High-performance NICs

What we call "high-performance" NICs have the ability, in a loaded system, to not lose any packets. They generally do this by using transmit and receive descriptor rings in main memory, which in turn point to packet buffers also in main memory.

High-performance NICs use bus-master DMA (direct memory access) to transfer packet data to and from main memory entirely independent of the CPU, using the descriptor rings as laundry lists of packet transmit and receive requests to carry out.

Thus, large scheduling latencies in software that service the NIC (e.g. io-pkt*) can be tolerated.