TCP Segmentation Offloading

One of the many tasks performed by the networking stack is TCP segmentation - turning a large chunk of data sent by an application into a series of packets small enough to fit within the maximum transfer size. The segmentation task involves performing checksums, making headers to match each segment, perhaps copying the data to assemble the packet, and transfering that packet to the network controller. This work is a significant part of the overhead of sending data over a network.

Some modern controllers, though, have the ability to do segmentation internally. In this case, the operating system passes in a set of template headers and a single, large chunk of data; the adaptor handles the rest. Much of the segmentation work goes away, and a number of smaller I/O operations turn into one big, fast transfer.

As of 2.5.33, the Linux kernel understands segmentation offloading, and the e1000 driver supports it; the work was done mostly by Alexey Kuznetsov and Chris Leech. Some results posted by Scott Feldman show what this change buys. In general, transfers do not go any faster, for a simple reason: the Linux network stack was already able to drive the interface at the speed of the wire. On a send of a long file, however, CPU usage dropped from 40% to 19%. This seems like an optimization worth having.

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