SLI Question

[FerofMilf]

just wondering do both video cards have to be the same for SLI to work? like if I have a 7950 Gt nvidia, can I get like a 8800? to add to the system? the 7950 is SLI ready not quite sure how it all works.

[Diamndzngunz]

http://www.evga.com/forums/tm.asp?m=151488

OK, I'm making this thread to correct some of the confusion I see floating around here with SLI. I'll also be happy to entertain any questions you may have or offer support should you need it just post and ask. My results have come from experience with SLI, and I've SLI'd every generation of cards since SLI was possible. Starting with SLI 6800Ultras, then to 7800GTX SLI, then 7900GT SLI, and now finally with my 8800GTX SLI. I've pretty much run the gauntlet with SLI as you can see

OK, so here's the F.A.Q.
-----------------------------------------------
Q = What do I need for SLI?

A = You will need a SLI compatible motherboard, a set of SLI compatible video cards, and a SLI bridge. (though in some cases you can enable software SLI without a bridge)

Q = Can I SLI a 8800GTS 320/640MB with a 8800GTS 512MB?

A = No, you cannot SLI them because the 8800GTS 320/640MB cards use the G80 core while the 8800GTS 512MB uses the G92 core. They are not compatible in SLI with each other.

Q = If I put 2 cards in SLI do I get double the memory from it?

A= No, you do not get double the memory. In order for SLI to function all data that is in one GPU's memory must also be mirrored in the other's. This is because the GPU's are co'rendering the same scene at the same point in time. In order for this to be possible they must both be processing the same data. So in SLI, since both banks of RAM are holding the same data, that leaves you with the total of ONE cards ram. So 2 cards that have 512MB of RAM means you have only a total of 512MB of RAM.

Q = Can I put 2 cards in SLI even though they have different clock speeds?

A = Yes. The way SLI works is that when you activate SLI the driver "looks" at the card's clock speed that is in the primary slot (top slot) and sets the clocks to both cards to that speed. SLI does not permit independant GPU clocks meaning that both cards must be running at the same frequency. So higher clocked card in the top slot = both cards run at the higher clock speeds. Lower clocked card in the top slot = both cards run at the lower clock speeds.

Q = So I can just put the higher clocked one in the top slot and be fine?

A = Yes, but only if you've tested the lower clocked card to be able to run at the speeds of the higher clocked card. For this reason I usually recommend to people that they put the lower clocked card in the primary position, since the other card is clocked higher and has already been tested from the factory to run at the higher speeds, you can rest assured that it will run at the lower cards frequency. You can then if you desire, overclock both cards back to the higher speed card if you find the lower clocked one to be able to handle it.

Q = What size PSU should I be looking for to use with SLI?

A = Considering that a overclocked quad core, 8800GTX SLI system peaks at about 540w under max load, you will only need a quality 650-750w unit to get the job done. I say "quality" because there is definitely a pecking order in the world of PSUs. And I can't stress it enough. The PSU is the heart and soul of your PC, and it is the single most important component in your PC, because everything depends on it and nothing works without it. So please do not skimp out on the PSU. That is not to say that you need to buy the most expensive unit out there, but you do get what you pay for. Cheap = cheap, remember that. Some high quality brands are PC Power & Cooing, FSP Source, Seasonic, Silverstone, Antec, Enermax and OCZ, Chances are if you buy a PSU from one of them you can count on it. There are others that are good too so if you have a question about a PSU, go ahead and ask

Q = Do I need to have cards from the same manufacturer for SLI to work?

A = No, you can SLI an EVGA 8800GT with an XFX 8800GT just fine. Clock speeds and bios can even be different it does not matter. As long as it's an 8800GT it will work with any other 8800GT in SLI.

Q = Can I SLI a 8800GT with an 8800GTS?

A = No. You cannot SLI different TYPES of cards. You must at least have the same type of card for SLI to work.

Q = What are these different SLI options I see? Alternate Frame Rendering 1, Single GPU...ect?

A = Those are different SLI performance modes, some work better with different games and some provide better compatibility.

Alternate Frame Rendering 1 = Think of the the frames as all being numbered 1,2,3,4,5...ect. All the driver does is designate what cards will render what frames. So in AFR1 the top card will render all the odd numbered frames (1,3,5,7....ect) and the bottom card will render all the even numbered frames (2,4,6,8...ect)

Alternate Frame Rendering 2 = The same as AFR1 but only reverses what card renders what frames. So in this mode the top card renders all the even numbered frames while the bottom card does all the odd numbered ones.

Split Frame Rendering = Kind of self explanitory if you think about it. All the driver does is cut the scene in half and makes the top card render the top half and the bottom card render the bottom half.

Single GPU = Most people think this means you are not using SLI, that is not true you are still using SLI, just that all frames are compiled and rendered on the primary GPU. You still get the benefit of SLI because the other GPU is still processing data for the primary GPU. This is what SLI defaults to and is the most compatible but AFR 1 and 2 offer better performance and they are better suited to balance the load between the GPUs. Split frame rendering offers the lowest performance of the rendering techniques.

Q = I just enabled SLI on my 8800GTX rig and my 3Dmark score only increased by 2000 points?

A= That is because of a CPU bottleneck. SLI configurations are massively bound by CPU speed. If you want big scores in benchmarks you better be prepared to overclock your CPU because even the mighty QX6850 quad core at stock speeds isn't nearly enough speed to push the 8800GTX's in SLI to their limits.

Now there is a flip side to the coin. You don't need a ridiculosly expensive CPU or have to overclock the daylights out of your CPU to benefit from SLI. SLI is best used in 2 ways (well 3 if you're a benchmark freak like me )

1st is if you play at high resolutions (1680x1050 or above) and what to be able to turn the eyecandy all the way up in the latest games (I'm talking 8xAA minimum, all bells and whistles for the graphics settings, 16xAF...ect) and you want it all to stay silky smooth.

2nd is if you are trying to get a little more life out of your rig before having to do a PC overhaul. Say maybe that 7900GT card is starting to feel the pressure of all those new games and their fancy new effects and want a cheap way to boost your frames per second. Well SLI is for you, adding a second card will help you tame some of the tougher games and give your PC a shot of adrenaline and give you that extra 6 month or year to save up for your next uber rig

Q = When I activate SLI I can't see the seconds cards clocks in Riva Tuner?

A = That goes back to my explanation of SLI not permitting independant clock speeds for the cards. The one set of sliders you do see are representing both cards. So adjusting it, you will be overclocking both cards in tandem.

Q = When I activate SLI my second monitor goes dead?

A = That is a limitation of SLI, when SLI is activated, you lose multi monitor support because of the way SLI is implemented is cannot suppot 2 monitors.

Q = What cards are compatible with Tri SLI (x3 SLI)?

A = Currently the 8800GTX, 8800Ultra and 9800GTX are the only cards that support Tri SLI (I will update this as newer cards become available or are announced)

I've just covered some of the things I see coming up here over and over again, so if you have a question and don't see it, please ask and I will be more than happy to answer or even add it to the FAQ.

[FerofMilf]

WOW! thank you VERY much sir.

GREAT INFO.

[FerofMilf]

OH NUTS!

can you help me figure out if my mobo is SLI compliant?

I own D975XBX NOT the (D975XBX2)

I read though the website and couldn't find any info http://www.intel.com/products/mother...5xbx/index.htm

http://www.intel.com/cd/channel/rese...ech/250939.htm

tested items?

[FerofMilf]

I looked though the manual and it dosen't say anything about SLI but it dose talk about using 2 video cards... Weird?

http://download.intel.com/support/mo...3153103_en.pdf