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Subjecte1000 full-duplex TCP performance well below wire speed
Dear LKML,

We've connected a pair of modern high-performance boxes with integrated
copper Gb/s Intel NICS, with an ethernet crossover cable, and have run
some netperf full duplex TCP tests. The transfer rates are well below
wire speed. We're reporting this as a kernel bug, because we expect a
vanilla kernel with default settings to give wire speed (or close to wire
speed) performance in this case. We DO see wire speed in simplex
transfers. The behavior has been verified on multiple machines with
identical hardware.

Details:
Kernel version: 2.6.23.12
ethernet NIC: Intel 82573L
ethernet driver: e1000 version 7.3.20-k2
motherboard: Supermicro PDSML-LN2+ (one quad core Intel Xeon X3220, Intel
3000 chipset, 8GB memory)

The test was done with various mtu sizes ranging from 1500 to 9000, with
ethernet flow control switched on and off, and using reno and cubic as a
TCP congestion control.

The behavior depends on the setup. In one test we used cubic congestion
control, flow control off. The transfer rate in one direction was above
0.9Gb/s while in the other direction it was 0.6 to 0.8 Gb/s. After 15-20s
the rates flipped. Perhaps the two steams are fighting for resources. (The
performance of a full duplex stream should be close to 1Gb/s in both
directions.) A graph of the transfer speed as a function of time is here:
https://n0.aei.uni-hannover.de/networktest/node19-new20-noflow.jpg
Red shows transmit and green shows receive (please ignore other plots):

We're happy to do additional testing, if that would help, and very
grateful for any advice!

Bruce Allen
Carsten Aulbert
Henning Fehrmann


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