I'm using TaskCompletionSource<> quite often. I have a network protocol design, where I receive many streams in one tcp/ip connection. I demultiplex those streams and then inform the corresponding "SubConnections" of new content.
Those "SubConnections" (which are waiting via await) then should continue in a new thread.
Usually I solve such issues by putting the TaskComplectionSource<>.Set call in an anonymous ThreadPool.QueueUserWorkItem method, like this:
ThreadPool.QueueUserWorkItem(delegate { tcs.SetResult(null); });
If I don't do this the corresponding await tcs.Task call will continue in the thread which called tcs.SetResult.
However, I'm aware of that this isn't the right way to do things. It's also possible to self-write a SynchronizationContext (or something) which will instruct the await call to continue in another thread.
My primary question here is: How would I do this in the "best practice" way?
My hope here is also to avoid the ThreadPool overhead, because it's quite high on Linux compared to just blocking a thread and waiting for a ManualResetEvent - even though the SynchronizationContext (or whatever) may also utilize the ThreadPool.
Please refrain from telling me that it's generally a bad idea to multiplex something in one tcp/ip connection or that I should just use System.IO.Pipelines, REST or whatever. This is my scenario. Thank you.
You can create the TaskCompletionSource using TaskCreationOptions.RunContinuationsAsynchronously (in .NET 4.6+):
var tcs = new TaskCompletionSource<Result>(TaskCreationOptions.RunContinuationsAsynchronously);
...
tcs.SetResult(...);
See e.g. this thread for more details.
Related
In the example of Socket.BeginReceive() your provided method will be called from a thread-pool thread when the asynchronous operation completes. The assumed behaviour seems to be that you will handle the callback and return, typically having called another async method.
But one could use this as a way to obtain a thread which you then use to manage that socket. Is retaining/monopolising this thread (does this have a proper name) going to cause problems or is it just another worker thread you can use as you want?
Note: I'm aware this isn't the modern paradigm for using Sockets and doesn't scale well
I am using ThreadPool in .NET to make some web request in the background, and I want to have a "Stop" button to cancel all the threads even if they are in the middle of making a request, so a simple bool wont do the job.
How can I do that?
Your situation is pretty much the canonical use-case for the Cancellation model in the .NET framework.
The idea is that you create a CancellationToken object and make it available to the operation that you might want to cancel. Your operation occasionally checks the token's IsCancellationRequested property, or calls ThrowIfCancellationRequested.
You can create a CancellationToken, and request cancellation through it, by using the CancellationTokenSource class.
This cancellation model integrates nicely with the .NET Task Parallel Library, and is pretty lightweight, more so than using system objects such as ManualResetEvent (though that is a perfectly valid solution too).
The correct way to handle this is to have a flag object that you signal.
The code running in those threads needs to check that flag periodically to see if it should exit.
For instance, a ManualResetEvent object is suitable for this.
You could then ask the threads to exit like this:
evt.Set();
and inside the threads you would check for it like this:
if (evt.WaitOne(0))
return; // or otherwise exit the thread
Secondly, since you're using the thread pool, what happens is that all the items you've queued up will still be processed, but if you add the if-statement above to the very start of the thread method, it will exit immediately. If that is not good enough you should build your own system using normal threads, that way you have complete control.
Oh, and just to make sure, do not use Thread.Abort. Ask the threads to exit nicely, do not outright kill them.
If you are going to stop/cancel something processing in another thread, ThreadPool is not the best choice, you should use Thread instead, and manage all of them in a container(e.g. a global List<Thread>), that guarantees you have full control of all the threads.
Asynchronous programming is a technique that calls a long running method in the background so that the UI thread remains responsive. It should be used while calling a web service or database query or any I/O bound operation. when the asynchronous method completes, it returns the result to the main thread. In this way, the program's main thread does not have to wait for the result of an I/O bound operation and continues to execute further without blocking/freezing the UI. This is ok.
As far as I know the asynchronous method executes on a background worker thread. The runtime makes availabe the thread either from the threadpool or it may create a brand new thread for its execution.
But I have read in many posts that an asynchronous operation may execute on a separate thread or without using any thread. Now I am very confused.
1) Could you please help clarifying in what situation an asynchronous operation will not use a thread?
2) What is the role of processor core in asynchronous operation?
3) How it is different from multithreading? I know one thing that multithreading is usefult with compute-bound operation.
Please help.
IO (let's say a database-operation over the network) is a good example for all three:
you basically just register a callback the OS will finally call (maybe on a then newly created thread) when the IO-Operation finished. There is no thread sitting around and waiting - the resurrection will be triggered by hardware-events (or at least by a OS process usually outside user-space)
it might have none (see 1)
in Multithreading you use more than one thread (your background-thread) and there one might idle sit there doing nothing (but using up system-resources) - this is of course different if you have something to compute (so the thread is not idle waiting for external results) - there it makes sense to use a background-worker-thread
Asynchronous operations don't actually imply much of anything about how they are processed, only that they would like the option to get back to you later with your results. By way of example:
They may (as you've mentioned) split off a compute-bound task onto an independent thread, but this is not the only use case.
They may sometimes complete synchronously within the call that launches them, in which case no additional thread is used. This may happen with an I/O request if there is already enough buffer content (input) or free buffer space (output) to service the request.
They may simply drop off a long-running I/O request to the system; in this case the callback is likely to occur on a background thread after receiving notification from an I/O completion port.
On completion, a callback may be delivered later on the same thread; this is especially common with events within a UI framework, such as navigation in a WebBrowser.
Asynchronity doesn't say anything about thread. Its about having some kind of callbacks which will be handled inside a "statemachine" (not really correct but you can think of it like events ). Asynchronity does not raise threads nor significantly allocate system ressources. You can run as many asynchronous methods as you want to.
Threads do have a real imply on your system and you have a hughe but limited number you can have at once.
Io operations are mostly related to others controllers (HDD, NIC,...) What now happens if you create a thread is that a thread of your application which has nothing to do waits for the controllers to finish. In async as Carsten and Jeffrey already mentioned you just get some kind of callback mechanism so your thread continues to do other work, methods and so on.
Also keep in mind that each thread costs ressources (RAM, Performance,handles Garbage Collection got worse,...) and may even and up in exceptions (OutOfMemoryException...)
So when to use Threads? Absolutly only if you really need it. If there is a async api use it unless you have really important reasons to not use it.
In past days the async api was really painfull, thats why many people used threads when ever they need just asynchronity.
For example node.js refuses the use of mulptile thread at all!
This is specially important if you handle multiple requests for example in services / websites where there is always work to do. There is also a this short webcast with Jeffrey Richter about this which helped me to understand
Also have a look at this MSDN article
PS: As a side effect sourcecode with async and await tend to be more readable
I have a BeginRead that calls a ReadCallback function upon completion. What I want to do in the callback is wait for a ManualResetEvent on a buffer to tell me if the buffer is empty or not, so I can issue a new BeginRead if I need more data. I have implemented this and it works. But my question is : is it safe to wait inside a callback?
I'm new to C#, if these were regular threads I wouldn't have doubts, but I'm not sure how C# treats callbacks.
Thank you.
APM callbacks are called on the thread-pool in all cases that I can think of.
That reduces your question to "Can I block thread-pool threads?". The answer to that is generally yes but it has downsides.
It is "safe" to do so until you exhaust the pool (then you risk deadlocks and extreme throughput reduction like >1000x).
The other downsides are the usual downsides of blocking threads in general. They cost a lot of memory to keep around. Lots of threads can cause lots of context switches.
Can't you just use await? I imagine your code to look like this:
while (true) {
var readResult = await ReadAsync(...);
await WaitForSomeConditionAsync(); //Instead of ManualResetEvent.
}
No blocking. Very simple code. There is no need to do anything special to issue the next read. It just happens as part of the loop.
My working model is something similar to producer/consumer.
Sounds like a good use for TPL Dataflow. Dataflow automates the forwarding of data, the waiting and throttling. It supports async to the fullest.
I am currently building an asynchronous console application in which I have created classes to handle separate areas of the application.
I have created an InputHandler class which I envisioned would await Console.ReadLine() input. However, you cannot await such a function (since it is not async), my current solution is to simply:
private async Task<string> GetInputAsync() {
return Task.Run(() => Console.ReadLine())
}
which runs perfectly fine. However, my (limited) understanding is that calling Task.Run will fire off a new (parallel?) thread. This defeats the purpose of async methods since that new thread is now being blocked until Readline() returns right?
I know that threads are an expensive resource so I feel really wasteful and hacky doing this. I also tried Console.In.ReadLineAsync() but it is apparently buggy? (It seems to hang).
I know that threads are an expensive resource so I feel really wasteful and hacky doing this. I also tried Console.In.ReadLineAsync() but it is apparently buggy? (It seems to hang).
The console streams unfortunately do have surprising behavior. The underlying reason is that they block to ensure thread safety for the console streams. Personally I think that blocking in an asynchronous method is a poor design choice, but Microsoft decided to do this (only for console streams) and have stuck by their decision.
So, this API design forces you to use background threads (e.g., Task.Run) if you do want to read truly asynchronously. This is not a pattern you should normally use, but in this case (console streams) it is an acceptable hack to work around their API.
However, my (limited) understanding is that calling Task.Run will fire off a new (parallel?) thread.
Not quite. Task.Run will queue some work to the thread pool, which will have one of its threads execute the code. The thread pool manages the creation of threads as necessary, and you usually don't have to worry about it. So, Task.Run is not as wasteful as actually creating a new thread every time.