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I got ASUS GTX 550 Ti and I want to SLI it with another graphic card. I heard that not all graphic cards are good idea to SLI, (or not all combinations) because sometimes the final performance could be even worse that with one graphic card.

Is that true?

What are the rules ? ( maybe chip-set needs to be same or something ? )

I was wondering if you can recommend me what Graphic card should I use as with mine. Should I use same one (GTX 550 Ti) ?

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The general outline I have seen is similarly spec'ed within the same series. So anything in the 5xx series in your case seems likely to work well. –  nerdwaller Nov 30 '12 at 16:16
2  
My understanding is that ideally you'd use two identical cards. –  Shinrai Nov 30 '12 at 16:17

2 Answers 2

up vote 1 down vote accepted

From the manual of the GTX 550 Ti: (page 15)

SLI is a revolutionary technology developed by NVIDIA that allows you to connect identical GeForce graphics cards together to deliver increased performance.


From their SLI FAQ:

1)

Can I mix and match graphics cards that have different GPUs?
No

2)

Can I mix and match graphics cards from different manufacturers?
Using 180 or later graphics drivers, NVIDIA graphics cards from different manufacturers can be used together in an SLI configuration. For example, a GeForce XXXGT from manufacturer ABC can be matched with a GeForce XXXGT from manufacturer XYZ.

3)

Can I mix and match graphics cards with different sizes of memory?
No. For example, an XXXGT 512MB cannot be paired with a XXXGT 1GB in an SLI configuration.

So basically, you would need:

  1. A similar card with the same GPU and same amount of memory.
  2. A spare PCIe slot on your motherboard.
  3. A motherboard which supports SLI (usually branded SLI certified).
  4. A SLI-connector
  5. And enough power to feed both cards (as in amperage on the +12v rails)
  6. Enough cables to feed power to the card (as 6 pin connectors)
  7. No more than two monitors, both connected to the primary card.

sometimes the final performance could be even worse that with one graphic card.

True. Not all games handle SLI or Crossfire well. This is why I recommend a single fast card over two older cards in SLI. However if you already have the older card or if you want to maximum speed by combining two modern cards then you can use SLI (with Nvidia cards) or Crossfire (AMDs version of SLI).

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@equivalent8 A popular game which DOES NOT support CrossfireX or SLI is Team Fortress 2, and all other Source Engine-based games ever since roughly a year ago when the Source Engine was updated to handle alt-tabbing better. So, if you like Valve games, don't bet that SLI will make performance better. –  Brutick Nov 30 '12 at 16:54
    
Hi, thx for this, I'm using this computer only for 3D modeling and rendering (3Ds Max, Cinema 3D, V-Ray) do you have any thoughts on this as well? –  equivalent8 Nov 30 '12 at 17:05
    
These programs seem to use CUDA. That is also from Nvidia, but quite different from SLI. (CUDA is about calculations, SLI about getting more graphical performance, which are real time restricted). THat is different enough to warrent a different question, and different solutions. (e.g. one where you can use some card for graphics, either a build in one from Intel, and AMD one or whatever) and one card dedicated to calculations.) –  Hennes Nov 30 '12 at 17:22
    
is there way to use 2 graphic cards for CUDA ? From what I was googling people say that SLI isn't good idea for SLI because SLI is slower than PCI, therefore they recommend to tell program to use half of processes one one card and another half on second card via PCI ... I have no idea if it's true... what do you think when I have two GTX 550ti cards and I wan't to speed up rendering ? –  equivalent8 Dec 3 '12 at 9:11
    
SLI is slower than PCI does not make any sense. SLI is a way to run the rendering on two or more cards. PCI and PCIe are standards to communicate over a bus (PCI) or over several point to point lanes (PCI-e). If you use CUDA or OpenCL then you are not rendering and SLI becomes irrelevant. Instead you are doing calculations. Those calculation can represent an image in ray tracing, but they could just as well be sciencetific calculation with no connection whatsoever to graphical output. The whole setup is different. –  Hennes Dec 3 '12 at 17:31

A few things to understand about the performance of SLI:

When people say performance may be worse. They may mean driver support is not be optimized for your specific use and this can lead to worse performance. Certain design software also won't utilize SLI at all and apparently neither will Valve games on the Source Engine. If you're playing games with this system you can expect the driver support will be acceptable for most mainstream titles as Nvidia is pretty good with staying up to date.

They could also be talking about "micro stutter." Micro stuttering is an interesting phenomenon as some people can't even see it, while others complain it makes games unplayable. Just do a quick google search in "discussions" for micro stuttering and you'll get the idea. Basically micro stuttering is a side effect of the way SLI renders frames. That is alternating frame rendering where card A renders frame 1, card B renders frame 2, card A renders frame 3 and so on. So if your monitor is plugged into card A when B renders a frame it needs to traverse the SLI bridge to be displayed on screen.

However, because of how alternating frame rendering works, in a well optimized program Nvidia can claim 100% performance increase. Which is true because a single card is only doing %50 of the work it would if it was solo.

Lastly:

Nvidia recommends you use the same exactly the same cards as the clock rate between aftermarket OEM's on the Shader, GPU, and VRAM can differ. However as a side note I and many others online have had success using the same GPU with different VRAM in SLI. I was able to use a 3gb 580GTX with a 1.5 580GTX (both from EVGA) without any problems. The memory is mirrored so it gave me a total of 1.5GB Vram, it just a waste of the extra VRAM.

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hi, thank you for this, as I mentioned in my comment in question bellow, I'm using this computer only for 3D modeling and rendering (3Ds Max, Cinema 3D, V-Ray) so I'm hoping SLI will help me with some performance. Got any ideas on this? –  equivalent8 Nov 30 '12 at 17:09

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