> For example, I have two different 3950U2B controllers in my machine right
> now. Each controller has two separate PCI functions. Each function reports
> MIN_GNT as 39 and MAX_LAT as 25. Multiply that times 4 and what do you
> get? Impossible to meet. Why are they so particular, well, each funtion is
> a separate Ultra2 wide SCSI controller and they operate entirely
> independantly of each other, and fully in parallel, so the four channels are
> capable of 320MB/s of data transfer. That number is so far above the PCI
> busses 133MB/s there isn't a chance in hell that the PCI bus could keep up
> with all four of them. But, that doesn't mean the MIN_GNT and MAX_LAT
> values are wrong, just that the devices are fast enough that the PCI bus
> can't possibly keep up with all of them.
Doug,
Thanks for your explanation of Adaptec interpretation of PCI.
The initial post was about the 7880U that looks like an Ultra Narrow
controller and, so, is only able to use 15 % of the PCI BUS bandwitch.
This controller is specifying a MAX_LAT of 1.92 micro-seconds.
My opinion is that any acceptable interpretation of MAX_LAT leads to
either this value being bogus, or that controller not being able to
share efficiently the PCI bus with _common_ PCI devices.
Your essay about 4 Ultra2 Wide (320 MB/s throughput) installed on a modest
133 MB/s PCI BUS is kind of off topic and makes no relevance.
(BTW, such a system is probably unique on the planet is likely something
that tests the limits of the PCI bus and certainly not the SCSI
controllers performances).
Regards,
Gerard.
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