[CentOS] Ethernet poor performance

Warren Young warren at etr-usa.com
Wed Jul 2 17:14:43 UTC 2008

Robert Moskowitz wrote:
> I get pings around 60ms.

Pings within the same LAN?  If so, that's slow even for 100BaseT.  It 
should be under 10 ms.

> When I switch the cards around, the addon card attached to my
> network, I get pings that alternate with one being ~1488ms and the
> next 488ms!  This is regular.

Have you removed the cabling from consideration?  And, do you know you 
have good cabling, tested as following the wiring standards?  Hand-made 
cables are particularly suspect, if made by someone who doesn't know 
what TIA/EIA 568 is, for example.

> I have played around with the IRQ for the 2nd card.  The 1st card I
> tried I was limited to 9,10, or 11.  The card I am using now allows
> 3, 4, or 7 (btw, I have disabled serial and parallel ports).

None of this should matter in the PCI world, unless you're using really 
old hardware, which didn't properly support IRQ sharing.

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

grep eth0 /etc/modprobe.conf

or scan through the output of the dmesg command, looking for stuff that 
appears near "eth0".

>> - 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).

That covers speed, but doesn't say anything about the duplex setting. 
You should see this mentioned near the Ethernet driver load lines in 
dmesg, too.

> ho, ho. MTU of 1500. Is this not doing MTU path discovery?

PMTU-D is done per-connection at the TCP layer, not statically down at L2.

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