[NTLUG:Discuss] Network question

[Scott Ripley]

Brian wrote:
> 
> A few days I posted a question as to why the network speed between two
> Linux boxes linked via two 10BaseT nics (one PCI, one ISA) was so
> abysmal (on the order of 1 or 2 Kbps). 
<SNIP>
> Is it possible for a PCI nic to "flood" an ISA nic?  Would it be worth
> my time to replace the ISA nic with a PCI nic?

As noted before, an ISA bus runs at 8Mhz, and PCI runs at 33Mhz. The
66Mhz that Kevin noted before was probably his mainbus or AGP port.
Also, the ISA bus is 16-bit while PCI is usually 32-bit (usually ~=99%,
some server mainboards have 64-bit PCI busses). In theory, an ISA 10B-T
NIC can completely saturate its connection, but that rarely happens.
Most ISA cards offload much of the work to the CPU, while PCI NICs
_generally_ have more onboard proccessing. Even having said this, the
rates you noted above are far too abysmal to be a result of this. There
are some ISA cards like the WD8003 (8-bit) that are notorious for bad
performance. If your card is one of these... Change it.
 
> FWIW, I discovered if I set the mtu on the PCI nic to 256, and the
> txqueuelen to 6, I could "put" to the ISA nic at a rate of about 540
> Kbps.  This seemed to the peak rate I could achieve by tweaking these
> two parameters.

Usually dropping the MTU will help with bad connections, as they don't
have to retransmit failed packets as often, and they retransmit smaller
amounts. 

In your earlier message, you noted that this happens after a couple of
hours of heavy network usage. The ISA card could be failing/overheating.
If this is the case, replace the NIC.

But...

Before replacing any hardware, I would check the cheaper/free (as in
beer) solutions first. 10B-T uses different pairs of wires for TX and
RX. Bad cables or ports can cause framing errors or dropped packets. You
could try switching the cables you use to connect each NIC to the hub.
If the problem changes, this could be a hint. Also, try using different
ports on the hub. This can rule out both before you have to spend any
money. 

Also, since you are going in to a hub (not a switch), make sure that
your NICs are set to half-duplex operation. Full duplex on a shared
media can cause strange things to happen on small networks (on larger
networks, it will generally just fail). If you are getting a huge
percentage of collisions, this could be the case. You can usually see
how the NICs are set up during boot in /var/log/messages,
/var/log/boot.log or /var/log/syslog depending on your distribution.

NOTE: on shared ethernet there will be a number of collisions. Ethernet
was designed to use CDMA (Carrier Detect Multiple Access) which uses
collisions to know when a NIC can't transmit. I don't know the exact
percentages of collisions to expect right off hand, but 50% would
definately be a bad thing.

Disclaimer: I pulled all of this from my feeble brain, and could be
completly wrong.

Good luck,

Scott 

 _____________________________________  ___
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