It's not offloading ALL the work onto the card. With 'offload' people often think about feature like computing checksums in hardware, but that's not a big deal

It's not offloading ALL the work onto the card. With 'offload' people often think about feature like computing checksums in hardware, but that's not a big deal on modern hardware. OpenOnload definitely leverages the cards capabilities, notably packet buffer handling which is exposed in their ef_vi API (there's a comment about this on the article's page); the flow steering (mentioned in the article) is a big help too. OpenOnload is a user space network stack built on top of ef_vi. But so much the win come from being userspace and tunable. OpenOnload also accelerate pipes, loopback UDP/TCP and epoll, all of which are unrelated to their hardware. Indeed, having unaccelerated fds in the same epoll set as accelerated fds kills performance because it needs to ask the kernel for the status of the unaccelerated fds. That said, I think you need to have a SolarFlare card for OpenOnload to work at all, even if you just use it for local communication. I've not tried it though.