crossfire bottleneck?

[Guest]

i have a motherboard (msi 970-g43) has just crossfire and not sli, will this bottleneck 2 r9 280x's? or not?

[Morgan MLGman]

Yes, because the second slot is PCI-E 2.0 x4. 10-15% performance decrease on the second card. You need at least PCI-E 2.0 x8 for the card to not be bottlenecked. It will work though, Crossfire is much more flexible than SLI (a.k.a. better) and it will work like that. Not as good as it could though.

[Oshino Shinobu]

Nvidia has some weird reasons for not allowing SLI on some boards. It supports Crossfire, so the board itself won't bottleneck them. 

 

EDIT: Looks like the second slot will be x4. There will be a slight bottleneck. 

[Jambls]

What CPU are you running?

[Oshino Shinobu]

Yes, because the second slot is PCI-E 2.0 x4. 10-15% performance decrease on the second card. You need at least PCI-E 2.0 x8 for the card to not be bottlenecked

The two blue slots on the board are PCIe 2.0 x16. They should run in x8/x8 in Crossfire. 

[Morgan MLGman]

The two blue slots on the board are PCIe 2.0 x16. They should run in x8/x8 in Crossfire. 

No. The second slot is physically PCIe 2.0 x4. I had one like this on my 970A-UD3 board. It allows crossfire, but you get a 10-15% bottleneck.

Specs of the mobo from the MSI site:

It's a cheap board with a 970 chipset so it has its limitations.

[Godlygamer23]

The two blue slots on the board are PCIe 2.0 x16. They should run in x8/x8 in Crossfire. 

MSI's page says it has one PCIe 2.0 x16 slot and one PCIe 2.0 x4 slot. The second slot is x16 in length only.

[Godlygamer23]

No. The second slot is physically PCIe 2.0 x4.

Electrically, technically since it physically is a PCIe x16 slot.

[Oshino Shinobu]

No. The second slot is physically PCIe 2.0 x4. I had one like this on my 970A-UD3 board. It allows crossfire, but you get a 10-15% bottleneck.

Specs of the mobo from the MSI site:

It's a cheap board with a 970 chipset so it has its limitations.

Ah, I see. I missread the specs. 

 

"- PCI_E2 supports up to PCIe x16 speed

- PCI_E4 supports up to PCIe x4 speed"

 

Didn't see that bit. 

[Morgan MLGman]

Electrically, technically since it physically is a PCIe x16 slot.

That's what I meant anyway, it doesn't have any more pins above x4 size in it It just looks like a regular x16 one.

[RuLeZ]

No. The second slot is physically PCIe 2.0 x4. I had one like this on my 970A-UD3 board. It allows crossfire, but you get a 10-15% bottleneck.

Specs of the mobo from the MSI site:

It's a cheap board with a 970 chipset so it has its limitations.

I doubt that it reaches 15% with "just" two R9 280x, I would say around 8% or even less

[Guest]

What CPU are you running?

 fx 8350

[Morgan MLGman]

I doubt that it reaches 15% with "just" two R9 280x, I would say around 8% or even less

13% bottleneck

18% bottleneck

So I guess it depends on the card, but up to 15% is a realistic expectation. As I said 10-15%.

[RuLeZ]

13% bottleneck

18% bottleneck

So I guess it depends on the card, but up to 15% is a realistic expectation. As I said 10-15%.

But the Fury X and the GTX 980 are a lot more powerfull than the R9 280x, thats why I think that 10-15% is overrating  And by the Fury X is just 8%, not 18%

[Morgan MLGman]

But the Fury X and the GTX 980 are a lot more powerfull than the R9 280x, thats why I think that 10-15% is overrating

It has nothing to do with the "power" but with the bandwith, 280X has actually a wider memory bus than a GTX 980 (384 bit vs 256 bit) afaik so because of that I'd expect similar results (not 100% sure if that's the reason though)

[RuLeZ]

It has nothing to do with the "power" but with the bandwith, 280X has actually a wider memory bus than a GTX 980 (384 bit vs 256 bit) afaik

The Fury X just had 8% of botteneck, you read the graphic wrong

[Morgan MLGman]

The Fury X just had 8% of botteneck, you read the graphic wrong

Yeah, my bad, still, it has HBM so I guess it might be a little different

[Godlygamer23]

It has nothing to do with the "power" but with the bandwith, 280X has actually a wider memory bus than a GTX 980 (384 bit vs 256 bit) afaik so because of that I'd expect similar results (not 100% sure if that's the reason though)

You are correct with the technical specifications between the 980 and 280X.

However, I wonder if the 280X is powerful enough to utilize the full bandwidth.

[Morgan MLGman]

However, I wonder if the 280X is powerful enough to utilize the full bandwidth.

I'm not sure, I guess regardless he still can expect a bottleneck, so I'd recommend upgrading the board to some other cheap solution that has more appropriate PCI slots like ASRock 970 Performance (he'll also be able to overclock on that board, 8+2 Power Phase and not that... monstrocity that's on the G43)