Scatter/gather async socket I/O in .NET

Question

I'm trying to use the Stream.BeginWrite Async I/O API in .NET for a high-throughput situation with many short messages. As such, a scatter/gather API will reduce the number of context switches (and CPU usage) tremendously. Does this API use the LPBUFFERS Win32 API at all? Is there an alternative API for Scatter/Gather I/O?

Answer 1

There is no way to do socket scatter/gather I/O in .NET. According to a MSFT blog post, there may be a similar API in .NET 4.5 (whatever that is...)

Answer 2

Looking at the .net sources, the accepted answer seems to be wrong.

SocketAsyncEventArgs has a BufferList attribute. When that is used, instead of the Buffer attribute that can only hold a single contiguous block of memory, operations can make use of scatter/gather DMA, as Socket.SendAsync(SocketAsyncEventArgs) uses WSASend internally, that

allows multiple send buffers to be specified making it applicable to the scatter/gather type of I/O

and Socket.SendAsync(SocketAsyncEventArgs) uses WSARecv, that

allows multiple receive buffers to be specified making it applicable to the scatter/gather type of I/O

I don't have the .net 3.5 sources handy, but BufferList exists since .net 3.5, so scatter/gather might have been supported since .net 3.5. The minimum OS requirements for WSASend and WSARecv exist are documented as Windows Vista / Server 2003.

N.B. I don't know what stream you are using, but NetworkStream.BeginWrite sends a single buffer to the WSASend, so you cannot use that for scatter/gathering.

Answer 3

I would be surprised if you could get to the scatter/gather api's from the BCL (it's for the l33t w1n32 haxx0rz, you know?), but there's always P/Invoke (which is suprisingly easy to use, I've found).

Answer 4

If you want to dig into the guts of the framework, there are a few ways to do it:

1) Reflector

2) MS recently opened up the source for debugging purposes, you can step into it with VS2008 if you enable the option under Debugging/Options/General

3) Koders.com seem to be hosting the framework source too:

http://www.koders.com/csharp/fidCE09E83BE706D0BD370658C3785E82D3A13FC2CE.aspx?s=flush()#L109