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IOCP is great for many connections, but what I'm wondering is, is there a significant benefit to allowing multiple pending receives or multiple pending writes per individual TCP socket, or am I not really going to lose performance if I just allow one pending receive and one pending send per each socket (which really simplifies things, as I don't have to deal with out-of-order completion notifications)?

My general use case is 2 worker threads servicing the IOCP port, handling several connections (more than 2 but less than 10), where the transmitted data is ether of two forms: one is frequent very small messages (which I combine if possible manually, but generally need to send often enough that the per-send data is still pretty small), and the other is transferring large files.

Cory Nelson
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Display Name
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2 Answers2

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Multiple pending recvs tend to be of limited use unless you plan to turn off the network stack's recv buffering in which case they're essential. Bear in mind that if you DO decide to issue multiple pending recvs then you must do some work to make sure you process them in the correct sequence. Whilst the recvs will complete from the IOCP in the order that they were issued thread scheduling issues may mean that they are processed by different I/O threads in a different order unless you actively work to ensure that this is not the case, see here for details.

Multiple pending sends are more useful to fully utilise the TCP connection's available TCP window (and send at the maximum rate possible) but only if you have lots of data to send, only if you want to send it as efficiently as you can and only if you take care to ensure that you don't have too many pending writes. See here for details of issues that you can come up against if you don't actively manage the number of pending writes.

Len Holgate
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  • What about streaming where latency is the most important factor (say control signal or other near-realtime information)? I'm thinking there it would be useful, since I want to initiate the next send without waiting for the previous one to be finished. – Display Name Sep 12 '14 at 19:19
  • Yes, that would be a scenario where it would be useful. IMHO it's always useful ;) if slightly more complex... – Len Holgate Sep 13 '14 at 15:56
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For less than 10 connections and TCP, you probably won't feel any difference even at high rates. You may see better performance by simply growing your buffer sizes.

Queuing up I/Os is going to help if your application is bursty and expensive to process. Basically it lets you perform the costly work up front so that when the burst comes in, you're using a little of the CPU on I/O and as much of it on processing as possible.

Cory Nelson
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