[CentOS] Ethernet poor performance

nate centos at linuxpowered.net
Wed Jul 2 17:15:23 UTC 2008

Robert Moskowitz wrote:

> Intel the first time, DLink (Realtek) currently. Both 10/100Mb

There's a lot of different Realtek chips out there, can you
send the output of lspci -v (and capture the network card only since
it spits out a lot of output). I've heard lots of bad things over
the years about Realtek cards though my personal experience with
them has been fine(last one I used I bought probably 6 years ago).

>> - What driver is it using?
> How do I tell?

Check in /etc/modprobe.conf, also you can include the output of

>> - Can you verify that the speed and duplex settings match on both ends of
>>   the connection?
> The switch has its 100Mb LED on. One of the switch ports has my
> Speedstream router which is only 10Mb, so we can believe the 100Mb LED.
> This is a dumb switch (my public network, so I am not going to plug into
> one of my Procurves).

Since it's a dumb switch there's probably no way to confirm duplex settings.
One thing you could try is hard setting the NIC to the various different
options and see if things improve. It could be that auto negotiation is
failing so they are running at mismatched duplexes. Which depending on
the quality of the switch will depend on the results. About 10 years ago
I had a 3COM network card and a Linksys switch that just refused to
co-operate. Of course 3COM blamed Linksys and Linksys blamed 3COM. Ended
up going with a netgear switch and that 'fixed' it.

You should be able to use the 'ethtool' command to force duplex/speed

> No and no.
> eth1 Link encap:Ethernet HWaddr 00:50:BA:42:82:49
> inet addr: Bcast: Mask:
> inet6 addr: fe80::250:baff:fe42:8249/64 Scope:Link
> RX packets:1709 errors:0 dropped:0 overruns:0 frame:0
> TX packets:1033 errors:0 dropped:0 overruns:0 carrier:0
> collisions:0 txqueuelen:1000
> RX bytes:535351 (522.8 KiB) TX bytes:117321 (114.5 KiB)
> Interrupt:10 Base address:0xa000

Quite unusual..

> ho, ho. MTU of 1500. Is this not doing MTU path discovery? But these are
> little PINGs so there should not be any frag problems (my WAN link is
> PPPoE, so you want pathMTU or have to hard config everything to
> 1415bytes per the pppoe man pages).

The only experience I have with MTU discovery is with jumbo frames, and
as far as I know MTU discovery is only available for TCP, not UDP or
ICMP. You could certainly manually set your MTU to 1415 with
ifconfig eth1 mtu 1415, but as you say the packets your sending are
tiny anyways so it probably won't have any impact.

>> - Try replacing the card itself? maybe it is bad.
> On my 2nd card already.

Of the same type? Try another brand/model? Get another Intel card,
or a broadcom-based card(3COM or something).

I wouldn't expect IRQ conflicts to be much of an issue with this type
of problem, such tiny amounts of traffic on the card. I can't even
remember the last time I had to deal with IRQs, 4-5-6 years? Things
seem to work pretty well these days.


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