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.
Related
I have my main Winforms application.
There are 6 Threads working in parrael + main thread, atleast that is what it ment to be .
I have created one Thread that is an Tcp Server, listening on specific port.
listenerThread = new Thread(new ThreadStart(AsynchronousSocketListener.StartListening));
listenerThread.Start();
I have also 5 different Threads that are doing different kind of stuff (for example updating database, counting averages/sums, being TCP Clients etc.)
My question is:
Is it possible that my TCP Server (which is working on one of 6 threads) wont read a message when one of those 5 different threads thread will take the computing power of CPU, and the TCP Server's Thread thread will have to wait ?
Another question: If that could happend, how could i avoid that ?
This is a summary of my comments above
"Is it possible that my TCP Server (which is working on one of 6 threads) wont read a message when one of those 5 different threads thread will take the computing power of CPU, and the TCP Server's Thread thread will have to wait ?"
Received data is buffered to an extent however if your code does not respond in an appropriate time then it could result in dropped data.
The same could be said in a single-core system and another app, say Prime95 is busy not playing nice and calculating prime numbers right next to yours.
Another question: If that could happend, how could i avoid that ?
When handling I/O and I'll focus on TCP here is to perform the minimal amount of processing in your data received handler irrespective of whether that handler is of the form IAsyncResult or async/await.
A good general flow is:
start an asynchronous read
read result of asynchronous read
place result in a queue
loop back to #1
Meanwhile you process the read results from #2 in a different mechanism whether that be a timer; GUI app-idle-loop; or a different thread so long as the thread processing the results has nothing to do with the flow above.
The reason being is that in a scenario involving reasonably high data transfer rates, if you were to read a block of data and then proceed to immediately update that Telerik datagrid UI showing 1000s of rows, there is a high chance that the next read operation will result in dropped data because you didn't respond in a sufficient amount of time.
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:
1
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 server in C#. The asynchronous example on msdn.microsoft.com suggests the following.
BeginAccept to listen for client (& start a new thread when client calls).
BeginReceive to receive the data from client ( & start a new thread to do it on).
BeginSend to reply to send data to client ( & start yet another thread to it on).
At this point there seems to be 4 separate threads, when from my (possibly naive) point of view there really only needs to be 2. 1 for the server to keep listening on and 1 for the conversation with the client. Why does my conversation with the client need 3 threads since I have to wait for a reply before I send and I won't be doing anything else while waiting to receive data from the client?
Cheers
BeginAccept does not start a new thread. It is attaching a handler to an OS level hook. No thread is going to be doing the meat of the work for this operation. The same is true for BeginReceive and BeginSend. None of these are starting new threads.
When the events that they are adding handlers for actually fire, then a thread pool thread is created to respond to the action happening. The CPU bound work done here should generally be quite low. What you'll see here is a lot of thread pool threads requested, but very little work being done by them, so they are sent back to the pool very quickly.
The thread pool is designed for this type of use. Rather than creating full threads for each event response (which would be expensive) you can create just 1-2 threads and continually re-use them to respond to all of these events in turn. The pool will only create as many threads as it needs to keep up with a sufficiently small backlog.
While your main thread will be marshalled around these operations, you should not have to "start a thread" yourself.
BeginAccept is a non blocking method - .NET will return immediately from it but invoke your callback on the thread pool when it fulfils its purpose asynchronously.
The threadpool is optimised outside of your control.
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.