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An Evaluation of an Attempt at Offloading TCP/IP Protocol Processing onto an i960RN-based iNIC
Ang, Boon Seong
HPL-2001-8
Keyword(s): TCP/IP networking; Intelligent Network Interface
Abstract: This report presents an evaluation of a TCP/IP offload implementation that utilizes a 100BaseT intelligent Network Interface Card (iNIC) equipped with a 100 MHz i960RN processor. The entire FreeBSD-derived networking stack from socket downward is implemented on the iNIC with the goal of reducing host processor workload. For large messages that result in MTU packets, the offload implementation can sustain wire- speed on receive but only about 80% of wire-speed on transmit. Utilizing hardware-based profiling of TTCP benchmark runs, our evaluation pieced together a comprehensive picture of transmit behavior on the iNIC. Our first surprise was the number of i960RN processor cycles consumed in transmitting large messages -- around 17 thousand processor cycles per 1.5kbyte (Ethernet MTU) packet. Further investigation reveals that this high cost is due to a combination of i960RN architectural shortcomings, poor buffering strategy in the TCP/IP code running on the iNIC, and limitations imposed by the I20-based host-iNIC interface. We also found room for improvements in the implementation of the socket buffer data-structure. This report presents profiling statistics, as well as code-path analysis that back up these conclusions. Our results call into question the hypothesis that a specialized networking software environment coupled with cheap embedded processors is a cost effective way of improving system performance. At least in the case of the offload implementation on the i960RN-based iNIC, neither was the performance adequate nor the system cheap. This conclusion, however, does not imply that offload is a bad idea. In fact, measurements we made with Alacritech's SLIC NIC, which partially offloads TCP/IP protocol processing to an ASIC, suggests that offloading can confer advantages in a cost effective way. Taking the right implementation approach is critical.
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