> Gerard Roudier wrote:
> > 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.
>
> The same numbers are used on Ultra narrow or Ultra-Wide cards. At 40MB/s,
> Ultra wide uses roughly 30% of the available bandwidth. But, I had that in
> my last email, I also stated in my last email that the value of 8 in the
> MAX_LAT register was wrong on the 7880 chips :)
I had 312 messages in my mailbox this evening and each of yours must be
counted as sized as 10 times the average message size. :-)
So, I may have missed that. I remember you spoke about old boards and
I did'nt know that the 7880 was so old.
[ ... ]
> > 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.
>
> The MIN_GNT and MAX_LAT values aren't about sharing. They aren't about
> fairness. They are about what it takes for that particular device to
No fairness requirement leads to PCI devices being allowed to speficy 254
as MIN_GNT and 1 as MAX_LAT that makes these PCI parameters irrelevant.
> operate at 100%. Anything else in these registers would be a lie and would
A device telling that it may achieve 100% when the PCI bus is dedicated
to itself is not a lie but _the_ truth, since any other device, even slow
will have some impact on performances.
> actually make it *harder* to get the correct performance out of the PCI
> bus. You don't want your cards being fair about PCI usage when specifying
> what they *want*, you want them to ask for what they need. Whether or not
> they get it is another matter.
You are playing with words in a insane way.
Here are my needs:
1) If I get 1000 M$ I will be 100% happy.
2) If I get 10 M$ I will be 99% happy.
3) If I get 1 M$ I will be 98% happy.
I _want_ 1000 M$, but I _need_ only 1 M$ to be quite happy. The
additionnal 999 M$ that I want are just wasting money for you with regards
to their efficiency to make me happy. So 3) is a lot more relevant for
you if you want to make me extremally happy. ;-)
In my opinion, PCI devices are expected to be as fair as I am when
specifying their needs, in order to allow the PCI BIOS to make a
reasonnably good work.
[ ... ]
Regards,
Gerard.
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