[vortex] DIFFERENT MAC ACCESS

[Bogdan Costescu]

On Tue, 4 Dec 2001, Markus Fraedrich wrote:

> It is possible to give one station a higher priority on the bus with setting
> the IFS of the other(s) station(s) to the maximum IFG (IFS+448BIT).

I only read once sometime ago some docs on the Scyld site which detailed 
how data is put on the wire, so I'm in no way expert in this. I believe 
that if you use a switch, the IFG only affects the physical link 
between the card and the switch, but will in no way be related to other 
links (between the same switch and other cards). I have no ideea about 
what happens when you use a hub.
Not related to IFG: I think that there are switches were you can define 
QoS for certain ports and/or types of traffic. This might in fact work 
much better for you.

> The higher priority is for realizing a mpeg stream, and should be only for
> packets of this streaming.

So you want to change IFG in real-time depending on whether the packet is 
part of a stream or not ? This sounds very bad - drivers are actually 
trying to get away from using I/O within the hot-path and you want to 
introduce (quite a lot) more I/O ?

> Im not sure if it is possible to reach this with the DPD1 setting of the NIC
> or if it is easier to implement this in a QoS Class (more Kernel specific).

I'm not familiar with the in-kernel QoS. However, it's done at higher 
levels than those that we discuss here - you will not be able to skip 
packets the are currently in the driver Tx queue, for example. Maybe 
you should consider Don's suggestion: replace the card with a faster one 
(Gigabit) - then even if you do QoS at higher levels, at least the driver 
Tx queue will get cleared faster.

Another suggestion: a hard real-time OS.

-- 
Bogdan Costescu

IWR - Interdisziplinaeres Zentrum fuer Wissenschaftliches Rechnen
Universitaet Heidelberg, INF 368, D-69120 Heidelberg, GERMANY
Telephone: +49 6221 54 8869, Telefax: +49 6221 54 8868
E-mail: Bogdan.Costescu@IWR.Uni-Heidelberg.De