I'm trying to create a socket server which can handle relatively large amount of clients. There are different approaches used for such a task, first is to use a separate thread for each incoming connection, and second is to use async/await pattern.
First approach is bad as there will be relatively small number of threads such as all system's resources will be lost on context switching.
Second approach from the first sight is good as we can have our own threadpool with limited number of worker threads, so dispatcher will receive incoming connections, add them to some queue and call async socket read methods which in turn will receive data from socket and add this data/errors to queue for further processing(error handling client responses, DB-related work).
There is not so much info on internal implementation of async/await I could found, but as I understood while using non-UI application all continuation is done through TaskScheduler.Current which is using runtime's threadpool and so it's resources are limited. Greater amount of incoming connections will result in no free threads in runtime's threadpool or amount will be so large that system will stop responding.
In this matter async/await will result in same problem as with 1-client/1-thread concern, however with little advantage as runtime threadpool's threads may not occupy so much address space as default System.Threading.Thread (I believe 1MB stack size + ~1/2MB of control data).
Is there any way I can made one thread to wait for some kernel interrupt on say 10 sockets so application will only use my explicitly sized thread pool? (I mean that in case there is any further data on one from 10 sockets, one thread will wake up and handle it.)
In this matter async/await will result in same problem as with 1-client/1-thread concern
When thread reach code that is running asynchronously then control is returned to caller so that means thread is returned to thread pool and can handle another request so it is any superior to 1-client/1-thread because thread isn't blocked.
There is some any intersting blog about asnyc/await:
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If it take a background thread before the data arrives, and when many connections waiting for data, there will be too many threads exist, causing performance degradation. is there an approach to wait for data without taking a thread?
Socket.BeginReceive(), and other asynchronous I/O methods in .NET, make use of the IOCP thread pool. The short version is that this is a very efficient way to manage I/O. There is practically no cost to wait for the I/O to complete, and even once it completes, your completion callback is called from a thread pool thread, tying up that thread only for as long as it takes for your callback to complete.
"IOCP" stands for "IO Completion Ports", a feature in the native Windows API. The basic idea is that you can have a single thread, or some small collection of threads, all ready to service the completion of a large number of I/O operations. This allows I/O operations to scale well into the hundreds of thousands, if not millions, of concurrent operations, while still only requiring a relatively small number of threads to deal with them all.
So, go right ahead use those asynchronous I/O APIs. They are the best way to write scalable I/O code.
(Aside: the Socket class in particular has a number of async options. Ironically, the methods ending in ...Async do not comply with the new(er) async/await paradigm in C#, but they are in fact the most scalable way to do I/O with a Socket, because not only do they use the IOCP thread pool, they also allow you to reuse your I/O state objects, so you can have a pool of those and minimize GC load.)
My question is more related to how WebSockets (on the client) work/behave with threads in .Net and what I am looking for as an answer would be more of a low level explanation on how the OS interacts with the .Net thread when it receives data from the server on its socket.
Suppose I have a client that opens 1000 sockets to a server asynchronously. It then sits there waiting for updates/events to come through. These events can arrive at different times and frequencies.
Assuming that every time data comes in via a socket, a thread needs to pick it up and do some work on it, am I correct to assume that IF all the 1000 sockets receive data at the same time I will then have 1000 threads (1 thread per socket) coming from the Thread Pool to pick-up the data from the socket? What if I wanted to have 3000 sockets open?
Any clarification on this is very much appreciated.
Assuming you are using the .NET Framework library WebSocket the received data will be returned on a thread from the ThreadPool (probably the IO Completion Thread Pool).
Thread Pool
When the thread pool is used you don't know how many different threads that will be active a the same time. The data is put on a queue and the thread pool works through it as fast as it can. You can control the min/max number of threads that it will use, but the way that the pool creates/destroys its threads is unspecified.
The above hold true for most asynchronous operations in .NET.
Excpetions
If you awaited the asynchronous receive operation in a synchronization context (for instance a UI thread) the operation will resume in the same context (UI thread), unless you suppress the sync context. In this case only one thread will be used and the receive operations will be queued and processed in sequence.
The .Net Socket async API manages threads automatically when using the BeginXXX methods. For example, if I have 100 active connections sending and receiving TCP messages, will be used around 3 threads. And it makes me curious.
How the API makes this thread management?
How all flow of connections are divided among the threads to be processed?
How the manager prioritizes which connections/readings/writings must be processed first?
My questions may not have sense because I don't know how it works and what to ask specifically, so sorry. Basically I need to know how this whole process works in low level.
The .Net Socket async API manages threads automatically when using the
BeginXXX methods.
This is not quite correct. APM Begin/End-style socket API do not manage threads at all. Rather, the completion AsyncCallback is called on a random thread, which is the thread where the asynchronous socket I/O operation has completed. Most likely, this is going to be an IOCP pool thread (I/O completion port thread), different from the thread on which you called the BeginXXX method. For more details, check Stephen Cleary's "There Is No Thread".
How the manager prioritizes which connections/readings/writings must
be processed first?
The case when there's no IOCP threads available to handle the completion of the async I/O operation is called TheadPool starvation. It happens when all pool threads are busy executing some code (e.g., processing the received socket messages), or are blocked with a blocking call like WaitHandle.WaitOne(). In this case, the I/O completion routine is queued to ThreadPool to be executed when a thread becomes available, on FIFO basis.
You have an option to increase the size of ThreadPool with SetMinThreads/SetMaxThreads APIs, but doing so isn't always a good idea. The number of actual concurrent threads is anyway limited by the number of CPU/cores, so you'd rather want to finish any CPU-bound processing work as soon as possible and release the thread to go back to the pool.
I'm writing a TCP Server in C# and I'm using the BeginXXX and EndXXX methods for async communication. If I understand correctly, when I use BeginXXX the request will be handled on in the threadpool (when the request is ready) while the main thread keeps accepting new connections.
The question is what happens if I perform a blocking action in one of these AsyncCallbacks? Will it be better to run a blocking operation as a task? Tasks use the threadpool as well don't they?
The use case is the following:
The main thread set ups a listening socket which accepts connections using BeginAccept, and starts listening on those connections using BeginReceive. When a full message has been received, a function is called depending on what that message was, in 80% of all cases, those functions will start a database query/insertion/update.
I suggest you use SocketAsyncEventArgs which is introduced in .net 4.5
Here's some reading material you can start with
click me
The question is what happens if I perform a blocking action in one of these AsyncCallbacks? Will it be better to run a blocking operation as a task?
If you do that too often or for too long then the ThreadPool will grow. Possible to the point where it will crash your App.
So try to avoid blocking as much as possible. But a little bit of it should be acceptable. Keep in mind that the ThreadPool will grow with 1 new thread per 500 ms. So make sure and verify that it will level out on some reasonable number of threads.
A blunt instrument could be to cap the MaxThreads of the pool.
Tasks use the threadpool as well don't they?
Yes, so your options are limited.
I have a piece of code (on a server) that uses async method to receive data on sockets like this:
asyncRes = connectionSocket.BeginReceive(receiveBuffer, 0, RECEIVING_BUFFER_SIZE,
SocketFlags.None, out error, new AsyncCallback(ReceiveDataDone), null);
In the handler (ReceiveDataDone) of the socket there are cases where Thread.Sleep(X) is used in order to wait for other things(questionable implementation indeed). I know this is a questionable design but I wonder if making such kind of code could explain an explosion of threads created in my application because of the other pending sockets in the server that have their ReceiveDataDone called. (when many connections are handled by the server the number of threads created figuratively explodes). I wonder how BeginReceive method on .NET sockets work, that could explain the huge number of threads I see.
You absolutely should not perform any kind of blocking action in APM callbacks. These are run in the ThreadPool. The ThreadPool is designed for the invocation of short-lived tasks. If you block (or take a long time to execute) you are tying up (a finite number of) threads and causing ThreadPool starvation. Because the ThreadPool does not spin up extra threads easily (in fact, it's quite slow to start extra threads), you're bottlenecking on the timing that controls how quickly the ThreadPool is allowed to spin up new threads.
Despite answering a different question, this answer I provided a while back explains the same issue:
https://stackoverflow.com/a/1733226/14357
You should not use Thread.sleep for waiting in ThreadPool Threads this causes the Thread to be blocked and It will not accept any further workitems for the time it is blocked.
You can use TimerCallback for such a use case. It will let the ThreadPool schedule other work on the waiting thread in the meantime.