Background On Issue: I recently upgraded to a 30” Dell monitor and started playing Battlefield 3 on 2056x1600 at 60 hz. The downside to this was that I had to take the graphics down to low in order to get frame rates that I could stand. So I decided recently to invest in a second HD 6950 to crossfire in order to get the graphics quality back to what I was used to. I inadvertently purchased a used 1 GB and my current card is a 2 GB but decided to XFire them anyway and see what happens (I had 14 day return time.) Also, after researching it online for a while, I was unable to find anything that said you could not crossfire the cards, only that both cards would utilize only 1 GB of VRam each. So I figure two 1GB HD 6950 in crossfire should beat a single 2GB HD 6950, right?
I have easily done 5 hours of reading and 15 hours of troubleshooting including any tweak/setting I could find online and cannot seem to get these things working properly. All of my equipment is Crossfire capable according to manufacturers. I have reconfigured them on the motherboard, and verified that both cards work independently. I have tried clean Catalyst driver swaps of all recommended versions including 11.10, 11.10 + 11.9 CAP 4, 12.1, 12.4, 12.10, 13.1, 13.1 + 12.11 CAP 2… each driver set gave me a little bit different result but no success. I have installed latest Direct X, BOIS on MOBO, & disabled Intel Hyperthreading. I have played with every graphics setting/clocking setting available in game, on CCC, and elsewhere as well as countless other BOIS/windows/etc. tweaks.
Nothing I can do will give me any marked improvement over disabling Crossfire and running off of the single HD6950 2GB. With certain drivers I was able to achieve a slightly better graphics setting, but with marginally better performance and usually stability problems (stuttering, etc.) According to Afterburner on screen, both cards hover around 50% rarely making it up to 70% or beyond (also, I only have access to GPU 2 memory at around 2 GB whereas I have seen Afterburner crossfire screenshots online where it shows both GPU 01 & 02 mem. usage.)
If anyone can offer any suggestions on how to get these cards working properly I would truly appreciate it. I have 2 days to decide whether or not to return the second card. I would be happy to provide logs/screenshots/video dumps.
Three related specific questions:
1.) Is this proof that one cannot crossfire a 1GB and a 2 GB card?
2.) When two cards are in crossfire, should GPU usage be in the 90%s while in game, similar to the GPU usage of a single card while in the same scenario?
3.) GPU-Z indicates that one card is in 16x PCIe and the other is in 8x PCIe. Could this be a problem and does anyone know how to fix it (mobo spec. says both slots are 16x.)
- Sapphire ATI Radeon HD 6950 (2) 1 GB & 2 GB
AMD Catalyst Driver Version, and Driver History
- 13.1, 12.11 CAP 2
- Windows 7 64-Bit, DX 11, .net 4 Framework
Motherboard or System Make & Model
- ECS X58B-A, latest BOIS (v 05/04/2010)
- Corsair TX750W
Display Device(s) and Connection(s) Used
- Dell 3007WFP (2056x1600, 60hz)
- Intel i7 x58
System Memory Type & Amount
- (3) Kingston DDR3 2gb
- Antec 900 Case, Coolermaster v8
I would return the card. I believe you would be better off getting a card with the same ram as the other. This will help with the instability you are facing. On both platforms nvidia and amd it creates issues from what I've read through the forums with people doing something similar.
I think the responses are somewhat on the right track with the RAM limitations but there are other things going on here... see below.
Well, the card is going back... I'm convinced that, while many say that ATI cards of different RAM capacity can be linked with crossfire, I believe this is not accurate, at least with the Sapphire HD 6950.
As I described briefly in the initial post, in MSI Afterburner as well as other hardware monitors, with the 1GB+2GB 6950 cards in crossfire I see only GPU2 memory, and in-game it is ALMOST ALWAYS around 2000MB. This directly refutes the claim that each card would operate at 1 GB of effective VRAM if put in crossfire.
Furthermore, with more settings tweaking I was also able to achieve 90%+ GPU usage on both cards by turning graphics settings on BF3 to "High" and then in "Custom" also turning on 2x MSAA. IN FACT, I was able to sustain good frame rates in high player count smaller maps at these settings (usually GPU2 mem usage was around 1500MB) which would never be possible with the single 6950 2GB, proving that the crossfire setup did provide for additional graphics proccessing capability. BUT in any large conquest map with many players and open areas, the GPU2 mem usage was always max.'ed out at +/- 2374MB, EVEN AT "LOW" GRAPHICS SETTINGS. When I reverted back to my 2GB HD 6950 ONLY (physically removed the other card) the memory usage in the same map with the same amount of players never exceeded 1200MB.
For these reasons, I believe that the HD 6950 (and possibly other ATI cards) with different memory capacity CANNOT BE UTILIZED WITH CROSSFIRE and I believe it is time to inform people of this so they do not have to waste the kind of time and money I did to figure it out.
CONCLUSION: DO NOT BUY A SAPPHIRE 1 GB ATI HD 6950 FOR CROSSFIRE WITH A 2 GB AS YOU MAY, AND PROBABLY WILL, EXPERIENCE PERFORMANCE ISSUES.
Other card manufacturers and ATI card models may have similar issues but I have not tested any other setups to vouche.