I am trying to understand async/await. I understand that you should not Await a CPU-bound method, but to help my understanding I am curious what happens if you do. Consider:
Public Async Function DoSomeTasks()
Await LongRunningCPUBoundMethod1()
LongRunningCPUBoundMethod2()
End Function
Public Async Function LongRunningCPUBoundMethod1() As Task
' Do stuff synchronously
End Function
Public Sub LongRunningCPUBoundMethod2()
' Do stuff synchronously
End Sub
How will the Task Scheduler handle the CPU resources? In what order will these methods execute? Will LongRunningCPUBoundMethod1 or LongRunningCPUBoundMethod2 execute first?
The thing to remember here is that Async/Await code is not necessarily multi-threaded. You can use them to help with multi-threaded code by awaiting items that start a separate thread, but what they really do is allow you to break up several tasks efficiently in the same thread.
This doesn't come without some overhead; switching between asynchronous tasks has a cost. When you await cpu-bound tasks, you've added that cost to the already cpu-intensive work, and therefore made things worse rather than better. However, if you combine this with code that starts the cpu-heavy tasks in a separate thread, and then uses a WaitHandle or a Task to send the results back, you might be fine again (depending on how many items you're awaiting relative to the number of available cores), because now you're taking advantage of the multiple cores in your CPU.
Additionally, let's look at this in context of .Net WinForms. It's important to remember here that you never want to do significant CPU work on main UI thread. Really, anything that blocks for more than a few milliseconds is problematic. If that thread is busy, the Windows Message pump doesn't run, you can't respond to events, and your user interface becomes unresponsive.
To understand Await in this context, think of it as if it breaks your method up into two parts (or more, if there is more than one Await). Everything up to and including the line with Await runs immediately, and everything after the await is hidden away by the compiler in a new callback method (called a continuation) that will be called with the same context (including variables local to the original method) and in the same thread when the Await has finished.
With this information, it should be clear that if you directly Await a cpu-bound method, you're still doing that work immediately on the UI thread, and your user interface is still in trouble. However, you can again account for this by starting the cpu-bound method in it's own thread. Await, in conjunction with Tasks, make this relatively easy to do without having to write a lot of new code. Certainly it's much better than the old DoEvents() technique.
Order of execution.
1.) LongRunningCPUBoundMethod1()
2.) LongRunningCPUBoundMethod2()
Heres how you could mess with the program flow and excution
var task = LongRunningCPUBoundMethod1();
LongRunningCPUBoundMethod2();
var result = await task;
// now result contains what was returned by LongRunningCPUBoundMethod1()
Sorry, I dont know how await/async affects CPU resources.
Related
I am trying to understand the coroutineScope() suspend function in Kotlin and I'm having a hard time understanding the exact purpose of this function.
As per the kotlinlang docs,
This function is designed for parallel decomposition of work. When any
child coroutine in this scope fails, this scope fails and all the rest
of the children are cancelled (for a different behavior see
supervisorScope). This function returns as soon as the given block and
all its children coroutines are completed.
But I feel this behavior can be achieved by launching a child coroutine and calling join on it.
So for example
suspend fun other() {
coroutineScope {
launch { // some task }
async { // some task }
}
}
This can be written as (scope is a reference to the scope created by the parent coroutine)
suspend fun other(scope: CoroutineScope) {
scope.launch {
launch { // some task }
async { // some task }
}.join()
}
Is there any difference between these two approaches since it looks
like they will produce same result and also seem to work in the same fashion?
If not, is coroutineScope merely a way to reduce this
boilerplate code of passing scope from parent coroutine and
calling join on child coroutine?
TLDR
Using CoroutineScope as in the example adds boilerplate code, is more confusing, error-prone and may handle cases like errors and cancellations differently. coroutineScope() is generally preferred in such cases.
Full answer
These two patterns are conceptually different and are used in different cases. Coroutines are all about sequential code and structured concurrency. Sequential means we can write a traditional code that waits in-place, it doesn't use callbacks, etc. and at the same time we don't get a performance hit. Structured concurrency means concurrent tasks have their owners, tasks consists of smaller sub-tasks that are explicit to the framework.
By mixing both above together we get a very easy to use and error-proof concurrency model where in most cases we don't have to launch background jobs and then manage them manually, watch for errors, handle cancellations, etc. We simply fork into sub-tasks and then join them in-place - that's all.
In Kotlin this is represented by suspend functions. Suspend functions are always executed within some context, this context is passed everywhere implicitly and the coroutines framework provides utils to use this context easily. One of the most common patterns is to fork and then join and this is exactly what coroutineScope() does. It creates a scope for launching sub-tasks and we can't leave this scope until all children are successful. We don't have to pass the scope manually, we don't have to join, we don't have to pass errors from children to their siblings and to parent, we don't have to pass cancellations from the parent to children - this is all automatic.
Therefore, suspend functions and coroutineScope() should be the default way of writing concurrent code with coroutines. This approach is easy to write, easy to read and it is error-proof. We can't easily leak a background task, because coroutineScope() won't let us go anywhere. We can't mistakenly ignore errors from background tasks. Etc.
Of course, in some cases we can't use this pattern. Sometimes, we actually would like to only launch a long-running task and return immediately. Sometimes, we don't consider the caller to be the owner of the task. For example, we could have some kind of a service that manages its tasks and we only schedule these tasks, but the service itself owns them. For these cases we can use CoroutineScope.
By using the scope explicitly we can launch tasks in the different context than the current one or from outside of coroutine world. We generally have more control, but at the same time we partially opt-out of the code correctness guarantees I mentioned above. For example, if we forget to invoke join() we can easily leak background tasks or perform operations in unexpected order. Also, in your case if the coroutine invoking other() is cancelled, all launched operations will be still running in the background. For these reasons, we should use CoroutineScope explicitly only if needed.
Common patterns
As a result of all that was said above, when working with coroutines we usually use one of these patterns:
Suspend function - it runs within the caller context and it waits for all its subtasks, it doesn't launch anything in the background.
Function receiving CoroutineScope either as a param or receiver - usually, that means the function wants to do something with the context even after returning (because otherwise it could be simply a suspend function). It either launches some background tasks or stores the context somewhere for a later use.
Regular function that uses its own CoroutineScope to launch tasks. Usually, this is some kind of a service that keeps its custom context.
At least to me, function which is suspend and receives CoroutineScope is pretty confusing, it is not entirely clear what to expect from it. Will it execute the operation in the caller context or in the provided one? Will it wait to finish or only schedule the operation in the background and return immediately? Maybe it will do both: first do some initial processing synchronously (therefore suspend), but also schedule additional task in the background (therefore scope: CoroutineScope)? We don't know this, we have to read the documentation or source code to understand its behavior. Your second example is unnecessary complication over a simple suspend function.
To further make my point consider this example:
data class User(
val firstName: String,
val lastName: String,
) {
fun getFullName(user: User) = ...
}
This example is far from perfect, but the main point is that it is confusing why we have to pass user to getFullName() if we call this function on a user already. We don't know whether it returns a full name of the passed user, the user we invoked the function on or maybe some kind of a mix? If that would be a member function not receiving a User or a static utility function receiving a User, everything would be clear. But a member function receiving a User is simply confusing. This is similar to your second example where we pass the context both implicitly and explicitly and we don't know which one is used and how exactly.
I have this requirement for a function that gets called periodically:
1. Get some input
2. Do 8 independent computations based on the input
3. Merge the results from these 8 computations and output the merged result
Since I've got at least 8 processors, I can do the 8 independent computations in parallel. So I created the following function:
fun process(in: InputType): ResultType {
runBlocking(Dispatchers.Default) {
val jobs = in.splitToList().map { async { processItem(it) } }
return jobs.awaitAll()
}
}
However, I've read in the documentation of runBlocking that it is "to be used in main functions and in tests."
This function is not the main function but is called way down in the call hierarchy in an application that does not otherwise use coroutines anywhere else.
What should I use to achieve this requirement if I shouldn't use runBlocking?
There is nothing wrong in using runBlocking() like this. The main point is to not overuse runBlocking() as a cheap way to convert regular code into coroutine one. When converting to coroutines, it may be tempting to just put runBlocking() everywhere in our code and that's all. This would be wrong, because it ignores structured concurrency and we risk blocking threads that should not be blocked.
However, if our whole application is not based on coroutines, we just need to use them in some place and we never need to cancel background tasks, then I think runBlocking() is just fine.
Alternative is to create CoroutineScope and keep it in some service with clearly defined lifecycle. Then we can easily manage background tasks, cancel them, etc.
I am already using implicit and fluent wait, but want to use thread.sleep, so want to know the syntax of it
Using Thread.sleep() is discouraged and there is no Performable for that in serenity.
Many testers pepper their web tests with Thread.sleep() statements, but this is the sub-optimal at best. It slows down the tests, makes them more brittle, and can hide genuine application issues. More insidiously, when our steps are artificially slowed by sleep statements, it takes longer not only to run and troubleshoot existing tests, but also to develop new ones.
https://johnfergusonsmart.com/handling-waits-and-asynchronous-pages-in-serenity-bdd/
I know my reply is very late and it's a very bad practice but I am posting here just for the sake of if it can be done. Also from your question it seems like you want to make a task of it. To do this you can make a anonymous task. For example
Integer wait = 500;
Task.where("{0} uses thread sleep",(actor)-> {
try {
Thread.sleep(wait);
} catch (InterruptedException e) {
e.printStackTrace();
}
});
You can wrap it inside a function or assign it to a Task variable. Just if you are wandering Task class have a overloaded method named where which takes a Consumer<Actor> as second argument instead of Performable.
Also you can make a normal task class by implementing Task and use thread sleep in performAs method.
But again considering you are using serenity I doubt it will come to the point where you would have to use Thread.sleep.
I've been reading up about concurrency in Kotlin and thought I started to understand it... Then I discovered that async() has been deprecated in 1.3 and I'm back to the start.
Here's what I'd like to do: create a thread (and it does have to be a thread rather than a managed pool, unfortunately), and then be able to execute async blocks on that thread, and return Deferred instances that will let me use .await().
What is the recommended way to do this in Kotlin?
1. Single-threaded coroutine dispatcher
Here's what I'd like to do: create a thread (and it does have to be a thread rather than a managed pool, unfortunately)
Starting a raw thread to handle your coroutines is an option only if you're prepared to dive deep and implement your own coroutine dispatcher for that case. Kotlin offers support for your requirement via a single-threaded executor service wrapped into a dispatcher. Note that this still leaves you with almost complete control over how you start the thread, if you use the overload that takes a thread factory:
val threadPool = Executors.newSingleThreadExecutor {
task -> Thread(task, "my-background-thread")
}.asCoroutineDispatcher()
2. async-await vs. withContext
and then be able to execute async blocks on that thread, and return Deferred instances that will let me use .await().
Make sure you actually need async-await, which means you need it for something else than
val result = async(singleThread) { blockingCal() }.await()
Use async-await only if you need to launch a background task, do some more stuff on the calling thread, and only then await() on it.
Most users new to coroutines latch onto this mechanism due to its familiarity from other languages and use it for plain sequential code like above, but avoiding the pitfall of blocking the UI thread. Kotlin has a "sequential by default" philosophy which means you should instead use
val result = withContext(singleThread) { blockingCall() }
This doesn't launch a new coroutine in the background thread, but transfers the execution of the current coroutine onto it and back when it's done.
3. Deprecated top-level async
Then I discovered that async() has been deprecated in 1.3
Spawning free-running background tasks is a generally unsound practice because it doesn't behave well in the case of errors or even just unusual patterns of execution. Your calling method may return or fail without awaiting on its result, but the background task will go on. If the application repeatedly re-enters the code that spawns the background task, your singleThread executor's queue will grow without bound. All these tasks will run without a purpose because their requestor is long gone.
This is why Kotlin has deprecated top-level coroutine builders and now you must explicitly qualify them with a coroutine scope whose lifetime you must define according to your use case. When the scope's lifetime runs out, it will automatically cancel all the coroutines spawned within it.
On the example of Android this would amount to binding the coroutine scope to the lifetime of an Activity, as explained in the KDoc of CoroutineScope.
Like it's stated with the message, it's deprecated in favor of calling async with an explicit scope like GlobalScope.async {} instead.
This is the actual implementation of the deprecated method as well.
By removing the top level async function, you'll not run into issues with implicit scopes or wrong imports.
Let me recommend this solution: Kotlin coroutines with returned value
It parallelizes tasks into 3 background threads (so called "triplets pool") but it's easy to change it to be single threaded as per your requirement by replacing tripletsPool with backgroundThread as below:
private val backgroundThread = ThreadPoolExecutor(1, 1, 5L, TimeUnit.SECONDS, LinkedBlockingQueue())
The language I am talking about is VB.
I'm struggling with this problem for over three weeks, and still a solution is not in sight.
The problem is the following:
I have got an intense calculation running in a sub procedure, while modifying the GUI a lot in the process. After a short period of time, the application freezes and is "not responding". After the calculation is finished, everything suddenly snaps to the point I have modified it. Nothing special about that.
But if I now try to start the sub with a thread, so the application doesn't freeze anymore, I can't access the GUI, because "it isn't created by the thread itself".
How do I get around this??
More concretely: How do I access information about the GUI (especially width and height of a PictureBox) and modify the GUI (especially setting a BackgroundPicture in a PictureBox)?
Any help is appreciated. Thank you!
You need to read up on how to use the Dispatcher (MSDN Threading Model)
If only one thread can modify the UI, how do background threads interact with the user? A background thread can ask the UI thread to perform an operation on its behalf. It does this by registering a work item with the Dispatcher of the UI thread. The Dispatcher class provides two methods for registering work items: Invoke and BeginInvoke. Both methods schedule a delegate for execution. Invoke is a synchronous call – that is, it doesn’t return until the UI thread actually finishes executing the delegate. BeginInvoke is asynchronous and returns immediately.
Small snippit of code from example in msdn article link:
startStopButton.Dispatcher
.BeginInvoke(DispatcherPriority.Normal,
New NextPrimeDelegate(AddressOf CheckNextNumber))