Where did all my packets go?

[torkell]

Okay, so recently I've been tinkering with Linux on an old IBM Aptiva (named Odyessus, after the guy in Greek mythology who had to go through all those trials to get back home). And I decided to enable Samba, that being the most sane way to share files between Linux and Windows 2000. And the networking performance was non-existant.

Turns out that while the network card in there (Netgear EA201) claims it can do 10M/full-duplex, my cheap'n'cheerful 5-port switch (10/100 full/half duplex autosensing) doesn't speak the same lingo as it and so runs in half-duplex mode. Which results in an absolute mountain of collisions that the card never hears about (because it thinks it can speak when the switch is speaking), and did you know TCP really doesn't like it when something like every other packet vanishes off to never-never land.

So, I forced the card to run in half-duplex, whereupon the collisions vanished execpt under extreme load, and network performance went through the roof. Networking is a very strange beast indeed.

Comments

[pewterfish.livejournal.com]

Eh, no major surprise there. There's no such thing as a free lunch: full-duplex performance in Ethernet is done by castrating the CSMA-CD mechanism and using the error-reporting back-channel to carry data. Net result, double the speed in uncongested networks, but truly dire performance if everything isn't just-so.