Why connect the cards in SLI and Crossfire?

[Hyp-X]

Lately, I've been thinking about why do nVidia and ATI bothered to make a custom connection between multiple cards in the system when there's PCIe.

I mean let's see in case of crossfire the interconnect has the bandwidth of about 330MB/s.
Using the PCIe 8x connection with it's 2GB/s bandwidth would be much better.
Actually it would be possible to send only half the data because one card usually produces half the data.

So why do they make incompatible, exotic and sucky connections instead of using what is already available?

 

[Dave Baumann]

Hold that thought...

 

[tEd]

Hold that thought...

how long....?

 

[Martin Eddy]

Hold that thought...

Don't ya just love DB.

 

[bigz]

Hold that thought...

 

[BRiT]

how long....?

Until Oct 5th.

 

[tEd]

Until Oct 5th.

I would have guessed next week. Crossfire launch article?

edit:
actually next week is in 2 minutes here so he may not have to hold the thought all that long

 

[Kombatant]

So why do they make incompatible, exotic and sucky connections instead of using what is already available?

Without giving it much thought, I'd say that the PCI-e bus would get saturated pretty easily (I imagine there's some texture replication in both cards' vram)

 

[Basic]

My guess(es):
Guaranteed bandwidth, and low latency (or at least low jitter in latency).
Not being dependent on good mb drivers for inter-PCIe communication.
Maybe there's some overhead that eats more out of the PCIe bandwidth than what you get as raw bandwidth between the cards.
Take your pick.

It doesn't seem like any of those reasons should be written in stone. But for now, with current motherboard HW/drivers, it might be good reasons.

 

[wireframe]

Using the PCIe 8x connection with it's 2GB/s bandwidth would be much better.Actually it would be possible to send only half the data because one card usually produces half the data.

Sounds reasonable bandwidth-wise when you consider that upcoming chipsets will allow for dual PCIe x16 instead of the current x8 mode that is imposed with dual cards. Looking at it from purely a bandwidth persepctive, that means you get a "free" additional 2GB/s in each direction to play with.

 

[Rys]

Hold that thought...

Depends on the direction of the wind, the driver revision you used and the position of the planets in your star sign, though....

 

[Pete]

how long....?

I think until tomorrow (the 26th).

But Rys' comment flew way over my head.

I was told (possibly by Rys) or read that SLI/Xfire don't use PCIe to combine images because of latency issues, IIRC, but we'll find out for sure in a few hours.

 

[IgnorancePersonified]

I thought the sli connector was a marketing or product differentation gimic alone.

Not sure about crossfire. dingle dangle dongle sounds a bit cumbersome to me.

 

[Graham]

http://www.theinquirer.org/?article=24720
according to that, the 6600LE can do SLI over pci-e.

Don't know how accurate it is though.

 

[SsP45]

I thought someone here in B3D showed that the SLI connector actually had 32+ GB/s of bandwidth.
________
Herbalaire Reviews

 

[IgnorancePersonified]

hmmm didn't see that. at 1st I heard roughly the same bandwidth as one pci-e lane, then proabbly about 10x that was the last I heard.

 

[ChrisRay]

http://www.theinquirer.org/?article=24720
according to that, the 6600LE can do SLI over pci-e.

Don't know how accurate it is though.

Yes technically any SLI capable card can do SLI over PCI-E. However disconnecting the bridge chip will cause performance slow downs. And the non SLI capable 6600's dont seem to scale as optimally as bridged solutions. It can technically be done it just isnt worth the deficit in performance.

 

[gunblade]

Yes technically any SLI capable card can do SLI over PCI-E. However disconnecting the bridge chip will cause performance slow downs. And the non SLI capable 6600's dont seem to scale as optimally as bridged solutions. It can technically be done it just isnt worth the deficit in performance.

That is just a lot more elegant that putting extra hardware like a TMDS receiver chips and a FPGA chips on the board to composite the image. The receiver and FPGA chips together will add close to $8 (depend on quantity) to the BOM, let alone an extra PCB design and SKU.

 

[Richard]

People who invest in SLI/XFire rate elegance and price a lot lower than raw speed. They're still important but if you're paying twice the price for a "3D accelerator" then your first expectation is not to get an elegant but slow part but instead balls to the wall performance.

 

[Karma Police]

I thought the new 80.xx drivers were going to allow bridge-less SLI.