreading.
Note that when many "slow" functions will be converted to the async/await syntax, you won't need to use TaskCompletionSource very much. They'll use it internally (but in the end somewhere there must be a TaskCompletionSource to have an asynchronous result)
The way I like to explain it is that the "await" keyword simply waits for a task to finish but yields execution to the calling thread while it waits. It then returns the result of the Task and continues from the statement after the "await" keyword once the Task is complete.
Some people I have noticed seem to think that the Task is run in the same thread as the calling thread, this is incorrect and can be proved by trying to alter a Windows.Forms GUI element within the method that await calls. However, the continuation is run in the calling thread where ever possible.
Its just a neat way of not having to have callback delegates or event handlers for when the Task completes.
I feel like this question needs a simpler answer for people. So I'm going to oversimplify.
The fact is, if you save the Tasks and don't await them, then async/await is "concurrent".
var a = await LongTask1(x);
var b = await LongTask2(y);
var c = ShortTask(a, b);
is not concurrent. LongTask1 will complete before LongTask2 starts.
var a = LongTask1(x);
var b = LongTask2(y);
var c = ShortTask(await a, await b);
is concurrent.
While I also urge people to get a deeper understanding and read up on this, you can use async/await for concurrency, and it's pretty simple.