SLI / CrossFire FAQs

[Maziar]

Yes,that's a very common question.I've talked about it in the first page
" note about Motherboards
Some Motherboards support Multi-GPU Technology at Dual x16 mode, some support it at Dual x8 mode and some support it at x16x4 mode Etc, what are the differences?
Well for getting the best performance out of Multi-GPU cards,you need a Motherboard that supports Multi-GPU at dual(Or more) 16x mode like Nvidia 790i ULTRA SLI, Nvidia790i SLI, Nvidia 780i SLI, Nvidia 680i( As i said, NVIDIA 790i ULTRA SLI ,790i SLI ,780i SLI and 680i SLI support 3-Way SLI too(Nvidia 680i LT SLI chipset has 2 PCI-E 16x slot which supports 2-way SLI only.),Nvidia 590i SLI, Nvidia nForce4 SLI 16x, ATI CrossFire Xpress 3200,Intel X38,X48 and X58.

Dual 8x is a very good configuration too,it doesn't have alot of difference(performance wise) with Dual 16x and its cheaper too.
Chipests like Intel P55*,Intel P45 Nvidia 750i SLI, Nvidia 650i SLI,NVIDIA 570i SLI, NVIDIA nForce4 SLI, ATI Radeon Xpress 200, Intel 975x and ONLY few Intel P35 boards(Like ASUS BLITZ FORMULA and ASUS BLITZ EXTREME) support Multi-GPU at dual x8 mode.

However,there are some expections too,for example EVGA has a 750i Motherboard(The model of it is
EVGA 750i SLI FTW)Which supports SLI at dual x16 mode unlike many 750i Motherboards which support SLI at dual x8 mode.

The last one is x16x4, this one is the weakest and doesn't perform very good compared to dual x8 or dual x16,so i won't recommend it for Multi-GPU configuration. Intel P35 and P965 chipsets support Multi-GPU at x16x4 mode.
However,the statement above is true with PCI-E 1.0 cards,because PCI-E 2.0 cards are different.And if you use 2(or more) PCI-E 2.0 cards in multiGPU mode and at x16x4 speed,they will perform close to dual x8/x16 mode.

Here is a good review which compares CrossFire performance between dual x16,dual x8 and x16x4
http://www.tomshardware.com/reviews/crossfire-pci-express,2095.html "

 

[armageddon528]

I now have 2 HD4870: First is Palit 4870 512mb; Second is Mac 4870 512mb.
I can not make them work in CrossFire mode!
What I have already done:
Flashed BIOS of Mac 4870 on XFX 4870.
Flashed BIOS of MotherBoard (i10 and 9)
Installed NEW Second Windows- Win7 x86 (First one is Win7 x64)
Installed fresh drivers from ati.amd.com website
Connect them with ONE CrossFire bridge (do not have second one)
Connect Power cables to both videocards
In Catalyst Control Centre (CCC) I put check on "Enable CrossFire"
Tested in 3 different games (Counter Strike Source, Call of Duty Black Ops, Star Craft II) and in AMD monitor I saw only ONE videocard load (and heat up), second one was just working at 0% load...
Maybe Mac 4870 does not support CrossFire with Hardware limits rather than Software (BIOS)?

 

[Maziar]

It seems that the PC 4870 card won't work with the Mac's 4870 because it requires Apple BIOS
http://forums.whirlpool.net.au/archive/1165623

 

[armageddon528]

Who knows... Probably, but why CCC turned on CrossFire? May be I need to install some "Profiles"? I heard about but do not know how they looks etc...

 

[Maziar]

Installing profiles mainly helps in changing your clocks,I don't think it's related to Crossfire or anything.
The best thing is to ask Apple about this.

 

[fusk]

Correct link to psu compatibility regarding crossfire
http://sites.amd.com/us/game/products/certified/Pages/certified-power-supplies.aspx

 

[Maziar]

Thank you edited and replaced the old one.

 

[Maziar]

Thank you for your suggestion
I talked about the importance of the resolution in "Is it really necessary to use SLI or CrossFire ?"

 

[cats_Paw]

I think it would be good to state, that SLI and Crossfire configurations suffer from micro shuttering (or something like that), wich is higher in AMD systems, giving crossfire more chance of a sudden FPS drop. It does happen with both SLI and Crossfire, its just better in SLI, and as far as i know, its due to drivers.

Wich can be a good reason to go a single card. It seems its less common nowdays thou (nvidia 400 generation and ati 5000 and up.).

 

[dogman_1234]

Seems like engineers have fixed that.

There will always be issues with multi-card configurations.

 

[Toine1]

What happens if the cards have different amount of memory?
For ATI cards;
One of the cards will raise/lower its clock speeds to reach the other card's speeds.
Could somebody change amount into speed!!!

Now my question: what if you are Crossfire a 1Gb HD4890 and a 2Gb HD4890, what Vram will be available 2Gb or just 1Gb.

 

[Maziar]

Done,thanks for the tip,I don't know why I wrote "memory" there
As for your question,It's already been answered in the FAQ
"Do SLI or CrossFire double the memory ?
This is one the questions that many people make mistake understanding it .
The answer is NO , SLI or CrossFire doesnt double the memory , You have a 512MB card , adding another card wont make your memory 1GB, it still will be 512MB,so both cards will use 512MB of RAMs but the memory won't double.
Another example :
A game needs 512MB card to run at maximum settings , and you have a 256MB card , and you think if you add another 256MB card , your memory will be 512MB and thats not true.


You may see this mistake in many sites."

 

[Toine1]

No, I will explain it on a another way!

Main ATI card is containing 2Gb
Secondary ATI do have 1Gb

What will be the available Video-Ram?

 

[nukemaster]

1gb. It always is limited to the lowest card. This assumes you have 2 cards that can work together.

 

[dogman_1234]

Those are the VRAM.

The BIOS on the Motherboard and the GPU Breadboards will drop both cards to 1 GB.

 

[shadstarn]

i installed new 2 hd6870 cards and a 750 w psu and connected these with bridge and in most games it runs slower then with one card .. how can that be? what is wrong with my setup and do you have any ideas?
also my psu has plugs for pci express 3 and 4 and i do not know what it means and how to connect using those plugs.. i connect using normal plugsa

this is my motherboard
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131339

i tried the game metro and it dos not improve much in that game
but in stalker clear sky and stalker prypriat it goes slightly slower with two cards then one and i actually red somewhere those games do support crossfire

what is wrong w my setup? do i need to do something in bios? should i reinstall drivers?
i red that two 6870 should kick ass compared to only one .. i had a site showing its spremacy over most ocnfigurations before it but yet i do not get any significant boost?

 

[dogman_1234]

Some games do not do well with multi-card setup. I do not know why, but it must be due to the rendering.

 

[Maziar]

Some games run better with CF turned off.
But games like Metro and Stalker should run well with CF enabled.Re-install the drivers and also download the latest crossfire profiles and see if it helps.

 

[Maziar]

Edited.
Added more cards in the GPU list.

 

[dogman_1234]

Hey Maziar.

If I were to setup a multi-card setup for FSX, and disable CF, would there be an significant difference?

 

[Maziar]

AFAIK, FSX doesn't benefit from either SLI or CF so disabling CF may even give you a more stable gameplay(with no possible graphic glitch)

 

[dogman_1234]

Would I see any improvements?

 

[Maziar]

No(unless they released a patch for it which I don't think they did)

 

[dogman_1234]

Thanks.

 

[shadstarn]

Thanks.

The profile applicatoin helped alot .. the one on atis site.

i widh ati would release a driver that made it possible to play without vsync because of hte tearing