Date: Wednesday, July 19, 2017
As per Intel's ARK Page for the 7900X, this CPU has a base frequency of 3.3GHz and Max Turbo Frequency of 4.3GHz with a TDP of 140 watts. However only two cores run at that 4.3GHz Turbo frequency under full load. This CPU is built using Intel's new Skylake-X architecture using a 14nm lithography. Yes, it's a big power-hungry CPU right out of the box running at stock clocks, but what happens when we try to overclock it? There have been rumors of its overclocking abilities being hindered by its motherboards power supply and cooling, but the story is a more than that, and quite frankly, may not hold much merit at all if you choose your platform wisely.
This article is not going look into benchmarks as that has already been attended to by others, but the overclocking abilities of this 7900X CPU and motherboards are still a bit misunderstood and we look into that a bit more.
We used the MSI X299 MSI Plus motherboard for our testing. It is currently in retail at Newegg for $260, and at Amazon for $261 with Prime Shipping. The Intel Core i9-7900X is listed for $1030 at Newegg, and $1062 at Amazon, but good luck finding those in stock.
We are also using 32GB of Corsair (CMK32GX4M4B3600C18) using its XMP profile at 3600MHz.
We are using a triple-fan Koolance radiator that is rated at 900 watts. It is hardly a "small" liquid cooling system and has served me well over the years when it comes to overclocking. I would not suggest this system is "the best," but it is a long way from AIO cooling and seeing that the radiator is about a foot long, it has a lot of volume and takes a while to heat-soak.
Our overclocking shown here is using all cores "locked" to run at the same clock, very much unlike the performance you would get relying on Intel stock Turbo clocks.
I run a 6950X in my personal system at 4.3GHz that I use for encoding 4K video using Adobe Premiere Pro, so out of the gate, I was aiming just a bit higher and figured 4.8GHz under full load would be the place the start. Damn the torpedoes.
To put it succinctly, 4.8GHz/3600MHz was not a reality with our specific 7900X under any kind of load. Starting any kind of CPU intensive task would BSOD the machine immediately, so we backed down to 4.7GHz. 4.7GHz was not to be had with this CPU either. While it was much more stable than 4.8GHz, a single instance of a 4K HandBrake encode would sink our OS almost immediately.
Where we ended up was at 4.6GHz with a 3600MHz memory bus, which we will talk more about below in what it took to achieve this. Keep in mind this is using three instances of HandBrake which does use the AVX instruction set which do cause CPUs to run much hotter when these instructions are being used. I think our load testing is a "worst case" real-world scenario.
The MSI X299 SLI PLUS is the third X299 motherboard, that I have had the pleasure of working with, and MSI has been the best so far. I am not going to discuss some of the lesser results of the other motherboards at this time, as I think this platform is still incredibly immature, was rushed out by Intel, and the motherboard manufacturers still need some time to get their ducks in a row on this one, much like what we are seeing with AM4 motherboards. That said, MSI seems to have its ducks lined up well with the X299 SLI PLUS.
These new Skylake-X processors have an Internal Voltage Regulator (IVR), and on some of these motherboards you can alter the voltage input to the IVR in the UEFI. If you decide to do this, I highly suggest that you pay a lot of attention to at-the-wall power usage and do not make big adjustments to IVR. I have seen our X299 systems pull near 500 watts when adjusting IVR input. As it stands now, I have not seen IVR input voltage adjustments make a huge amount of difference it getting stable overclocks. So I put this here as just a word of warning.
On our MSI system we are seeing idle at-the-wall usages of 160 watts. Under the full load with the 4.6GHz overclock, shown above, we registered ~350 watts. These 7900X CPUs have the ability to pull a LOT of wattage, so again, be aware.
Dialing in the 7900X overclock with granularity will pay off in spades in terms of stability. You absolutely want to feed the CPU as little voltage as possible as these CPUs run tremendously hot. I know that many of you do this already, and it is the way it should be done, but recent Intel CPUs have been much more forgiving when it comes to heat production, so just do not think that the 7900X follows that trend.
This 7900X is very much akin to what we have seen with AMD Ryzen 7 CPUs. The 7900X heats up quick beyond the 4.3GHz mark as you try to scale the clock.
You might notice that CPUz above is not reporting vCore voltage correctly. Checking while writing this shows me that CPUz v1.80 has been made live on their site and is now available for download, and does report vCore correctly. This will make your tweaking at home much easier.
For our 4.6GHz overclock above, we did need to set vCore to 1.20v in the UEFI, and we did need to set the CPU Load Line Calibration Control to its highest level, which happens to be "Mode 8" on the MSI X299 SLI PLUS. I would suggest that CPU LLC is another area in UEFI tweaking that could net you better temperature and overclocking results if you spend the time to tweak these, and you will likely be required to use LLC to get stable "edge of the envelope" overclocks. Setting this particular 7900X to 1.25v vCore cost a good bit of wattage and heat, as I talked about above, the voltage will ramp the CPU's heat output exponentially.
As for our Corsair RAM setting, I simply set the XMP profile and I did not try to overclock the RAM beyond this level. Seeing that we were scoring around 60GB/s in benchmarks, I figured that was good enough for what we were doing at the time with overclocking.