PCIe Scaling

Before we move on to our conclusion, we wanted to provide some insights on PCIe scaling. We used the X99 platform, which is capable of running two video cards at PCIe 3.0 x16 speeds, but with our Core i7-5820K processor, one of our cards was limited to PCI 3.0 x8 speed. When our benchmarks results weren't held back by a CPU limitation or a poor SLI implementation, we routinely saw the second card operating at around 95% load, while the top card was pegged at 99%. Could this have been holding back the SLI scaling we observed in our game benchmarks? To determine whether it was indeed a factor, we ran a single GTX 980 Ti through four of our most demanding games at 4K, but this time used the second PCIe slot to force it to run at x8 speed.

PCIe scaling

Starting with Metro: Last Light on the left, we see that performance actually dropped about 4% at x16 versus x8, but this is probably just an artifact of standard benchmark variation. On the other hand, Crysis 3 performance improved 3% with 16x bandwidth, and Battlefield 4 improved 2.5%. The Witcher 3 performed identically at both settings, suggesting that there is a different bottleneck holding back performance in this game. For all intents and purposes, we'd call this a wash. Yes, PCIe 3.0 slots running at x16 probably do offer on average a 1-2% improvement in game performance versus slots at x8, but it's not enough of a difference to warrant spending a lot of extra money on.

The VRAM Question

Before we get to our conclusion, we wanted to share just a few observations about VRAM usage at 4K. There's been speculation that 4K gaming will drive VRAM requirements way up, perhaps above the 4GB that comes standard on most high-end card on the market today. Of the games we tested, only one actually exceeded 4GB at 4K, Battlefield 4, which used 4.5GB. It also used 3.5GB at 1440p. We've seen BF4 use far less than 3GB at 1440p on 3GB cards, suggesting that this game keeps a lot of data in cache that doesn't necessarily improve game performance. Far Cry 4 was another game that pushed the VRAM limit, hitting 3.6GB at 4K, and with this game, we think that amount of VRAM is really required. 

As for our other games, the VRAM usage at 4K was far lower, ranging between 2GB and 3GB, suggesting that it's fairly game-dependent, and more importantly, a well-coded game doesn't need a ton of VRAM to look good at 4K. It could be argued, for instance, that Crysis 3 and The Witcher 3 are just as good looking as BF4 and Far Cry 4, despite needing about half as much VRAM at 4K. That's little consolation when game developers continue to rely on larger and larger VRAM reserves in coding their games, so overall, we do think that 4GB will become a limiting factor in 4K gaming soon enough.

Therefore, taking into consideration all the data we collected, we really wouldn't recommend anything less than a true 4GB card for 4K (ruling out the GTX 970, for instance), and the 6GB that the GTX 980 Ti is equipped with is probably the sweet spot right now. 

Conclusion

Well, we took on the 4K challenge, and overall came away pretty impressed at what's possible on today's hardware. With two GTX 980 Ti cards paired up in SLI, all eight of our games were playable at 4K. That being said, SLI scaling was all over the place, ranging from 18% in The Witcher 3 all the way up to 90% in several of our games, and averaging around 70%. Sometimes this was limited by SLI itself, but sometimes it was actually CPU-limited, even on our beefy six-core processor. Still, SLI was critical to achieving great 4K performance, as only half of the games in our test suite were truly playable with a single GTX 980 Ti: Grid 2, Tomb Raider, Crysis 3, and to a lesser extent Far Cry 4. Metro performance was beyond slow, and Battlefield 4 and The Witcher 3 just didn't hit playable framerates with a single GTX 980 Ti at 4K. Thief seemed to be bugged at 4K, exhibiting incorrect rendering, and SLI didn't solve this problem.

Before we get to our overall conclusion, we want to say something about game performance at 1440p versus 4K. In short, every game worked fantastically at 1440p, even on a single card, providing much better performance than at 4K. This isn't too surprising given that in general 4K is twice as demanding. What really surprised us, however, was that image quality really didn't differ that much when jumping to 4K despite so many more pixels on screen. Of the eight games we tested, only two looked noticeably better at 4K: Metro: Last Light and Crysis 3. As for the rest of our games, it seems they just don't have in-game assets that can truly take advantage of the extra resolution. They look very good at 1440p, and they look identical at 4K.

So, overall, here are the take-away lessons we'd share with anyone looking to build a gaming system in the year 2015:

  1. Playing at 4K requires about twice as much rendering power as playing at 1440p
  2. You need two very powerful cards in SLI (or Crossfire) to get playable performance in all games at 4K
  3. In most games, playing at 4K on a 4K monitor simply isn't distinguishable from playing at 1440p on the same monitor, due to limitations in today's game engines

And that brings us to our somewhat surprising conclusion: striving to achieve playable 4K performance in today's games just isn't worth it. With the advent of 144Hz and G-Sync-enabled 1440p screens that significantly improve the overall gaming experience in ways that 4K cannot, a die-hard gamer is much better off buying a 1440p screen over a 4K screen, and enjoying the lower heat output, lower energy use, and lower cost of running a single card at higher refresh rates (not just higher framerates, mind you!). There are reasons to go 4K, for instance if you need the screen real estate for other PC uses, or if you're investing in a 4K home theater setup, but for the typical gamer using a desktop PC, we think a 144Hz 1440p monitor, like the amazing Acer XB271HU, is the way to go.

As always, we had fun putting all these benchmarks together, and hope you learned something new to help inform your next PC purchase. For ideas on putting together a balanced gaming system, feel free to check out our PC Buyer's Guides, updated monthly. Want to pick up your own pair of GTX 980 Ti cards? We recommend the MSI GTX 980 Ti Gaming and the EVGA GTX 980 Ti Hybrid!

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