See, one GeForce GTX 580 is already a power-house in terms of performance, with two cards we already started noticing CPU-limitation in certain titles, e.g. the two GPUs would like to go faster yet the processor (an overclocked Core i7 965 @ 3.8 GHz) is, with somewhat dated titles, not able to keep up with the GPUs 100%.
With GPU performance nearly doubling up each two years this is going to become a problem in the years to come. See, a lot of games these days do have multi-threaded processor support, yet often that is limited to two or three CPU cores. The processor industry is making a huge shift to multi-core processing and with the serial architecture of the processor it poses an issue for multiple and high performance GPUs gaming wise.
In short, we really need faster per core processors to deal with that issue, unfortunately processor manufacturers are only focused on more cores. In the upcoming year for example AMD scheduled production of 16 core CPUs in the second quarter of 2011 and launch with widespread availability in the third quarter. Yep, AMD has revealed its latest Interlagos processor featuring up to 16 CPU cores, of course initially HPC / Server targeted... but that's where we are headed.
Why this tremendous comment on processors in a GPU article? Well, 3-way SLI on the GeForce GTX 580 allows us to look into the future. Two to three years from now what you'll see will be the new high end single 'GPU' performance. Showing you the great potential so much raw horsepower really has also shows drawbacks, in terms of CPU limitation.
So before starting up the article, rest assured we'll be showing you performance of our standard benchmark suite, that means some older titles, some modern titles and some DX11 titles. There's some shock, some awe and some coolness to be found in this article.
Let's head on over to the next page where we'll walk you through, well... everything.
Multi-GPU mode explained
Okay, first we are going to walk through our standard bit explaining multi-GPU gaming. We tend to get a little repetitive with this chapter, but honestly newcomers often do not know how, when and what multi-GPU gaming is and how it functions.
Both NVIDIA's SLI and AMD's ATI Crossfire allow you to combine/add a second, third or fourth similar generation graphics card (to add in more GPUs) to the one you already have in your PC. This way you effectively try to double, triple or even quadruple your raw rendering gaming performance (in theory).
Over the years one thing has proven itself though, the more GPUs you add, the worse the scaling becomes, though two GPUs in most scenarios is ideal.
How do multi-GPU setups work? Think of a farmer with a plough and one horse. One horse will get the job done yet by adding a second or third horse, you'll plough through that farmland much quicker and (hopefully) more efficiently as the weight of that plough is distributed much more evenly. That's roughly the same idea for graphics cards. One card can do the job sufficiently, but with two or more you can achieve much more in less time. And that's what you want in your PC, graphics frames rendered with the least amount of time equals performance.
So along these lines, you could for example place two or more ATI graphics cards into a Crossfire compatible motherboard, or two or more NVIDIA GeForce graphics cards in SLI mode on a compatible SLI motherboard.
A Crossfire compatible motherboard is pretty much ANY motherboard with multiple PCIe x16 slots that is not an nForce motherboard.
An SLI certified motherboard is an nForce motherboard with more than two PCIe x16 slots or a certified P55 or X58 motherboard. If your motherboard does not have the SLI certification mentioned on the box, it's likely not SLI compatible. Keep that in mind. For 3 and 4-way SLI your motherboard will need additional NF200 chips. Thus make very sure your motherboard has support for these extensive modes. Otherwise the driver will refuse to activate.
Once we seat the similar graphics cards on the carefully selected motherboard we just bridge them together, with a supplied Crossfire connector or in NVIDIA's case, an SLI connector. Then install/update the drivers, after which most games can take advantage of the extra horsepower we just added into the system.
These are some pretty nifty parts and bear in mind, when you opt for multi-GPU gaming, always have your gear right. You'll need that quality power supply, you'll need that beefy motherboard and processor, and then you'll need a chassis with some very good airflow keeping the graphics cards nicely chilled down.
Alright then, these are the facts. With 3-way SLI the more 'dated' games will run into that dreaded CPU bottleneck fairly fast unless you really make it heavy on the GPUs, that means resolutions of 1920x1200 and above and flicking on everything you can image quality wise. With the more GPU limited titles like Metro 2033, Battlefield BC2, Crysis and even Far Cry 2 with say 8xAA 3-way SLI does show noticeable gains. But as stated you do need to seek it at the extremely high resolutions.
On the topic of high-resolutions, there are more situations where you can reap the benefits of multi-GPU modes. 3D stereoscopic gaming for example with a modern title can cut your framerate in half. Also, gaming with three monitors where you literally increase the GPU load by threefold, these are situations where a setup like shown today makes sense.
What doesn't make sense obviously is the massive amount of money you need to spend on a setup like this :) The three cards alone will set you down roughly 1400 EUR, and then you still need to build a PC around it. Make no mistake, you'll need heavy duty gear. A top of the line 3-way SLI compatible motherboard, a nice overclockable processor, as you really want some more processor performance, and then obviously we need to have a chat about power consumption and your power supply. The very bare minimum you'll need to get is a kilowatt PSU, and you'll need quality of course. In the worst case scenario with the GPUs overclocked and our processor running at 3.8 GHz we here and there touched 950W (!). So we say, get yourself a good 1200 Watt PSU, and make darn sure that the PSU you choose has these 6-pin AND three 8-pin PCIE PEG connectors to feed the graphics cards.
When it comes to CPU bottlenecking you might wonder... why not overclock that processor any higher? Well, with the complexity of three GPUs you need to find yourself a stable symbiosis of components. We could say overclock to 4.4 GHz yet would need to alter CPU cooling to LCS or better and that PSU would need to be rated higher as well. Remember, overclocking requires additional voltage on the processor, if you need to step from 1.3v towards 1.5v or even higher your overall peak power consumption will rise significantly. An additional 100 to 200 Watts would not be weird at all. And that's why we kept the processor at our 1.35v / 3.75 GHz sweetspot as overclocking it dramatically higher would endanger system stability by pushing the 1200 Watt PSU close to its limits.
So then, in conclusion as always, this is x-factor stuff, extreme, nearly silly stuff to do and show. But people out there do purchase setups like shown today. At least you now know the pros and cons. Our recommendation is simple, if you have the hots for a multi-GPU setup based on the GeForce GTX 580, we recommend a 2-way SLI setup at best. With 3-way you'll run into heaps of curiosities like power consumption, noise levels and then the CPU bottlenecks - that last one is just sheer dreadful.
Flipside of the coin, 3-way SLI can be really helpful with 3D Stereoscopic gaming and a triple monitor gaming setup. The drivers, as always, are done well, we did not have any issues significant enough to mention, so from NVIDIA's side it's all good. But on a single monitor we do think that the infrastructure and sheer cash needed outweighs the extra performance you'll get for it in return. But that doesn't mean it's not tremendously cool stuff to play around with though.
Oh and 4-way SLI you wonder? Worse... that's 2-3% performance increase if you are lucky, with 3DMark Vantage as the exception. So yeah, stick to 2-way SLI we say -- unless you need to break 3DMark records of course
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