Re: new bottleneck section in wiki

[Keld J?rn Simonsen <keld@dkuug.dk>]

On Wed, Jul 02, 2008 at 02:03:37PM -0500, Matt Garman wrote:
> On Wed, Jul 02, 2008 at 12:04:11PM -0500, David Lethe wrote:
> > The PCI (and PCI-X) bus is shared bandwidth, and operates at
> > lowest common denominator.  Put a 33Mhz card in the PCI bus, and
> > not only does everything operate at 33Mhz, but all of the cards
> > compete.  Grossly simplified, if you have a 133Mhz card and a
> > 33Mhz card in the same PCI bus, then that card will operate at
> > 16Mhz. Your motherboard's embedded Ethernet chip and disk
> > controllers are "on" the PCI bus, so even if you have a single PCI
> > controller card, and a multiple-bus motherboard, then it does make
> > a difference what slot you put the controller in.
> 
> Is that true for all PCI-X implementations?  What's the point, then,
> of having PCI-X (64 bit/66 MHz or greater) if you have even one PCI
> card (32 bit/33 MHz)?

My understanding is that this is not true for all PCI-X busses, only for
some.

> A lot of "server" motherboards offer PCI-X and some simple graphics
> chip.  If you read the motherboard specs, that simple graphics is
> usually attached to the PCI bus [1].  So what's the point of having
> PCI-X slots if everything is automatically downgraded to PCI speeds
> due to the embedded graphics?

I think there are some mobos that have both PCI-X and PCI busses.

> I read some of the high-level info on the Intel 6702 PHX PCI-X hub
> [2].  If I understand correctly, that controller is actually
> attached to the PCI express bus.  So to me, it seems possible that
> PCI and PCI-X could be independant, and that PCI-X will compete with
> PCI-E for bandwidth.

Yes, that is possible.

> [1] The ASUS M2N-LR has PCI-X (via the Intel 6702PHX) and an
> embedded ATI ES1000 video card.  The ES1000's specs say it has a PCI
> bus interface.
> ES1000: http://ati.amd.com/products/server/es1000/index.html
> 
> [2] http://www.intel.com/design/chipsets/datashts/303633.htm

best regards
keld