2x 7970 CrossFire PCIe 3.0 @ dual x8/x8

[LegolasElf]

Hello,

I'm sorry if this has already been posted somewhere but I have second GIGABYTE 7970 on the way which I am going to CF with my current one. I have the following mobo:

http://www.newegg.com/Product/Product.aspx?Item=N82E16813131790

So I can see that I will only be able to do PCIe 3.0 @ dual x8/x8

Im not very familiar with the PCIe bus but how is 3.0 at x8/x8 different from 3.0 @x16/x16 (im not sure if this is even possible)

Or how is 3.0 @ x8/x8 different from 2.0 @ x16/x16

Will my performance of 3.0 @ x8/x8 be worse than 2.0 @ x16/x16 or 3.0 @ x16/x16?

Also, from what I gather, my SandyBridge does not support PCIe 3.0 (would need an Ivy Bridge CPU to enable PCIe 3.0 bandwidth) so I will be running at PCIe 2.0 x8/x8 at all times. How does PCIe 2.0 @ x8/x8 compare to PCIe 3.0 @ x8/x8? Marginal performance difference?

Thank you for any help

 

[Bobnova]

8x/8x 3.0 is only minimally hampered. 1-2% if I recall correctly, at most. Probably less.
It's the same as 16x/16x 2.0.
8x/8x 2.0 you'll lose a couple more percent, but really nothing worth worrying about IMO.

 

[Ajay57]

Well +1 ^^Bob there is little difference to worry about over nothing really and to prove it just watch this Video! When you finished they are other ones saying the same thing. AJ.

1,

Just slot your card in the Mobo and enjoy it!

 

[LegolasElf]

Ok so right now a the moment with my current CPU overclock in my sig and a single 7970 GHz card, my 3DMark11 Performance score is 9479. What kind of ballpark score am I looking at after I slot in the second 7970 and enable crossfire?

 

[Bobnova]

Looks like 14k or so in 3d11 performance.

 

[LegolasElf]

Looks like 14k or so in 3d11 performance.

Im seeing a lot of threads of people seeing absolutely no performance increases using crossfire. I am aware of the fact that not all games today are optimized for crossfire. Still makes me nervous though.

 

[Bobnova]

I can't speak for that as I haven't used a 7970 for anything other than benching.
In benchmarks, they scale pretty nicely.

 

[Ajay57]

Well do what the guy did in the video you already half way home, just slot it in install what you need! Then fire it up and rerun the test again like he showed in the video, will sure give you a answer? AJ.

 

[wagex]

they should scale plenty well...
http://www.anandtech.com/bench/Product/588?vs=508

looks like my theory was correct

 

[Ajay57]

Well what do you know Bob on the money 14k is about right for the score. so have a look at this! It should give you a rough idea LE my friend?

1, http://www.techpowerup.com/174864/MSI-dominates-3DMark-11-Performance.html?cp=2

Posted by EarthDog!

 

[LegolasElf]

they should scale plenty well...
http://www.anandtech.com/bench/Product/588?vs=508

looks like my theory was correct

Per this post, it shows the CF cards as AMD Radeon HD 7970 CF - 2x925MHz/5.5GHz 3GB GDDR5. 5.5GHz is the memory clock? I see that the stock clock on my 7970GHz card is 1500MHz. Was that overclocked so far as 5.5GHz???

Sorry I'm a noob.

 

[Knufire]

5.5Gbps, not GHz.

 

[LegolasElf]

Ah I see, sorry.

 

[Bobnova]

To be even more specific: 5.5Gbps/pin (times 32 pins per chip, or 384 pins in total), or 5.5GHz GDDR5 speed.

GDDR5 has two clock signals arranged 90* out from each other, both of which send data when the clock goes HIGH and when it goes LOW. One bus is for commands and command related communication, the other is for actual memory data.
The end result is that information is being passed at 4x the base clock speed, 6GHz (4x1.5GHz) in the case of your card.

 

[M33Cat]

My 2nd 7950 is the air, I could get it by Thursday if I'm lucky, this ones the gigabyte 3x fan @ 900, it will go nicely with my powercolor 2xfan @ 880.

I'll be running them @900MHz and in 8x/8x mode

 

[LegolasElf]

Few last questions guys: 1 CrossFire bridge or 2? Whats the difference between using 1 vs 2? Redundancy? Whats good practice?

 

[wagex]

i know with older cards using two bridges for two cards would cause problems, idk if they fixed it or not. the second set is for tri fire / quad fire

 

[Bobnova]

Really? I've always run two when I had enough bridges and slots for 'em.

 

[wagex]

Really? I've always run two when I had enough bridges and slots for 'em.

yea its been since the 48xx series since ive ran crossfire so things could have changed, and it didnt always mess up just out of the blue it would start artifacting and stuff on me lol.

 

[Janus67]

I've only used 1 before with no issues.