Intel Skylake-X: Intel's new six, eight and ten-core CPUs reviewed

Intel's new Skylake-X line-up demonstrates just how good competition is for the PC hardware market. In the wake of Ryzen 7's exceptional value, Intel has been forced to react. The impact of AMD's return to the market is ongoing but in the short term, $700 off the cost of a ten-core CPU and $400 off the sticker price of an eight-core chip is definitely a step in the right direction. On top of that, further adjustments to pricing may be required in the wake of the imminent arrival of AMD's Threadripper - a 16-core monster for 10-core Intel money.

In the here and now, we're going to be looking at the four CPU key releases compatible with Intel's new Basin Falls enthusiast platform, all running on the new X299 chipset. The price to performance sweet spot comes in the form of the six-core Core i7 7800X - available for just £30/$40 more than Intel's mainstream, quad-core i7. There's a hefty jump to the $599 eight-core chip - the Core i7 7820X - and a significant leap again to the $999 ten-core Core i7 7900X.

Bizarrely, Intel has also released a quad-core part for the Basin Falls platform - the Core i7 7740X - which is, essentially, a slightly overclocked version of the existing 7700K and released under the Kaby Lake-X umbrella. Priced so closely to the 7800X, it has no real point on an enthusiast platform geared towards power users, its lack of relevance eclipsed only by the existence of an i5 version. Quite why these parts exist at all is a bit of a mystery: perhaps if you're looking towards a many-core upgrade path in the months or years to come, it might make sense to invest primarily in the platform. However, when surrounding aspects of an X299 build cost so much, the paltry extra £30/$40 required for the 7800X over the 7740X essentially destroys that argument.

However, back in the day, we found that a quad-core i7 could actually outperform its many-core variants in many gaming scenarios, so we were happy to include it in the line-up for review. Our focus here is on gaming performance, but we're including a quick batch of benchmarks here to give you some idea of scalability across the range. Intel's XTU benchmark is included along with Cinebench R15's single and multi-core performance. These are essentially synthetic tests of little real-life value, but our Handbrake h.264 and HEVC benchmarks are based on a real-life workload - we're encoding at 4K using the presets we use for serving high-quality video downloads at digitalfoundry.net, and the x264 and x265 encoders used by the tool are the best in the business (and open source too).

Intel Skylake-X Review! Core i9 7900X! Core i7 7820X/ 7800X/ 7740X!

XTU and Cinebench offer few surprises - single-core performance is much of a muchness across the many-core chips, with the more highly clocked Core i7 7740X taking point. Benchmarks here were taken with the standard all-core turbo disabled, but the processors still seemed to run at max turbo speeds regardless, giving a very close grouping, with only the 7800X falling a touch behind.

Multi-core results from Cinebench show almost linear scaling when frequencies are matched at 4.4GHz, but it's interesting to note that Handbrake's video encoding starts to see a law of diminishing returns kick in the more cores and threads you throw at the same task. More processing resources means better results of course, but there's no linear increase in results - and ironically, this is the precise inverse in terms of the pricing situation.

We tested each processor in an MSI X299 Gaming M7 ACK motherboard, paired with four sticks of Corsair Vengeance DDR4 rated for 3000MHz - but running without issue with a 200MHz overclock. Corsair also supplies the RM1000i power supply and a Hydro 100i GT twin-fan closed loop cooler. The Core i9 7900X in particular is heavy on power draw, especially when overclocking, so ensuring your surrounding components are up to scratch is essential - we'd also recommend an eight+four pin CPU power input on your motherboard if you're going to push a high-end chip.

In terms of gaming, we've since tightened up our testing procedures to ensure that the CPU takes point as much as possible as the bottleneck in performance. We aim to concentrate on the most CPU-intensive areas of our titles, while offering a repeatable test that we can roll out across all processors. Our results aren't designed to show performance in a usual gaming scenario (where GPU is usually the defining factor), more to show relative maximum theoretical performance using a cross-section of challenging game engines.

Actually isolating areas of gameplay that constitute a thorough CPU work-out isn't easy - and there's still the sense that with high-end processors like this, we may still be GPU-bound in some areas. There are plenty of scenarios where we can get the best out of all of these powerful processors though. We've relocated our Crysis 3 GPU bench area to a section of the Welcome to the Jungle stage that can challenge even a ten-core i7, and we've binned off Rise of the Tomb Raider's inbuilt GPU-centric benchmark for actual gameplay that pushes CPU hard in the notorious Geothermal Valley.

We do keep our gallop through Novigrad City in The Witcher 3 though - just like Crysis 3, this area of gameplay can see a mainstream i5 hit 100 per cent utilisation across all cores. Far Cry Primal remains an excellent workout for single-core performance in a modern engine, while Assassin's Creed Unity and The Division are both many-core aware, but tend to max out on a standard quad-core i7. Meanwhile, Oxide Games' Ashes of the Singularity CPU stress test does exactly what it says on the tin - however many cores your processor has, it'll do its level best to make good use of them.

Skylake-X Benchmarks! Core i9 7900X vs 7820X vs 7800X vs 7740X!

The core takeaway from our first run of benches is that every i7 in the Basin Falls line-up - and by extension, other modern i7s like the 6700K and 7700K - are phenomenally good gaming CPUs. With the exception of the Ashes of the Singularity CPU stress test (designed to push the game engine beyond usual limits) all of these chips produces excellent gaming results. On top of that, performance minimums are all above the crucial 60 frames per second. And this is all with no overclocking - although all-core turbo (an out-of-the-box OC usually enabled by default) may be active here, despite disabling it in the BIOS. And that guarantee of tip-top performance really is what has made the i7 brand such a winner over the last few years: Core i5 gets you to 60fps for the majority of titles, while i7 takes care of the most challenging fare.

But to what extent do the many-core chips actually improve upon the quad-core experience? There has been talk of degraded gaming performance on Skylake-X and you will note that the the quad-core i7 7740X is managing to equal or best the six-core i7 7800X in a couple of tests. The interconnectivity fabric between the cores in the big chips has been revamped, and this has impacted game performance on some titles. But you can see that generally, more modern game engines tend to benefit from the additional cores - but the improvements are incremental at best, best felt in the lowest performing areas of those titles.

If you're ever looking for one single game on which to judge a prospective CPU purchase, Crysis 3 and its Welcome to the Jungle stage should be your number one destination. It thrives on whatever CPU resources you can throw at it, and the many-core chips love it with minimums in the 80s (vs 66fps on the quad-core 7740X). Mainstream CPUs don't stand a chance here, with even an overclocked i5 7600K at 4.8GHz unable stay north of 60fps (45ps minimum - really).

Far Cry Primal is clearly the big disappointment here for the six, eight and ten-core i7s - the title suffers from a double-whammy in not only being unable to match quad-core i7 performance, but also suffering from Skylake-X's occasionally diminished performance. However, titles like Rise of the Tomb Raider and Crysis 3 demonstrate that while detrimental in some games, Skylake-X still offers a small boost in other tests. But to what extent is degraded game performance actually a real issue? Only like-for-like, clock-for-clock metrics will do, so our next tests involves overclocking the eight-core 7820X to 4.4GHz and comparing that to similarly clocked Intel processors from prior enthusiast releases.

We're still in the process of updating our AMD stats, and Ashes of the Singularity and Rise of the Tomb Raider have had Ryzen optimisation updates, so we've omitted our library results here. However, as we were looking at comparing eight-core overclocked performance, why not factor in Ryzen 7 too? After all, while we have 4.0GHz 1800X performance metrics here, the £290/$330 Ryzen 7 1700 should overclock to the same level (depending on your cooler). The results are fascinating overall, not least because even if you're still using a circa-2014 Haswell-E processor, you're still competitive with Skylake-X clock-for-clock. Indeed, in Ashes of the Singularity and Far Cry Primal, you're marginally ahead of Intel's latest and greatest.

Not only that, but you're also dealing with a mature platform if you're using a Haswell-E or Broadwell-E processor. During testing, we encountered stutter in Assassin's Creed Unity caused by Turbo Boost 3.0 (disabling it removed the stutter completely) and there were also intermittent issues with The Witcher 3, regardless of the processor we used - even the Kaby Lake-X Core i7 7740X had the problem, and that doesn't use the new many-core interconnectivity fabric. It's the sort of issue we expect to see cleared up in BIOS updates further on down the road, and the chances are you may not see it all, bearing in mind that most gameplay is GPU-limited - but regardless, it did catch us by surprise.

While the Intel results cluster together with each platform capable of claiming victory on an individual basis, it's the Ryzen 7 results we found most interesting - for two reasons. Firstly, while Ryzen 7 can duke it out with eight-core Intel chips in terms of productivity and computational tasks, it's still clearly in quad-core Intel territory for gaming - and based on these results, a Basin Lakes six-core chip at £350/$390 is the better gaming choice. However, in several titles, Ryzen 7 isn't that far behind its eight-core rivals, and remember that a Ryzen 7 1700 is just £290/$330 up against the £599/$549 Core i7 7820X. A 4.4GHz OC is nice for big chips like these Intel monsters, but in actual fact, they can be pushed further, so how does that change things up?

We found it fairly easy to overclock all of the Basin Falls Intel offerings, but what's remarkable about these chips is that despite being large processors running on a really tight 14nm FinFET fabrication process, there's a fair degree of overclocking headroom, even on top of the all-core turbo you'll get out of the box. However, caution is required. These processors - the i9 7900X in particular - draw anything up to 400W depending on how much voltage you put through them. We recommend a meaty power supply, a motherboard with four+eight pin CPU power inputs, plus an all-in-one liquid cooling solution - preferably one with a 360mm radiator, though our Corsair H110i GT did manage to contain the heat during gaming and intense video encoding. And we'd also make sure you keep an eye on VRM temperatures on your motherboard, though if you're dissipating CPU heat effectively, you should be okay. A basic rule of thumb is that voltages above 1.25v will cause some extreme consumption and heat generation, so it's best to stay significantly beneath that threshold.

The X299 boards and processors are designed for enthusiasts, so overclocking to the limit is going to be more common, but the question is really the extent to which you can expect improved results. We achieved 4.6GHz on the i7 7800X and (amazingly) the i9 7900X, while the i7 7820X seemed happy at 4.8GHz - a remarkable turnout. Meanwhile, the smaller 7740X processor hit 5.0GHz at 1.3v - though again, a substantial cooling solution will be required to sustain that. There are gains, but they are not hugely impressive with only the power-hungry Crysis 3 and the single-thread monster that is Far Cry Primal seeing much in the way of appreciable gains.

The bottom line is that while overclocking will increase your minimum frame-rates (where the CPU's role in gaming is usually most felt when things aren't running so well) the increased return you're getting perhaps isn't worth the bother in most games. And one thing we'd like to stress is that decent DDR4 memory can have more of an impact on gaming performance than overclocking. As the table below demonstrates, faster memory offers more performance than overclocking alone and only by pairing the two together do you get best results. However, bearing in mind the downsides of overclocking on the Basin Falls platform, shifting the emphasis to overclocking memory - or investing in faster modules - may lead to better gaming results. One of the strengths of the platform is that memory bandwidth in excess of 4000MHz is achievable, so scaling up there could throw up some fascinating results.

Intel Skylake X: Core i7 7740X/ 7800X/ 7820X/ Core i9 7900X - the Digital Foundry verdict

For now, Intel hosts the most capable consumer-level CPU on the market in the form of the remarkable Core i9 7900X, while both the six and eight-core 7800X and 7820X continue the firm's run of handing in hugely capable enthusiast-level processors. Don't go in expecting massive increases in gaming performance over a quad-core i7, but if you're heavily into content creation and a spot of Battlefield 1 or whatever, Skylake-X still has much to offer. Similarly, if you're running a high frequency monitor, a many-core processor will make a difference in select titles.

Skylake-X's mixed fortunes in gaming performance aren't quite what we wanted to see - and all the evidence suggests that the existing Haswell-E and Broadwell-E processors are still great performers - but the sense is that more forward-looking engines will make more of the new architecture. Meanwhile, the overclocking potential is also remarkable, even if the real-world improvement to gaming is minimal beyond the now-standard all-core turbo deployed by all board manufacturers.

But for gaming, while there are some noticeable gains, there is no real knock-out blow for the mainstream Core i7 chips, while comparisons with Haswell-E and Broadwell-E show that Intel's older many-core processors can still do the business. The Skylake-X price-cuts to the eight and ten-core processors are nice, but there is the sense that the enthusiast premium is still too much bearing in mind the change in market conditions, while continuing to cut-off PCI Express lane availability continues to come off as quite mean - something that must surely be coming to an end now AMD is offering access to all lanes for all processors across its range.

Regardless, out of all the chips reviewed here, while the 7900X and 7820X are perhaps the sexiest offerings, the six-core Core i7 7800X at £360/$390 is the only processor I would seriously consider buying for myself - it offers a lot more processing power than a quad for a relatively small premium, beats Ryzen on gaming, and it should be broadly competitive in productivity tasks too, though you may need to overclock to get the job done there.

But even then, a Skylake-X purchase in the here and now isn't a slam dunk - and there is the sense that the X299 chipset has still yet to mature, just as it took Ryzen time to fully bed in. Turbo Boost 3.0 seems like a good idea on paper, but for gaming at least, it seems to either do nothing or introduce additional stutter - in fact, general gaming stability in CPU-bound conditions doesn't seem quite so robust as prior Intel chipsets whether TB3.0 is active or not. This is borne out by the Core i7 7740X, which seems to deliver the same performance or worse than a similarly clocked i7 7700K - something we just shouldn't be seeing. Given time, we expect the kinks in the platform to be ironed out, and if you're in the market for a big chip processor, perhaps waiting a little while is for the best with AMD's Threadripper just around the corner.