Power Consumption & Overclocking
Intel isn't using a soldered integrated heat spreader (IHS) for Skylake-X and Kaby Lake-X. Instead of the metallic solder most enthusiasts want to see, heat moves from the die to the IHS through inexpensive thermal interface material, which is just a fancy name for common thermal paste. This decision has implications for both our power consumption measurements and overclocking efforts.
First, we had to use our industrial-grade Alphacool Eiszeit Chiller 2000 cooler to achieve meaningful test results. Air cooling is out of the question for Core i9-7900X, and even a decent all-in-one liquid cooling solution won’t get far.
Power consumption is measured after the voltage converters and CPU, using points on the motherboard. The 230A we saw during our overclocking tests prompted us to add a fan to the motherboard. We only had one platform for this story, so we played it safe with extra cooling around the LGA interface.
These numbers are generated using stock motherboard settings; any significant under-volting had no effect, except to cause stability problems.
At idle, the Core i9-7900X comes in below its predecessor and AMD’s Ryzen models. Getting there required a firmware update to correct problems with P-states, among other issues plaguing this platform right up until launch.
Even lightly threaded workloads like AutoCAD 2015, which uses just a few of the Core i9-7900X’s cores, catapult Intel’s -7900X to the back of the pack in power consumption metrics.
This is almost certainly due to higher clock rates than previous-gen HEDT CPUs like the Core i7-6950X. Taking into account Skylake-X's increased performance, though, the Core i9-7900X remains a more efficient solution. One percent more power consumption gets you 29 percent more 2D performance and a massive 39 percent more 3D performance. Both tasks profit from Turbo Boost Max Technology 3.0 in a big way.
If more than three cores are used, the Core i9-7900X’s advantage shrinks considerably. It still beats the Core i7-6950X by approximately 17 percent, but the power consumption increase is a lot larger (around 10 percent).
Pushing all of the Core i9-7900X’s cores with Prime95 or LuxRender propels power consumption to incredible heights. You do get 48 percent more rendering performance in LuxRender, but at the expense of 58 percent-higher power use. This approach has the elegance of a sledgehammer. Then again, if you need speed at any cost, Core i9-7900X is top-notch.
Overclocking and Stability
We now know that the Core i9-7900X’s performance to power consumption ratio turns negative as you utilize more of its on-die resources. Of course, this has to be factored into your overclocking plans, since many coolers can't cope with the heat dissipated by a >200W processor.
Stable overclocking, defined as reliable operation under Prime95 for prolonged periods of time without hitting a temperature limit, wasn’t possible beyond 4.4 GHz. Reports of >5 GHz with all cores active should be taken with a grain of salt. We did boot into Windows at 5.1 GHz, but running actual applications resulted in either a BSOD or a motherboard emergency shutdown.
We did manage to achieve a stable 4.8 GHz overclock under the single- and multi-core Cinebench R15 benchmarks. However, our cooling solution was probably the decisive factor there. Realistically, 4.5 GHz should be achievable with an all-in-one liquid cooler. Let’s take a look at the comparison curves for Cinebench R15 with all cores active:
At a Vcore of 1.4V, the system stayed stable for 10 benchmark runs. Intel's Core i9-7900X consumed an average of 261W, while individual peaks jumped as high as 293W. A test at 4.8 GHz using 20 instances of a year-long shading computation for a rooftop photovoltaic array, including profit calculation, pushed power consumption all the way to 335W. The motherboard shut down after we started Prime95 without limiting AVX. The last recorded value was 364W.
The performance and power consumption curves yield some interesting findings. Performance scales with clock rate in an almost linear fashion. Power consumption increases faster, but not as rapidly as we feared that it might.
The more pronounced curve is due to relatively high values at lower frequencies, where the “background noise” (load caused by other system components) is more noticeable.
So, what’s the final verdict on overclocking Intel's Core i9-7900X? It depends on the quality of your cooler and its ability to move heat off the CPU as quickly as possible.
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