PCI-EXPRESS bandwidth vs GPU bus width

[icmacdon]

Hello all,
I have been doing some research on trying to find out if graphics cards with a greater bus width require more PCI-Express bandwidth than those with a narrower bus, example R9 290 (x) 512 bit vs GTX 980 256 bit. Technical explanations would be welcomed. Also would VRAM on board necessitate having higher bandwidth? I am specifically targeting my setup of R9 290x Vapor X 8gb crossfire and the impact (if any) that bus width, vram and xdma communication over the pcie bus would have on performance. All help appreciated.

 

[chenw]

The bus width of VRAM on cards is entirely internal, it has absolutely no bearing on the PCI bandwidth.

As long as you have at least X8 lanes on each GPU on at least PCI-E 2.0 version, you should not notice any PCI-E bandwidth bottlenecks on any card.

 

[Sakkura]

Memory bandwidth is internal on the card, PCIe bandwidth is external, when the card needs to communicate with the CPU or system memory.

I don't think there's any direct link between GPU memory bandwidth and the required PCIe bandwidth, but of course higher GPU performance in general would tend to increase the required PCIe bandwidth.

The interesting thing would be bridgeless Crossfire, because then the cards have to communicate across PCIe. But I haven't seen any indication of PCIe 3.0 x8/x8 having any negative impact on Crossfire configurations. And AMD presumably wouldn't have dropped the bridge if it would lead to performance loss.

 

[icmacdon]

Thank you for explaining that to me. So it seems that XDMA may have some impact only?

 

[Sakkura]

Thank you for explaining that to me. So it seems that XDMA may have some impact only?

Yeah. But since the old Crossfire bridge could actually only transfer at 900 MB/s, it shouldn't be an issue. PCIe 3.0 x8 has a lot more bandwidth available.