[rescue] Bus Speed v I/O rules of thumb?

[Joshua D Boyd]

On Fri, May 17, 2002 at 12:23:54PM -0700, Francisco Javier Mesa-Martinez wrote:

> Of course a bus is a shared medium, so the more stuff you hang to it, the
> more the performance is going to suffer. That is why putting 2 scsi boards
> + 2 FDDI + a fast ethernet + .... on a single bus is not a good idea

I would expect that a SS10 level machine would be able to handle that sort
of bus load.  25mhz*4bytes wide == 100megabytes/second== 800megabit/second. 
Assuming that there is no DMA (is there DMA?  I don't remeber), then those
devices will require 940megabits/second to run flat out.  Obvious problem,
assuming no DMA and no bus overhear, we are still 140megabit short. In 
reality, chances are unlikely that more than 2 network interfaces would be 
running flat out, and so it is likely that at any given time, no more than 
say 500megabits would be asked for from the machine, which shouldn't cause it 
to break a sweat, unless the bus overhead is much worse than I would have
expected (I expect about 25%).

At least, that is my thinking.  I could be wrong.[1]  

-- 
Joshua D. Boyd

[1] After several reedits to correct math errors, I realized that DMA isn't
    usefull here, rather it would be something like I2O, which is unlikely
    to be used.