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Cluster Computing Journal

4 (1):7-18, March 2001.
© Kluwer Academic Publishers

Speculative Defragmentation - Leading Gigabit Ethernet to True Zero-Copy Communication

Christian Kurmann
Laboratory for Computer Systems, Swiss Federal Institute of Technology (ETH), CH-8092 Zürich, Switzerland

Felix Rauch
Laboratory for Computer Systems, Swiss Federal Institute of Technology (ETH), CH-8092 Zürich, Switzerland

Thomas M. Stricker
Laboratory for Computer Systems, Swiss Federal Institute of Technology (ETH), CH-8092 Zürich, Switzerland

Abstract
Clusters of Personal Computers (CoPs) offer excellent compute performance at a low price. Workstations with 'Gigabit to the Desktop' can give workers access to a new game of multimedia applications. Networking PCs with their modest memory subsystem performance requires either extensive hardware acceleration for protocol processing or alternatively, a highly optimized software system to reach the full Gigabit/sec speeds in applications. So far this could not be achieved, since correctly defragmenting packets of the various communication protocols in hardware remains an extremely complex task and prevented a clean 'zero-copy' solution in software. We propose and implement a defragmenting driver based on the same speculation techniques that are common to improve processor performance with instruction level parallelism. With a speculative implementation we are able to eliminate the last copy of a TCP/IP stack even on simple, existing Ethernet NIC hardware. We integrated our network interface driver into the Linux TCP/IP protocol stack and added the well known page remapping and fast buffer strategies to reach an overall zero-copy solution. An evaluation with measurement data indicates three trends: (1) for Gigabit Ethernet the CPU load of communication can be reduced processing significantly, (2) speculation will succeed in most cases, and (3) the performance for burst transfers can be improved by a factor of 1.5 over the standard communication software in Linux 2.2. Finally we can suggest simple hardware improvements to increase the speculation success rates based on our implementation.

Keywords
zero-copy communication, networking, speculation, gigabit Ethernet, packet defragmentation

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