Re: TCP IP Offloading Interface

Roland Dreier
2003-07-13T16:08:16Z

David> TOE is evil, read this:

David> http://www.usenix.org/events/hotos03/tech/full_papers/mogul/mogul.pdf

David> TOE is exactly suboptimal for the very things performance
David> matters, high connection rates.

David> Your return is also absolutely questionable. Servers
David> "serve" data and we offload all of the send side TCP
David> processing that can reasonably be done (segmentation,
David> checksumming).

David> I've never seen an impartial benchmark showing that TCP
David> send side performance goes up as a result of using TOE
David> vs. the usual segmentation + checksum offloading offered
David> today.

David> On receive side, clever RX buffer flipping tricks are the
David> way to go and require no protocol changes and nothing gross
David> like TOE or weird buffer ownership protocols like RDMA
David> requires.

David> I've made postings showing how such a scheme can work using
David> a limited flow cache on the networking card. I don't have
David> a reference handy, but I suppose someone else does.

Your ideas are certainly very interesting, and I would be happy to see
hardware that supports flow identification. But the Usenix paper
you're citing completely disagrees with you! For example, Mogul writes:

"Nevertheless, copy-avoidance designs have not been widely adopted,
due to significant limitations. For example, when network maximum
segment size (MSS) values are smaller than VM page sizes, which is
often the case, page-remapping techniques are insufficient (and
page-remapping often imposes overheads of its own.)"

In fact, his conclusion is:

"However, as hardware trends change the feasibility and economics of
network-based storage connections, RDMA will become a significant
and appropriate justification for TOEs."

- Roland