Hi,

I have a weird problem with a 10/100Mbit-Realtek-NIC (Chip on the card reads "8139D", lspci reports "RTL-8139/8139C/8139C+")

If used in combination with a Supermicro X7SBE, the network card receives about 100Mbit/s, but is only able to transmit around 30Mbit/s. The NIC reports Full Duplex 100Mbit/s.

I have a few of these available for testing, and this behavior is reproducible - but only if used together. Any other NIC (other Realtek, VIA, Intel) will work just fine, as does the 8139D if placed on another board.

What I've done so far:

1. Looked into IRQ-related stuff. Doesn't seem to be an issue.
2. Upgraded the BIOS - no change.
3. Tried tweaking with ethtool. The card only supports very basic stuff like autoneg on/off and fixed speed settings. Changed these settings around a bit - no effect.
4. Changing PCI parameters. Changing the PCI-Slot. Nothing.
5. Tried different Kernels / Debian Linux Versions. Nope. (Currently on Squeeze with 3.2.0 bpo)
6. Tried to use 8139cp instead of 8139too. Didn't work, 8139cp doesn't grab the NIC.

Multiple reboots and smashing my head on the keyboard also didn't help.

I'm at a bit of a loss here, any hints appreciated :>.

not an answer, but have you tried to adjust the settings of the card using mii-tool? I vaguely remember having to use it instead of ethtool with some realtek chipsets. also, have you done a trace with tcpdump to maybe see if packets are getting dropped at the card?

No errors or funny stuff.

Nope. Read something about mii-tool being deprecated, so I didn't try. Should I?

It could be hardware related. Does ifconfig report errors or dropped frames? If the card is connected to a port on a managed switch, check the port statistics for errors.

This may sound silly, but have you tried replacing the network cable? Cable defects are surprisingly common.

i've had to use it on older cards that ethtool wouldn't work with. specifically a linksys lne100tx from ~2000.

Cable is fine.

mii-tool doesn't have a lot of options though. I could only do what I was already able to do with ethtool.

Everything looks fine. No errors. Got some overruns once, but they weren't reproducible.

No errors on the switch side either? Since the problem is related to outbound traffic any L1/L2 errors would be recorded at that end.

What are you using to test throughput? iperf?

Does CPU utilization increase noticeably during tests?

Didn't monitor the other side (currently just a laptop via crossover) yet; will do later and look for errors there.

Nope, just copying random data with scp.

Almost no change in cpu load when copying.

Since you're only seeing poor performance with this particular brand and type of NIC, it clearly is related to the hardware or the driver, or how those interact with other components.

Still, it would be interesting to see how it performs with iperf or any similar testing tool, since that kind of test basically excludes absolutely all other hardware, subsystems and drivers as possible problem sources.

iperf confirmed the measured data rates. RX ~95 Mbit, TX ~30Mbit

A -vv tcpdump on the receiving side showed something interesting though: An unusually high amount of incorrect TCP checksums. Currently googling possible reasons.

Sooo... Some suggest turning checksumming off.

Problem: I get an "operation not supported". As posted before, ethtool does have many options, but the Realtek NIC supports only the really basic stuff. How can I change these parameters?