SLI Setup and Testing Configuration
We got our hands on a set of GTX 980 reference cards to run through paces in 3-Way and 4-Way SLI.
The idea of multi-GPU gaming is pretty simple on the surface. By adding another GPU into your gaming PC, the game and the driver are able to divide the workload of the game engine and send half of the work to one GPU and half to another, then combining that work on to your screen in the form of successive frames. This should make the average frame rate much higher, improve smoothness and just basically make the gaming experience better. However, implementation of multi-GPU technologies like NVIDIA SLI and AMD CrossFire are much more difficult than the simply explanation above. We have traveled many steps in this journey and while things have improved in several key areas, there is still plenty of work to be done in others.
As it turns out, support for GPUs beyond two seems to be one of those areas ready for improvement.
When the new NVIDIA GeForce GTX 980 launched last month my initial review of the product included performance results for GTX 980 cards running in a 2-Way SLI configuration, by far the most common derivative. As it happens though, another set of reference GeForce GTX 980 cards found there way to our office and of course we needed to explore the world of 3-Way and 4-Way SLI support and performance on the new Maxwell GPU.
The dirty secret for 3-Way and 4-Way SLI (and CrossFire for that matter) is that it just doesn't work as well or as smoothly as 2-Way configurations. Much more work is put into standard SLI setups as those are by far the most common and it doesn't help that optimizing for 3-4 GPUs is more complex. Some games will scale well, others will scale poorly; hell some even scale the other direction.
Let's see what the current state of high GPU count SLI is with the GeForce GTX 980 and whether or not you should consider purchasing more than one of these new flagship parts.
SLI Setup and Configuration
Running more than two graphics cards in your system requires a bit more planning than just running SLI. First, if you are targeting 4-Way configurations, and have the standard dual-slot graphics cards, you are going to be limited to a handful of motherboard and platforms that support it. There are Z97 motherboard available today that can run 4-Way SLI but they integrated a third-part PCI Express bridge chip to turn the x16 lanes of PCIe provided by Haswell and allow it to feed four different graphics cards at x8 bandwidth. Kind of like black magic.
By far the more popular platform for 3-Way and 4-Way GPU users is the Intel E-series of processors. Sandy Bridge-E, Ivy Bridge-E and now Haswell-E. These use the X79 or X99 (HSW-E) chipset and supply much more PCIe bandwidth from the processor, allowing native support for x8 PCI Express 3.0 in most cases. Our testing setup uses an Intel Core i7-3960X along with an ASUS X79 motherboard to support 4-Way SLI, both electrically and physically.
From a software point of view, setting up SLI is really easy, even for lots of GPUs. Once you plug in all the cards are you going to run, and then install the right driver (344.16 in our case), enabling SLI is as simple as selecting a radio button.
As long as you have the right SLI bridge connecting all of your cards (supplied with your SLI-ready motherboard) you are ready to go.
Testing Setup
If you have been following the graphics card and GPU reviews at PC Perspective for any length of time, you likely realize that we do our reviews quite differently. Rather than relying exclusively on something like FRAPs and an average or average + minimum frame rate, our testing process is quite a bit more complex. We capture the actual game footage coming out of the system and run some post-processing on the recording to measure frame rates as well as frame times. This allows us to measure real-world gaming experiences as well as show smoothness and frame time consistency in a far superior way as compared to other software based solutions.
Rather than paste in our entire process here, instead I will link you to our initial GeForce GTX 980 article and it's page that details the PC Perspective Frame Rating process. Give it a read if you are new to the site; it will make the coming pages make a lot more sense.
| Test System Setup | |
| CPU | Intel Core i7-3960X Sandy Bridge-E |
| Motherboard | ASUS Rampage IV Extreme |
| Memory | Corsair Dominator DDR3-1600 16GB |
| Hard Drive | OCZ Agility 4 256GB SSD |
| Sound Card | On-board |
| Graphics Card | NVIDIA GeForce GTX 980 4GB |
| Graphics Drivers | NVIDIA: 344.16 |
| Power Supply | Corsair AX1200i |
| Operating System | Windows 8 Pro x64 |
Our test system remains the same for this 3-Way and 4-Way SLI testing and because of the power efficiency of the GM204 GPU, we didn't have to worry about upgrading our 1200 watt power supply to a 1500 watt unit. In fact, as you'll see on the last page, we barely were able to hit over 800 watts in testing!
Some Thoughts On Scaling Measurement
An astute commenter on this story helped me realize that I had not explained the scaling measurements of the multiple GPUs and how the "optimal" scaling rates would look. In the following pages you'll see discussion of our performance results that mention specific scaling percentages. What is important to realize is that as you add GPUs to your system, the maximum (or optimal) scaling rate changes. For example, going from 1 card to 2 cards, your theoretical maximum performance scaling is 100%. When you go from 2 to the 3 cards though, that maximum is 50%.
Here is an example of how scaling would work in 4-Way SLI if everything worked perfectly. You are seeing performance of 14.7 FPS (Crysis 3 at 4K) with a single card, 29.4 FPS for two cards, etc.
When looked at from a step-by-step perspective, this how the optimal result looks. With a pair of GTX 980 cards you would love to see 100% scaling over a single card. The move to 3-Way SLI should result in 50% more performance when compared to the 2-Way SLI result. Finally, the move to 4-Way SLI should result in 33% additional performance.
On the following pages, when you see a scaling rate of 20% at the 4-Way SLI level, then you know that is actually decent scaling considering the maximum theoretical level.
These are my favorite
These are my favorite articles.
Couple of requests:
Could you also include an evaluation of 4K-3D?
There we would expect an ideal of 50% of the frame rate of 4K, what will we actually see?
Also, what about the subjective factor? Yes, I see number going up on a chart, but does that translate into a “wow” factor? Same could be asked about 2K vs 3K vs 4K if the resolution does not translate into a better graphical experience due to textures etc. Numbers can’t tell that story.
Hey Ryan, I heard you talking
Hey Ryan, I heard you talking about sli profiles on the podcast. I can play Shadow of Mordor in 4k by going into the Nvidia Control Panel and enabling ‘Force Alternate Frame Rendering’ in the individual program setting. Running the benchmark after brought the fps up to 70 avg vs 40 fps before.
This is the first time I’ve tested with a game, I’m wondering if someday you can go back and test that setting with games vs using their sli profile.
Thanks
You guys really need to run
You guys really need to run skyrim with something like Kountervibe ENB or Opthfeldt ENB, a weaksauce card can run vanilla skyrim at good fps. Just a thought anyway.
Enb has issues with SLI and
Enb has issues with SLI and crossfire last I checked. Otherwise it’s something I would like to see as well.
Have you considered putting
Have you considered putting together a Xeon build with dual or quad socket CPUs to alleviate the CPU bottleneck for testing multi-card solutions?
Have you thought of testing
Have you thought of testing using the NetStor NA255A? That would allow for much cooler temperatures due to much better air flow. Your bandwidth going to/from the processor might be reduced but I think it is still rated at 128Gbps. I know it is an expensive expansion cabinet, but anyone willing to pay upwards to $4000 depending on video card, probably will be willing to pay $2200 for the expansion cabinet if it improves things.
Just n2 cool everything with
Just n2 cool everything with xeon CPU’s 64g ram and 4way sli…. be done with it… if one has the money…. multi cards onnly improve the experiance. not by much but can help make it better…. fuck it all
i would like to see a 3 way
i would like to see a 3 way SLI nvidia gtx titan black is it possibele
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I am just glad to see that my
I am just glad to see that my quad TITAN’s are smashing the hell outta these cards. I have no issues with scaling…..albeit when using 4k that is as the lower resolutions do not even push a quad configuration to use max clocks of the OC’d GPU’s. Let alone using “all” of the cards for that matter at lower resolutions.
I am easily getting over 100fps with everything in BF4 maxed, but AA off. And with DSR I get to play at 4k with a higher than average refresh rate as well. 100-120hz 4k anyone….?
And to me…honestly it seems
And to me…honestly it seems as if something is wrong with either the system setup, or a software issue. The scaling you guys are getting as opposed to others’ is just down right ugly. And those Frame Times….Oh my goodness. I’d have to say that it would be an unpleasant experience there. Something I have no issues with here as well.
http://us.hardware.info/reviews/5623/3/nvidia-geforce-gtx-980-sli–3-way-sli–4-way-sli-review-benchmarks-battlefield-4
BF4 right here.
Sure, I’m late to the party.
Sure, I’m late to the party. But it should be noted that whenever one card has data that isn’t physically on it’s own memory, that memory MUST be shared via PCI Express.
PCI Express is very fast compared to your LAN, or Cable Modem. But compared to a modern video card’s memory bandwidth? It’s nothing.
So, the more that must be shared, the quicker the PCI Express bus becomes the bottleneck.
BUT, this doesn’t have to be the case. A game that takes SLI into consideration can split workloads that don’t have hazards (one doesn’t depend on data the another must complete first), then minimal memory must be synchronized across the bus. Now, I’m merely speculating–as I have no experience writing AAA games–but if I were to guess, I would say that this is the reason for the large disparity between 0% gain, and 90% of ideal gain for those SLI setups. It all comes down to how those workloads are organized.
So with all of that in mind, it might be useful to compare SLI (even 2X) with differing PCI Express lane speeds.
The motherboard featured has four x16 lanes, and one x8. So try two cards, with one card on 8x (meaning the synchronization will occur at the lowest common denominator of 8x). Also try making the 8x card the “primary” card and see if that changes anything to see if the primary card gets the most data and the secondary gets only supplementary. (I don’t know what the memory mirroring/sharing strategy is for SLI.)
Lastly, a switch handles routing PCI Express traffic, and while the motherboard seems like a beast, it’s possible that chip is unable to satisfy the throughput of ALL lanes simultaneously. So motherboard chipsets can definitely be the bottleneck in these scenarios.
PCI Express 4.0 is slated for late 2016, and doubles the bandwidth of lanes. It stands to reason a typical PCI Express switch’s throughput will increase. So one could, in the future, see if the same cards on a newer 4.0 motherboard ends up with higher throughput. Even though the cards can’t run faster than 16x, it’s less likely that the switch would be overwhelmed.
This is fun stuff. If you’ve got the time and hardware, I’d definitely recommend trying these variations. The knowledge could help you in future write-ups.
Would Palit GeForce GTX 980
Would Palit GeForce GTX 980 Trio ‘s be good in SLI? What would be the best configuration (e.g. 2-way sli, 3-way sli)?
For gaming, stick to the
For gaming, stick to the fastest single core card you can find (980TI OC’ed on water) to about 1400. If you need high res 120+ FPS use two of these on air or water. Dont go 3 way on SLI, just a big hassle. My setup is 4X Palit 980 TI on water, but not using SLI but for GPU render using Cuda.
I’m currently running 4X
I’m currently running 4X Palit 980 TI on water cooling and my scaling is 100% increase pr. card. There is more to multiGPU than SLI and gaming. I’m now using Octane render and this is scaling almost perfectly, even up to 8-12X GPUS. This uses any Cuda core available (even from different cards) and does not use SLI functions.
Not impressed with Nvidias SLI controls, as 3+ GPU setup in SLI often will cause trouble in games as many cant even handle more than one card properly. As a result, you have to physically disable some of the graphics cards on the motherboard, a really akward and not well thought through solution for daily use.
Can you explain the following
Can you explain the following numbers:
From my point of view I get:- 5937 points 1-way SLI
- 11628 points 2-way SLI (95.86% increase compared to 1-way)
- 17160 points 3-way SLI (189.03% increase compared to 1-way)
- 22315 points 4-way SLI (275.86% increase compared to 1-way)
How do you come to:
I find this really useful
I find this really useful because i was considering if to buy another graphics card (sli) and it seems like 2 way has the biggest performance increase than having 2 or more and for the money your going to spend its wise just to get 2 way sli for a performance increase, thx
I’d like to know how many of
I’d like to know how many of you that say more than 2 cards is pointless, has actually tried it? If you did try it and didn’t use proper settings via 3rd party software for it, well it’s no surprise why your experience sucked. 3/4-way is the only way to go if you run 3 screens, Enjoy high graphics and high fps+Hz not 60/60 more like 90/75 minimum.