Wednesday, March 11, 2009

Gigabit Ethernet

... is actually quite fast. So fast - in fact - that the bottleneck on both PCs under my desk is currently the PCI bus into which the network cards are plugged. I had no idea that the bus ran at 33MHz and was only be able to transfer 32 bits per cycle (math says: 33M * 32b = 1056Mb). Todo: is this duplex?

There's a very useful FAQ regarding Gigabit Ethernet available here.

In a test with 4 network cards (2 in each machine) I saw the following:

w) 1A -> 2C (93Mb/s)
x) 1A -> 2D (902Mb/s)
y) 1B -> 2C (93Mb/s)
z) 1B -> 2D (294Mb/s)

1A: Gigabit embedded on motherboard (1000 link at switch)
1B: Gigabit on PCI bus (1000 link at switch)
2C: Fast embedded on motherboard ( 100 link at switch)
2D: Gigabit on PCI bus (1000 link at switch)

Machine 1 was sending large UDP datagrams. Machine 2 was not listening, it was just up so that ARP could get the MAC addresses of its adapters (without which, we could not send a datagram).

tests w + y appeared to be throttled by the router as it managed a mixed 100/1000 route
test x was great and showed that an onboard gigabit controller can actually do its job
test z showed the PCI bus being the bottleneck, allowing 3x faster than Fast, but 3x slower than Gigabit.

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