Efficient Linux sockets (DMA/zero-copy)
There's been some talk on linux-kernel recently about providing an API for something along these lines, but the sticking point is that you can't DMA from general userspace buffers to the network card, because:
- What looks like contiguous data in the userspace linear address space is probably not-contiguous in physical memory, which is a problem if the network card doesn't do scatter-gather DMA;
- On many machines, not all physical memory addresses are "DMA-able". There's no way at the moment for a userspace application to specifically request a DMA-able buffer.
On recent kernels, you could try using vmsplice and splice together to achieve what you want - vmsplice the pages (with SPLICE_F_GIFT) you want to send into a pipe, then splice them (with SPLICE_F_MOVE) from the pipe into the socket.
AFAIK you are using the most efficient calls available if you cant use sendfile(2). Various aspects of efficient high performance networking code is covered by The C10K problem