Ethernet performance using TSE in uClinux

I've been doing some research on Zero-Copy stacks and haven't found any implementation for Linux that I could try to use. All I've found is sendfile() and a suggestion to use an mmapped file as a buffer as a hack to make use of sendfile() to remove the user-to-kernel copy normally done on send. I haven't been able to get this working because I get this error trying to use mmap for a size greater than a couple KB, even though malloc works fine to allocate the space:

Error mmapping the file: Cannot allocate memory

Regardless, memcpying a 50KB buffer takes 32us, which is still only a small part of the overhead I'm seeing, so I don't think zero-copy will help all that much. Still, I'd be happy to hear any suggestions of how to implement it for uClinux on NIOS II.

So far the only way I've found to get decent bandwidth is to send large UDP datagrams.