Search code examples
c#.netsocketsiocp

How does .NET handle IOCP thread safety?

I'm playing with SocketAsyncEventArgs and IO Completion Ports. I've been looking but I can't seem to find how .NET handles race conditions.

Need clarification on this stack overflow question: https://stackoverflow.com/a/28690948/855421

As a side note, don't forget that your request might have completed synchronously. Perhaps you're reading from a TCP stream in a while loop, 512 bytes at a time. If the socket buffer has enough data in it, multiple ReadAsyncs can return immediately without doing any thread switching at all. [emphasis mine]

For the sake of simplicity. Let's assume one client one server. The server is using a IOCP. If the client is a fast writer but server is a slow reader, does IOCP mean the kernel/underlying process can signal multiple threads?

1 So, socket reads 512 bytes, kernel signals a IOCP thread
2 Server processes new bytes
3 socket receives another X bytes but server is still processing previous buffer

Does the kernel spin up another thread? SocketAsyncEventArgs has a Buffer which by definition is: "Gets the data buffer to use with an asynchronous socket method." So the buffer should not change over the lifetime of the SocketAsyncEventArgs if I understand that correctly.

What's preventing SocketAsyncEventArgs.Buffer from getting corrupted by IOCP thread 2?

Or does the .NET framework synchronize IOCP threads? If so, what's the point of spinning up a new thread then if IOCP thread 1 blocks the previous read?

Solution

  • I've been looking but I can't seem to find how .NET handles race conditions.

    For the most part, it doesn't. It's up to you to do that. But, it's not clear from your question that you really have a race condition problem.

    You are asking about this text, in the other answer:

    If the socket buffer has enough data in it, multiple ReadAsyncs can return immediately without doing any thread switching at all

    First, to be clear: the method's name is ReceiveAsync(), not ReadAsync(). Other classes, like StreamReader and NetworkStream have ReadAsync() methods, and these methods have very little to do with what your question is about. Now, that clarified…

    That quote is about the opposite of a race condition. The author of that text is warning you that, should you happen to call ReceiveAsync() on a socket that already has data ready to be read, the data will be read synchronously and the SocketAsyncEventArgs.Completed event will not be raised later. It will be the responsibility of the thread that called ReceiveAsync() to also process the data that was read.

    All of this would happen in a single thread. There wouldn't be any race condition in that scenario.

    Now, let's consider your "fast writer, slow reader" scenario. The worst that can happen there is that the first read, which could take place in any thread, does not complete immediately, but by the time the Completed event is raised, the writer has overrun the reader's pace. In this case, since part of handling the Completed event is likely to be calling ReceiveAsync() again, which now will return synchronously, an IOCP thread pool thread will get tied up looping on the calls to ReceiveAsync(). No new thread is needed, because the current IOCP thread is doing all the work synchronously. But it does prevent that thread from handling other IOCP events.

    All that will mean though, is that if you have some other socket the server is handling and which also needs to call ReceiveAsync(), the framework will have to ensure there's another thread in the IOCP thread pool available to handle that I/O. But, that's a completely different socket and you would necessarily be using a completely different buffer for that socket anyway.

    Again, no race condition.


    Now, all that said, if you want to get really confused, it is possible to use asynchronous I/O in the .NET Socket API (whether with BeginReceive() or ReceiveAsync() or even wrapping the socket in a NetworkStream and using ReadAsync()) in such a way that you do have a race condition for a particular socket.

    I hesitate to even mention it, because there's no evidence in your question this pertains to you at all, nor that you're even really interested in having this level of detail. Adding this explanation could just confuse things. But, for the sake of completeness…

    It is possible to have issued more than one read operation on a socket at any given time. This would be somewhat akin to double- or triple-buffered video display (if you're familiar with that concept). The idea being that you might still be handling a read operation while new data comes in, and it would be more performant to have a new read operation already in progress to handle that data before you're done handling the current read operation.

    This sounds great, but in practice because of the way Windows schedules threads, and in particular does not guarantee a particular ordering of thread scheduling, if you try to implement your code that way, you create the possibility that your code will see read operations completed out of order. That is, if you for example call ReceiveAsync() twice in a row (with two different SocketAsyncEventArgs objects and two different buffers, of course), your Completed event handler might get called with the second buffer first.

    This isn't because the read operations themselves complete out of order. They don't. Hence the emphasis on "your" above. The problem is that while the IOCP threads handling the IO completions become runnable in the correct order (because the buffers are filled in the order you provided them by calling ReceiveAsync() multiple times), the second IOCP thread to become runnable could wind up being the first thread to actually be scheduled to run by Windows.

    This is not hard to deal with. You just have to make sure that you track the buffer sequence as you issue the read operations, so that you can reassemble the buffers in the correct order later. All of the async options available provide a mechanism for you to include additional user state data (e.g. SocketAsyncEventArgs.UserToken), so you can use this to track the order of your buffers.

    Again, this is not common. For most scenarios, a completely orderly implementation, where you only issue a new read operation after you're completely done with the current read operation, is completely sufficient. If you're worried at all about getting a multi-buffer read implementation correct, just don't bother. Stick with the simple approach.