990FX chipset SLI Query

[Krish931]

Okay so I'm currently running an AMD system. Specs are an ASUS M5A99FX Pro R2.0 mobo (AMD 990FX/SB950 Chipset), FX-8350 CPU, Dual Zotac GTX970s in SLI, 16 gigs of memory, a Corsiar CX750W power supply. I built this system a couple of months back (my first build), and I've been getting pretty solid performance, so I didn't really think much about any potential, for lack to a better word, discrepencies, in this build.

 

The question I have stems from an article I read about the FX-8350 and the 990FX chipset two days ago (link given below). Correct me if I'm wrong, but as I understand it, AMD's 990FX chipset has 40 PCIe lanes, of which 38 are usable, which means that ideally, if I'm using two GPUs in SLI, both should run at x16.

 

Now, the problem I am facing is, one of my 970s is running at full x16, whereas the other, which should run at x16 if I'm correct, is running only at x4.

 

I am absolutely unaware of why this could happen. And while I'm not complaining about the performance I'm getting from my rig, I still feel a bit perturbed knowing that if both GPUs run at x16, I may stand to benefit from it, rather than the current x16-x4 configuration they're running at at the moment.

 

Any help on resolving this will be greatly appreciated.

 

Oh and this is the link to the article I read

 

http://www.guru3d.com/articles-pages/amd-fx-8350-processor-review,4.html

 

And here are some screenshots I took that may help elaborate my query.

 

Thanks

 

 

[Glenwing]

Okay so I'm currently running an AMD system. Specs are an ASUS M5A99FX Pro R2.0 mobo (AMD 990FX/SB950 Chipset), FX-8350 CPU, Dual Zotac GTX970s in SLI, 16 gigs of memory, a Corsiar CX750W power supply. I built this system a couple of months back (my first build), and I've been getting pretty solid performance, so I didn't really think much about any potential, for lack to a better word, discrepencies, in this build.

The question I have stems from an article I read about the FX-8350 and the 990FX chipset two days ago (link given below). Correct me if I'm wrong, but as I understand it, AMD's 990FX chipset has 40 PCIe lanes, of which 38 are usable, which means that ideally, if I'm using two GPUs in SLI, both should run at x16.

Now, the problem I am facing is, one of my 970s is running at full x16, whereas the other, which should run at x16 if I'm correct, is running only at x4.

I am absolutely unaware of why this could happen. And while I'm not complaining about the performance I'm getting from my rig, I still feel a bit perturbed knowing that if both GPUs run at x16, I may stand to benefit from it, rather than the current x16-x4 configuration they're running at at the moment.

Any help on resolving this will be greatly appreciated.

Oh and this is the link to the article I read

http://www.guru3d.com/articles-pages/amd-fx-8350-processor-review,4.html

And here are some screenshots I took that may help elaborate my query.

Thanks

Which slots are you using? You've probably got it in an x4 slot. Just because they fitted an x16-length plastic socket on the slot doesn't make it an x16 slot in actuality.

[Krish931]

The motherboard has 4 PCIe 2.0 x16 slots, based on what I read in the manual, of which it says two run at 'dual x16/x16 mode', while the others run 'at x4 mode, compatible with PCIe x1 and x4 devices'. It goes on to say that in SLI, use slots number _1 and _3, which I have done.

 

Edit : I've attached a photo of the motherboard manual, if it helps.

[Glenwing]

The motherboard has 4 PCIe 2.0 x16 slots, based on what I read in the manual, of which it says two run at 'dual x16/x16 mode', while the others run 'at x4 mode, compatible with PCIe x1 and x4 devices'. It goes on to say that in SLI, use slots number _1 and _3, which I have done.

 

Edit : I've attached a photo of the motherboard manual, if it helps.

 

If you've plugged them into the 1st and 4th PCIe slots (PCIe 2.0 x16_1 and PCIe 2.0 x16_3) then there shouldn't be a problem. Most likely the software is reading it wrong or the configuration is changing dynamically to save power or something if the bandwidth isn't being fully utilized. NVIDIA doesn't allow SLI to be enabled in x4 configurations, so if you're able to enable SLI at all then you're alright and it's just a software-reading hiccup.

[Krish931]

I am able to enable SLI yes. And if it's a software hiccup as you said, then I suppose it should be alright. But then again, do you know why two separate software could be reading it the same way? Cause then it would lead me to believe what you stated before that it's probably changing the config to save power. Does that mean that it's only when the GPUs are under load that it kind of 'switches' to an x16-x16 config?

 

Forgive me for sounding so jittery but this has been bugging me for the past two days now.

 

Thanks again for your replies

[Glenwing]

I am able to enable SLI yes. And if it's a software hiccup as you said, then I suppose it should be alright. But then again, do you know why two separate software could be reading it the same way? Cause then it would lead me to believe what you stated before that it's probably changing the config to save power. Does that mean that it's only when the GPUs are under load that it kind of 'switches' to an x16-x16 config?

 

Forgive me for sounding so jittery but this has been bugging me for the past two days now.

 

Thanks again for your replies

 

It does the same thing on my system now that I remember; I started ignoring it a long time ago lol. It should switch to at least whatever configuration is necessary for the situation. In gaming realistically you won't need more than 2.0 x8 so don't be surprised if it doesn't go any higher than that

[Krish931]

Okay. I can breathe easy now.

Thanks for your time. Much appreciated

[Krish931]

UPDATE : So I opened up the case and was cleaning out the dust from the components, when I decided, for whatever reason to try and swap the positions of both the cards after a bit of cleaning. I did so, cleaned up whatever I could and fired up the system.

 

Lo and behold, both cards are now running at x16 in SLI

 

I suppose sometimes the best fix is to just clean up the parts in your build

 

Here are some screenshots. Hope this helps anyone with the same problem.

 

 

[sgloux3470]

And that is why you always read your motherboard manual.