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)?
>
> 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?
>
Server class boards have multiple PCI buses (2 or 3 usually), so the
PCI-X slots are on a different bus.
> 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.
>
Newer workstation (and probably server boards) have the PCI-X slots on a
PCI-E "bus" (hence you can actually get boards with PCI-X slots for a
reasonable price nowadays). In actual fact PCI-E doesn't have a bus as
such, all the connections are point-to-point so there's no bandwidth
contention at that stage. You may get contention afterwards though,
depending on how the PCI-E lanes are connected to the processor. AMDs
HyperTransport is supposed to be very good for this, whereas Intel
systems tend to suffer more (or did - I've not looked at this recently
so they may have moved on now).
Cheers,
Robin
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