[Pc_Support] ABS 8-Port GbE Switch for $51.99 - $20 Rebate ...

[Bryan J. Smith]

Damien McKenna <dmckenna at thelimucompany.com> wrote:
> OK, newbie qn here, but what difference does the frame size make?

At 1,500 byte standard frames, an 10Base node must process at least
1,000 full-sized frames in a second using the full data transfer rate
(up to 14,880 smaller frames per second if payloads are smaller). 
That increases to at least 10,000 frames in a second for FE.  At GbE,
that's almost 100,000 frames in a second!

By using 9,000 byte Jumbo Frames, that reduces the number to under
15,000 frames in a second -- more on-par with FE.

Overhead of incoming UDP/IP datagrams and, especially, TCP/IP
segments can seriously overload the system, resulting in dropped
frames for a node with today's commodity GbE hardware and their small
buffers.  Without 802.3x flow control, this can utterly _destroy_
performance.

The overwhelming majority of commodity TCP/IP off-load engines are
for _transmission_only_.  They are _not_ used for receiption, where
it's really needed.  To get such, you need a $600+ GbE HBA and some
well cached/buffered equipment.

Until then, the best recommendations, in order of cost ...
- 802.3x
- NICs/switches with larger caches/buffers (NICs difficult to find)
- Jumbo Frames, all nodes
- Jumbo Frames, VLANs with localized traffic (separate routing)
- Jumbo Frames, VLANs with layer-3 switching (wire-speed routing)

GbE can be and is quite often implemented like crap.  You do _not_
want to use SOHO networking equipment even in a small business.  More
than a few nodes and you can _toast_ any reason for GbE, reducing it
to even worse than FE performance if you don't even have all
nodes/switches with 802.3x.

Lack of flow control with cheap GbE hardware is the new
"collision-like nightmare."


-- 
Bryan J. Smith     Professional, Technical Annoyance
b.j.smith at ieee.org      http://thebs413.blogspot.com
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*** Speed doesn't kill, difference in speed does ***