PCI Express card slot assignements

neiltstratton · 23 Jun 2015

Hi all - I have serached high and low for advice on this but have only succeeded in confusing myself.
I build media servers for my own use in live events and have a question that has bugged me for years.
I have an Asus Sabertooth motherboard, and my graphics card is a Radeon 7970. I have two further graphics cards which are used to capture camera and computer. The latency on the camera capture is pretty bad to be honest even with all the best components. I was wondering if someone could steer me as to where my cards should be sited on the motherboard?
The 7970 sits in the main PCIEx16 slot.
The other cards I have are a Blackmagic Decklink SDI Card Type:PCI Express 1 lane, compatible with 1,4,8 and 16 PCIe slots
I also have a Datapath RGBE1s which is a PCI Express x4 card.
The motherboard spec is
2 x PCIe 3.0/2.0 x16 (Single at x16, dual at x8/x8)
1 x PCIe 2.0 x16 (max at x4 mode, black) *1
3 x PCIe x1
The PCIe 2.0 x16 slot shares bandwidth with PCIe 2.0 x1_3 slot and USB3 E1/2 ports at back. The PCIe 2.0 x16 slot default setting is in x1 mode.
Can anyone give me some advice on the best siting for the cards?
I would be very very grateful indeed as my brain is in knots trying to work this out
Many thanks
Neil

Lady Fitzgerald · 23 Jun 2015

With that board, you don't have enough PCI-e lanes to go around on without losing your rear USB 3.0 ports or losing a bit of performance on your 7970. The 7970 goes in the red x16 slot. The Blackmagic can go in either the top or second x1 slot.

The only way you will be able to run the x4 Datapath without reducing GPU peerformance is in the bottom slot, in which case, you would have to reset the slot to x4, which means you would lose the third PCI-e x1 slot and two USB 3.0 ports on the rear. If you need those rear USB 3.0 ports, then you would have to run the x4 Data path in the white x16 slot. That will cause your GPU to run on only eight lanes (x8). However, the performance loss won't be 50%. At worst, it would probably be only 10-20%.

PCI-e lane distribution can be difficult to sort out. Some boards offer more flexibility than others. Some boards use additional chips to run ports like USB 3.0 and SATA; others steal lanes from PCI-e slots. Because I needed more lanes than a standard board could support (24 lanes), I went with a x79 board to get 40 lanes. Even then, I had problems being able to run as many cards as I wanted (I wound up getting two external TV tuners instead of two internal TV tuner cards) .

GokAy · 24 Jun 2015

I would think x8 for that GPU would not reduce performance with a single card setup. It is easy to test, no?

There is a chart here for a GTX 980 (post #4) - GTX 980 in a PCIE x8 slot - Graphics Cards - Linus Tech Tips
6% loss from PCIe 3.0 x16 to 2.0 x8, 7970 should be much less imho.

neiltstratton · 24 Jun 2015

Thank you so much for your replies. Helped me a lot.
I can sacrifice the extra usb ports so will put the Datapath in the 3rd slot and run at x4.
The Blackmagic card is the one I am trying to improve performance on.It's only a x1 card so dont see any advantage in placing in a x16 slot. Is this correct?
If I leave in a x1 slot the GPU will run at x16.I tested on shared lanes of x8 and as you say no noticeable difference.
Thanks for your help
Neil

Lady Fitzgerald · 24 Jun 2015

neiltstratton said:

...The Blackmagic card is the one I am trying to improve performance on.It's only a x1 card so dont see any advantage in placing in a x16 slot. Is this correct?...

It is correct.

neiltstratton · 24 Jun 2015

Thank you Jeannie