Intel Core i9-7900X ‘Skylake-X’ 10C20T CPU Indepth Analysis

1. Introducing Skylake-X: Core i9-7900X2. Overclocking the Core i9-7900X3. Testing Methodology4. CPU Tests 1: Cinebench, Handbrake, x265 Encoding5. CPU & Memory Tests: 7-Zip, Kraken, Sandra6. Gaming Tests 1: 3DMark, GTA V, Metro LL7. Gaming Tests 2: Ashes, GRW, Total War8. 1440P & 4K Gaming Tests: GRW & GTA V9. Technical: Power Draw, Efficiency, Temperatures10. Closing Thoughts11. View All Pages

We leave the system to idle on the Windows 10 desktop for 5 minutes before taking a power draw reading. For CPU load results, we read the power draw while producing five runs of the Cinebench multi-threaded test as we have found it to push power draw and temperature levels beyond those of AIDA 64 and close to Prime 95 (non-AVX) levels. Even five continuous loops of Cinebench results in a short run time on high-performance CPUs which influences the validity of the temperature reading, so we run 5 minutes of AIDA64 stress test to validate data.

The power consumption of our entire test system (at the wall) is shown in the chart. The same test parameters were used for temperature readings.

Power Consumption

Power draw readings are accurate to around +/-5-10W under heavy load due to instantaneous fluctuations in the value. We use a Platinum-rated Seasonic 760W PSU (with 8-pin plus 4-pin power connectors where possible) and install a GTX 1070 video card that uses very little power.

The Core i9-7900X is a power guzzler, which partially explains its superb performance metrics. The 10-core Skylake-X system draws 73W (32%) more power than the next most power-hungry stock system which is the Core i7-6950X. Add almost 100W (almost 50%) onto the power draw of a Ryzen 7 1800X system to get to the Cinebench load usage for the Core i9-7900X. AIDA64 load sees the i9-7900X throttling its clock speed back to around 3.6GHz, down from 4GHz all-core, to reduce power consumption in the synthetic stress test.

Switching focus to overclocked results, nothing can match the 4.6GHz 1.20V Core i9-7900X when it comes to load power draw. With the system pulling more than 400W from the wall, many of us who said >1kW PSUs are unnecessary in the modern day may be about to eat our own words. Make sure that beefy PSU has 8-pin plus 4-pin CPU power connectors, as this level of power draw points to significant stress on a single 8-pin connector, not to mention the motherboard VRM. Despite its lower VCore (1.20V versus 1.275V), the 7900X draws 53W (15%) more power than the 4.2GHz overclocked 6950X in Cinebench. Looking at the 4.05GHz Ryzen 7 1800X, the increase in power draw to our overclocked 7900X is a sizeable 142W (54%). A 4.8GHz 4C8T Kaby Lake 7700K uses less than half the power of the 7900X.

I guess that spells a convincing end to those jokes about AMD being power-hungry. Judging by the power consumption numbers, Intel has taken a brute force approach with as much finesse as a bulldozer to achieving ground-breaking performance with the Core i9-7900X. Personally, I am all for higher power draw if the performance warrants it. However, it is also clear that many consumers value efficiency, not to mention a reduction in thermal energy pumped into their chassis or cooling loop or workspace.

Performance per Watt in Cinebench, which we will loosely regard as rendering power efficiency, is better on the i7-6950X and Ryzen 7 1800X compared to the i9-7900X. Overclocking results in reduced performance efficiency for all CPUs and the 7900X is hit hard in this metric. Ryzen 7 1800X continues to look like a solid performance per Watt option even when overclocked. AMD's chip backs up that claim with strong all-out performance in Cinebench, though nowhere near the Intel 10-core CPU levels.

Temperatures

Temperature recordings were taken using a 280mm Corsair H110i GT all-in-one liquid cooler with its two fans running at full speed (around 2300 RPM). Ambient temperatures were held around 23°C (and normalised to 23°C where there were slight fluctuations).

We read the Tdie temperature for Ryzen which accounts for the 20°C temperature offset. The charted temperatures are without the 20°C offset added on. Ryzen idle temperatures are omitted due to inaccurate readings caused by sensor drift.

There is not much more to say about thermal performance of the Core i9-7900X that has not already been said – it's far from a cool-running chip. While that's fine in theory – you want higher performance, you'll have to accept higher operating temperatures – the reason for the poor thermal performance is what will leave a sour taste in enthusiasts' mouths. This is not simply a case of ground-breaking performance resulting in high load temperatures, it's largely driven by Intel's decision to use cheaper thermal interface material (TIM) beneath the heatspreader rather than the superior solder approach.

Reasons for Intel's decision to use TIM rather than solder are unclear; the per-unit costs differential between TIM and solder is likely to be insignificant on a $999 processor, even when factoring in the economy of scale. The large Skylake-X die is unlikely to present concerning failure mechanisms for solder, either, so perhaps the health and safety procedures and certification processes at Intel's factories are where TIM really shows its cost benefit over solder. Or perhaps Intel simply does not care that TIM starts to show limitations only when the vendor's processors are run outside of specification (overclocked).

Whatever the reason, it is not a move that has stood well with enthusiasts. Consumers were disappointed by the poor thermal performance of the Kaby Lake 7700K to the point where DIY de-lidding kits became popular. That's a whole new level of seriousness when trying to force the heatspreader off a $999 processor. I do hope that Intel realises the issues presented by using a lower-quality thermal design for HEDT parts and addresses the situation in future product iterations.

It's not just overclockers feeling the pain here, as prosumers and workstation buyers who value low-noise operation at stock operating parameters are also realising the caveats of a limited thermal design. The sub-60°C stock-clocked load results shown in our chart may seem good for the Core i9-7900X. However, when factoring in the 2300RPM fans used on a high-end 280mm AIO cooler to achieve those results, thermals look far less impressive.

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