high collisions rates REVISITED

[Vernon Schryver]

In article <9104250127.AA28210 at nazgul.physics.mcgill.ca>, loki at NAZGUL.PHYSICS.MCGILL.CA (Loki Jorgenson Rm421) writes:
> 
> 	OK... no one gave me any particularly useful info about what to
> look for insofar as our net problem goes (recap: high collision rates
> on 4D IRIS servers (2-10%) compared to SUNS (0.1% on servers and clients)
> according to "netstat -i").


3.3 and all previous versions of IRIX count both "deferals" and
"collisions" in the Coll column in `netstat -i`.  A deferal happens when a
machine decides to transmit and discovers that the media is already busy.
A collsion happens when a transmitter notices that some other station is
transmitting while it is also transmitting.  Only collisions are counted in
the Coll column in 4.0.

This ancient characteristic (I'm reluctant to call it a "bug" not just
because I did it but because the deferal rate seems useful) was noticed
during testing of new models that can transmit arbitrarily long strings
of packets with 9.6 usec gaps.  They drove the "Coll" rate up to 90% on
other machines.

10% is not a high collision rate, even if you don't count deferals.


Vernon Schryver,  vjs at sgi.com