Netperf and Sun Gigabit Ethernet (or other Full-duplex modes of transmission)

Todd Jobson (
Fri, 19 Jun 1998 05:28:13 -0400

Netperf'ers :

I am testing the throughput of a Sun E10000 using their Gigabit Ethernet
adapters (resold alteon cards) using netperf.

A couple of questions about how netperf bases it`s throughput #'s (Mbps).

Assume that for these questions / statements, that I am only using a
cross-over cable between 2 NIC's on 2 separate systems.

- Is the throughput (Mbps) # based strictly on Unidirectional writing
of data and timing it's completion, using some statistical passing
of results... between client and server ?? Would this be considered
half-duplex ?? In either case, if I run netperf on both systems
simultaniously (reporting 190 Mbps on both systems, from each netperf),
does this mean that I add up the 2 system throughput #'s to get the
'FUll DUPLEX' throughput ?? (380 Mbps ??), or does Netperf take into
account and use the timing of both the sending and receiving sockets,
based on the total # of bytes transmitted and received (handshaking)
even in a unidirectional transfer ??

- Given that this Sun VGE driver operates only at a FULL duplex mode,
does this preclude me to the belief that only having one system
transmitting data to another (in a single direction), that this only
can be called 'half-duplex', thus not being allowed to consume more
than half of the bandwidth ??? I don't believe so, but please correct
me if I am wrong. I understood the dynamics of ethernet throughput to
simply allow for a maximum amount of data (bandwidth) to be carried on
the media overall (total of both sides eg. 100Mbps sending, 900 Mbps
receiveing ; if true Gbps were achievable).

This leads me into the question of Collision detection not being
a factor (or even possible) when using 'Full Duplex' ethernet .?
Does this mean that even though no collisions/errors are reported
by the device driver, that collisions could still be occuring ??, or
does this mean that collisions are physically impossible, given that
two separate 'channels' are multiplexed over the media ??? Is this
one of the reasons that 'Full Duplex' ethernet is usually (if not
always) wired so that a switch is the connecting device between nodes,
rather than allowing several nodes to transmit simultaniously without
collision detection ?? Please enlighten me on all of the nitty-gritty
details, as I have not yet received any 'good' explanations as of yet.

For the Sun folks, what are the 10 or 15 'vge' driver parameters exactly
used for ??? Is there an internal doc or docs that describe various
tested configurations ??? I've been told to do different things by
both support and the configuration manuals...

Please respond as soon as possible, since the customer(s) at lucent
are becoming VERY uneasy after spending sooo... much money, not to
get the expected bandwith from Gigabit Ethernet (unless the tools are
deceiving us) ??


Todd Jobson
Technical Manager
Sun Professional Services