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POSocket.BeginSend does not complete synchronously despite plenty of buffer space
 

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  1. POSocket.BeginSend does not complete synchronously despite plenty of buffer space
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    <p>Why does <code>Socket.BeginSend</code> go asynchronous so quickly and so unpredictably? I know it's supposed to be asynchronous, but it can also complete immediately, in which case <code>IAsyncResult.CompletedSynchronously</code> is set. I would expect <code>CompletedSynchronously</code> to be true until the 8KB send buffer is filled up. The test case below (cleanup code stripped for brevity) demonstrates that <code>BeginSend</code> goes asynchronous on (typically) 3rd iteration after buffering only 6 bytes of data:</p> <pre class="lang-cs prettyprint-override"><code>TcpListener listener = new TcpListener(IPAddress.Any, 35001); listener.Start(); Socket client = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp)) client.Connect(new IPEndPoint(IPAddress.Loopback, 35001)); Socket server = listener.AcceptSocket()) for (int i = 1; i &lt;= 10; ++i) { bool sent = false; IAsyncResult result = client.BeginSend(new byte[] { 1, 2, 3 }, 0, 3, SocketFlags.None, r =&gt; sent = true, null); Assert.IsTrue(result.CompletedSynchronously, // fails typ. on 3rd iteration String.Format("Asynchronous on iteration {0}", i)); Assert.IsTrue(sent); client.EndSend(result); } </code></pre> <p>The exact iteration when it goes asynchronous is somewhat random. When process starts, it's 3. Afterwards it's 1 for all new sockets. I've seen other numbers sporadically. I know the 8KB socket send buffer is there, because <code>Socket.Send</code> (with <code>Socket.Blocking = false</code>) stops at about 8KB.</p> <p>Method <code>Socket.SendAsync</code> behaves similarly except it always completes asynchronously (i.e. no attempt at direct buffer write). See my <a href="https://stackoverflow.com/questions/14345297/when-does-socket-sendasync-complete-synchronously">related question about Socket.SendAsync</a>.</p> <p>I need <code>BeginSend</code> to be mostly synchronous for performance reasons (asynchronous callbacks are slower). Not to mention the unpredictability ruins unit tests and benchmarks for my low-level communication code.</p>
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    1. USRobert Važan
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    1. COThe question doesn't make sense in general. The rate at which a modern processor can generate data is easily two orders of magnitude higher than it could ever be sent across a network. The only way you are going to get synchronous sends is when you artificially delay your BeginSend calls. The point of BeginSend is to not do that.
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      1. USHans Passant
    2. CO@Hans Notice that I am calling `Socket.EndSend` in the loop. That means the previous send is already completed. There's nothing to wait for and therefore no need for asynchronous completion.
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      1. POSocket.BeginSend does not complete synchronously despite plenty of buffer space
      1. USRobert Važan
    3. CO@Hans Also I wouldn't be that sure about processor speed. My older benchmarks clearly show that `Send` with `Blocking = false` is significantly faster than `BeginSend`. It might be caused by overhead of many small packets targeted at large number of IPs.
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      1. USRobert Važan
 

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