fxp driver performance expectations

Paul Marciano pm940 at yahoo.com
Thu Jun 15 20:20:51 UTC 2006

--- Danial Thom <danial_thom at yahoo.com> wrote:
> You couldn't do 100Mb/s with em on a 100Mb/s line
> with min packets, because there are gaps between
> packets so its impossible.

Thanks for the detailed reply Danial.

By 100Mbps I mean line-rate: 148809 packets/sec for
64-byte Ethernet frames + IPG and preamble appended.

The 10/100 fxp NIC is on straight PCI-33.

The 1000 em NIC is on PCI-Express x1.  It can do
100Mbps line-rate (148809pps) in 100Mbps mode.  In
1000Mbps mode it can do ~700Kpps, so the bottleneck
isn't the FreeBSD IP stack.

> Realize that fxp parts are only 32bit/33mhz so
> the bus is a factor. Although its a 1Gb/s bus,
> thats only when bursting, so its really
> substantially less. With shorter packets you
> have more setups and I/O and therefore more
> overhead on the bus.

Yes indeed.

> fxp performs similarly to an em controller when
> they are both on a 32bit/33mhz bus in Freebsd 4.x.
> 5.x is about 20% slower than 4.x, but I expect the
> drivers to be about the same for 5.x as well.

Thanks for that.  I realize that comparing a PCI-33
NIC to a PCI-Express NIC isn't fair.  I don't have a
PCI-33 Gig NIC - why I need outside info.

> Are you using a traffic generator, or are you
> relying on some server to return packets?

Ixia traffic generator.

> We have customers with fxp interfaces on freebsd
> 4.x pushing 90Mb/s+ (while doing a lot of other
> processing also), so its certainly possible.

Yes, I can get 100Mbps using larger frames on the fxp
- but it's the performance limit on the smallest
frames, with the largest per-frame overhead, that I'm
trying to discover.

Thanks again for the reply - I appreciate the info.


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