Conclusion

As we already mentioned this is the first article of a series of SLI scaling analysis with different CPUs and different numbes of graphics cards as well as different graphics cards. So far there are already some rather interesting findings. One rather obvious thing is the fact, that consistent scaling regarding GPU performance can only be found with theoretical benchmarks. Referring to 3DMark we see that the overall socre is between factor 1.57 and 1.61 (depending on our CPU preset) higher than with one card. Translated into the raw GPU score we see an scaling factor of 1.82, which is usually about the maximum scaling you get with SLI. A closer look at Unigine Heaven reveals that in case of the basic preset scaling factor is as low as 1.2 at stock frequencies and 1.18 with the CPU at 4.5 GHz. When it comes to the Extreme Preset then we see that the GPU has much more influence again, which is acutally rather obvious, since there is much higher load on the VGA(s). Nevertheless it's interesting to see that overclocking the CPU to 4.5 GHz makes the scaling factor go from 1.59 - with stock clocks - to 1.71, meaning the CPU is also limiting the maximum score.

Games on the other hand are a completely different story. Some of you might be asking, why in the name of ... we ran an SLI with low resolution. Well, browsing all the results shows, there is actually some scaling to going on and we find it interesting to see that even at low resolutions certains games benefit from a second card. Another point is that adding a second card makes dead sure, the VGAs aren't limiting in any case, meaning CPU limitation becomes even better visible. But lets go back to talking about the results now. Scaling factors at low resolutions vary from 1 to 1.47. 1 basically means, the fps didn't change at all and 1.4 is quite a mssive jump, which bears the question why. The game in question is Call of Duty Black Ops 2 of which we know, graphics load is very low. If we have a closer look at the scores we see that with the CPU overclocked to 4.5 GHz causes fps to go up by more than 47 percent, which clearly indicates that this is a perfet example for CPU scaling. Judging by our selection of games this is a spike and not what can be expected from any game.

Now lets focus on high resolution and high details, where the games cause quite some load on the two graphics cards. In this case the lowest scaling factor we find with Sleeping Dogs and the CPU at 4.5 GHz, where fps are by factor 1.01 higher than with one card. On the other hand there is Crysis 3, which produces some values that look rather ridiculous at a first glance. If we look at the scaling factor with the Intel Core i7-4960X at stock clocks then there is a scaling factor of 2.67 and at 4.5 GHz this factor goes up to 2.88. We've cross checked this value about 20 times in the meantime and we always get the same result. Our explanation on this goes as follows. Since there must be a multiplying factor we assume, there is a CPU as well as GPU bottleneck being opened. We think that when we add two cards in the system the benchmark is not VGA limited anymore, which is causing the CPU to kick in. We think this is acutally possible since the scaling factor is even higher when we overclock the CPU 4.5 GHz, which means the CPU must have a rather drastic effect on the endresult.

To close this conclusion we try to leave you guys with at least some sort of buying advice. Basically if you do have the funds to buy two high-end graphics cards and then use them in SLI (apparently) it makes sense to combine these two cards with a powerful CPU, since you're quite likely to run into CPU limitation on certain games. On the other hand the question remains how much sense it makes to buy two high-end cards in the first place, since not too many games are actually well optimized for multi-card setups. Overall you'll definitely end up with a highly futureproof setup but the benefit comes at quite a cost. 

Page 1 - Introduction	Page 9 - Call of Duty Black Ops 2
Page 2 - Test Setup	Page 10 - Sleeping Dogs
Page 3 - 3DMark Fire Strike	Page 11 - The Elder Scrolls V: Skyrim
Page 4 - Unigine Heaven 4.0	Page 12 - Metro: Last Light
Page 5 - BattleField 4	Page 13 - Power Consumption
Page 6 - BattleField 3	Page 14 - Scaling Factor
Page 7 - Bioshock Infinite	Page 15 - Conclusion
Page 8 - Crysis 3