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How does a computer utilize dual GPUs? And how does it compare to a more powerful GPU?

post #1 of 11
Thread Starter 
Just a little curious as to how a computer utilizes two graphics cards and if it's efficient. I've never used a sli setup and will most likely not go that route for a while. However I suspect in a year or so I may want to upgrade to a new CPU that could handle a more powerful sli setup. Get another 7870 which by that time could be less than 200$ or so.

So how does a sli setup work? It can't be as simple as attach two cards together and they run off each other can it? I know a sli setup can possibly use more voltage, that's a given. Let's say instead of a sli 7870 setup I just upgrade to a 7950/7970. Is such an upgrade even justifiable?

The more info someone can shed on it the better, thanks for helping out my curiousity.
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post #2 of 11
Well... seems like you still have a lot to learn tongue.gif

SLi is ONLY for nVidia

Ati uses Crossfire


SLi has to be identical chipsets, Ati can have 2 different cards in CF.

1 vs 2 cards and 2 vs 3 and etc... is called "scaling" (something else you can google for wink.gif )

So sure it has a lot of benefits, but also some downsides. (stuttering, bad driver/game support, adequate power supply needed, etc)


SLi works like one card does the calculations of getting the image on the screen, while the second one calculates PhysX.

But nowadays the graphics cards are so powerfull they can calculate both (image+physx) in 1 card, and even faster than some setups.


And even with this reply, there is still A LOT more to tell...
   
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post #3 of 11
Quote:
Originally Posted by ASUSfreak View Post

Well... seems like you still have a lot to learn tongue.gif

SLi is ONLY for nVidia

Ati uses Crossfire


SLi has to be identical chipsets, Ati can have 2 different cards in CF.

1 vs 2 cards and 2 vs 3 and etc... is called "scaling" (something else you can google for wink.gif )

So sure it has a lot of benefits, but also some downsides. (stuttering, bad driver/game support, adequate power supply needed, etc)


SLi works like one card does the calculations of getting the image on the screen, while the second one calculates PhysX.

But nowadays the graphics cards are so powerfull they can calculate both (image+physx) in 1 card, and even faster than some setups.


And even with this reply, there is still A LOT more to tell...

dude no sli does not work like that. that example that you put there is what its called a dedicated physx card. that is not sli.

 

sli and xfire do alternate frame rendering. one card renders the even frames and the other card renders the odd frames.  thats just the basics and there is alot more to it. *OP i suggest that you use the search option on the forum or on google to get a real detail description on multi gpu and how it really works.

    
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post #4 of 11
Thread Starter 
I did search, couldn't exactly find what I was looking for. Maybe I'll google search some topics for more specific answers since I haven't really got any here yet either. Thx anyway.
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post #5 of 11
    
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post #6 of 11
Thread Starter 

Repped. Thanks a bunch. Read up on some performance results and all that, now reading about how it actually works.

Now, as asked earlier. How does CPU performance factor into a crossfire setup? If the 7870 is a decently powerful GPU by itself, would a crossfire setup require just about the strongest CPU out there? Or would the 8120 be powerful enough to run a dual setup already? I'm asking so I can get an idea of how powerful a cpu would be needed.
Edited by Xerosnake90 - 9/16/12 at 10:55am
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post #7 of 11
Quote:
Originally Posted by Xerosnake90 View Post

Repped. Thanks a bunch. Read up on some performance results and all that, now reading about how it actually works.
Now, as asked earlier. How does CPU performance factor into a crossfire setup? If the 7870 is a decently powerful GPU by itself, would a crossfire setup require just about the strongest CPU out there? Or would the 8120 be powerful enough to run a dual setup already? I'm asking so I can get an idea of how powerful a cpu would be needed.

There is a lot of info to take in on the subject, that is for sure. Like the guy above said, the most common way CF/SLI works these days is called AFR (alternate frame rendering) where each card renders a frame in sequence. This is the easiest to implement but also the most prone to stuttering issues due to the load not being equal between frames, ex. frame 1 is a wall which card 1 renders in 10ms while frame 2 is a complex scene which card 2 renders in 30ms, you can start to see the little hitches in smoothness if you pay attention. Nvidia uses some frame metering techniques to smooth this while AMD does not, so some people who are sensitive to rate changes like SLI better than crossfire. The problem is also lessened when more than two cards are involved, 3-way setups are typically the smoothest although they may not have the best scaling. The other main type is SFR (split-frame rendering) which hasn't been used in years, it can be smoother than AFR but is less commonly implement in games and drivers. Naturally developers follow the path of least resistance because the SLI/CF crowd is pretty small compared to the full player base of any given game, so AFR is the preferred method although it is not the best quality.

Scaling is the next important issue. Depending on the drivers, system itself, and the game being played you will almost never see two identical cards put together being twice as fast as a single card of the same type. Anything over 80% scaling is decent, while some things like synthetic benchmarks -3Dmark11 for example- can see 95%+ scaling. The more cards you add, the worse the scaling is especially as you add more GPU power than you actually need for the resolution of your display setup. 4-way setups are usually plagued with problems because they are simply very rare and the drivers from both major companies tend to only give cursory support to that type of setup.

Another thing to remember is that your Vram is not doubled, a 2GB card combined with another 2GB card still only offers you 2GB of usable graphics memory, because the same data must be mirrored between the two. That is why dual GPU cards claiming the full memory on them is simply a marketing tactic to make it sound better, for example the GTX 690 may have 4GB of physical memory on it, but since both GPU's fill their Vram with the same data, there is only effectively 2GB to use and if you happened to have something that used more than 2GB worth of textures you would run into the Vram limit, not at 4GB like they put on the front of the box in big bold letters.

CPU performance, ah theres the question. It depends to be honest, mainly on the game, graphical settings, and resolution. This is the simplest I can put it and still be somewhat accurate: the difficulty lies with how fast the CPU can provide information to the GPU's, the faster your GPU setup gets the faster the CPU has to work. The solution to a CPU bottleneck is to increase resolution or graphical detail, if you cause the frame rate to slow down because the cards render frames slower, the CPU doesn't have to work as hard to keep the graphics cards running at full steam. In the end, as long as the game plays how you want it to it really isn't a big concern. Some people stress over not getting the extra 10fps a faster CPU would give them, others do not. I tend to not worry as long as it plays how I want it to at the settings I like.

As far as crossfiring your 7870 in the future? Nobody can say except you, when that time comes look at benchmarks of the games you want to play and determine if its worth it or better to just buy a single more powerful card.
Edited by Scorpion49 - 9/16/12 at 11:14am
post #8 of 11
What resolution are you using ?

I don't play games, but always use the best single card you can afford, sell the 7870 and replace with the latest generation, IMHO, only a huge hi-res display will need two cards.

ATI/AMD can use two slightly different cards from the same series 5870/5850/5830, not just any mix.

HTH

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post #9 of 11
Quote:
Originally Posted by manolith View Post

dude no sli does not work like that. that example that you put there is what its called a dedicated physx card. that is not sli.

sli and xfire do alternate frame rendering. one card renders the even frames and the other card renders the odd frames.  thats just the basics and there is alot more to it. *OP i suggest that you use the search option on the forum or on google to get a real detail description on multi gpu and how it really works.
yup
    
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post #10 of 11
Thread Starter 
Quote:
Originally Posted by 2002dunx View Post

What resolution are you using ?
I don't play games, but always use the best single card you can afford, sell the 7870 and replace with the latest generation, IMHO, only a huge hi-res display will need two cards.
ATI/AMD can use two slightly different cards from the same series 5870/5850/5830, not just any mix.
HTH
dunx

My resolution is at 1280x720. I'm running on a 42 inch hdtv at the moment and may or may not upgrade to a dedicated monitor that runs better resolutions. I'm not a fan of smaller screens that may look slightly prettier.
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