Skylake – Overclocking – Power Consumption and Voltage Scaling
Intel just released the new Skylake CPUs and so far the CPUs seem to be great for overclocking. I already had the chance to test some 6700Ks during the ASUS Absolute Zero event few weeks ago to see how they scale with extreme cold – impressive! Depending on the individual chip quality 6,0 – 6,8 GHz is possible on LN2 and quite easy to achieve using a good motherboard. But how good and how high do these chips scale with normal cooling? I used one of my samples for a voltage scaling and power consumption analysis.
I won’t explain the whole overclocking process of Skylake here because I’m already working on a separate OC-Guide. However, Skylake overclocking in general is really simple. Basically all you have to do is increasing the CPU core voltage and CPU multiplier. There are some additional features you should play with such as loadline calibration and C-States but that’s about it.
- ASUS Maximus VIII Hero
- LEPA G1600 PSU
- 2 x 4 GB Kingston Predator 3300 C16
- Noctua NH-D15 (1 Fan, 2 Watt)
- Nvidia 8400GS (16 Watt 2D Load)
- Corsair F60 SSD (2 Watt Load)
- Noiseblocker 120 mm Fan (2 Watt)
- Energy Cost Meter to measure the power consumption
The two memory sticks will run at 3200 C16 for the test. For high memory frequencies you have to increase some of the additional voltages. These are the settings I used:
Considering the system components, the idle load of the system has to be higher than 22 Watt because of the GPU, SSD and system-fans. The LEPA G1600 PSU might not be the best choice because PSUs usually have a bad efficiency below 20 % load so this is kinda a worst case situation. Most PSUs will drop to around 70-75 % efficiency below 20 % load – even if they are 80+ gold certified. However, we’re only talking about few Watts difference.
Skylake Overclocking – Voltage Scaling
Both, 6600K and 6700K have a very high overclocking potential. Even though the die size is smaller than Haswell, the CPUs don’t have temperature issues on high load. While temperature is something you can compensate with a good cooling solution or delidding, you have no influence on the individual chip quality.
I tested my 6700K in 100 MHz steps from 4100 MHz to 4700 MHz to see how much additional voltage you need from step to step. Each step was tested by 1h of Prime95 with 1344K FFT. I tested in 5 mV steps to be sure this is the lowest voltage I could reach with my chip. The cache frequency was always at 4100 MHz.
Starting from 4100 MHz at 1.136 Volt, the CPU needs additional 32 mV per 100 MHz until 4400 MHz. Afterwards we can see a step of 48 mV followed by two steps of 64 mV. The result is a quite high core voltage of 1.408 V for 4700 MHz. We can see the core voltage is following an exponential growth.
Even with the very strong Noctua NH-D15 the CPU hit around 82 °C core temperature at 4700 MHz. I noticed that this CPU is getting unstable above 85°C, no matter how high you push the voltage.
So this is the limit with this chip unless I would swap the TIM between DIE and IHS. Considering the fact that this is just an average chip – I’m not disappointed.
Skylake Overclocking – Power Consumption
The power consumption is an essential part when it comes to overclocking. It will increase the temperature of your CPU, room temperature and eventually your electricity bill.
Taking a look at the stock power consumption I noticed that the core voltage was way too high. 1.392 Volt result in a power consumption of 191.8 Watt during Prime 95 8K FFT load. This even exceeded the power consumption of the same CPU manually overclocked to 4600 MHz. After some additional testing I found out that the CPU would still run stable at 1.168 Volt running 4200 MHz – saving almost 50 Watt on load.
While you can undervolt your CPU to save some energy on load it seems like the stock setting is the best in idle. I tried a lot of different configurations of Speedstep, C-States and the Voltage mode, but I could not match the 53,3 Watt of the Auto setting.
The power consumption curve looks pretty similar to the voltage scaling and also shows an exponential growth. The stock turbo clock of 4200 MHz is possible at 1.168 MHz, which results in a power consumption of 143 Watt. Compared to idle this is an increase of 81.3 Watt. 4300 MHz are possible with a very small increase of 5.5 Watt. Going from 4300 MHz to 4400 MHz you already have to accept the double increase – 10.6 Watt. From my point of view each step up to 4600 MHz is worth it. However, the step from 4600 MHz to 4700 MHz results in almost 30 Watt additional power draw, which is probably too much considering the extra performance you will get in return.
I usually create overclocking-guides for each generation and each generation I come across the same kind of comments. “Why does my CPU not clock down anymore in idle?”… “I don’t want the extra power draw because of the high voltage in idle”…”I don’t want to use the Override mode, I want to use Offset because of the lower power draw”.
Basically people think that a low voltage and low core clock in idle will result in a huge difference. However, this is not true. 99 % of the power consumption is a result of the utilization.
I tested 5 different combinations – playing with C-States, Speedstep, Windows Power Saving Modes and vCore Mode. As you can see in the chart below, there is no big difference. The power consumption during 8K Prime95 load is almost the same in all scenarios – only Offset results in a slightly higher power draw on load. Then again, Offset results in the lowest power consumption in idle because of the decreased voltage.
The core clock in idle also doesn’t really affect the power consumption. There are only 2 Watt difference between 800 MHz and 4600 MHz in idle – safe to say that you can always use the full speed.
Both, voltage scaling and power draw are following an exponential growth. Depending on the CPU quality you should be able to hit 4.5 – 4.7 GHz at a healthy voltage and power draw with the current Skylake CPUs. However, at a certain point the increased frequency will draw too much power and the additional frequency will not pay off.
You also don’t have to worry about any power saving options or voltage modes. My advice is to keep Speedstep enabled, disable C-States and use the Override voltage mode. This will be the easies way to overclock and you won’t experience disadvantages over the other options.
Consindering the 50 Watt I saved by undervolting my CPU, I strongly advice to avoid Auto CPU core voltages.
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