During the time of the first generation Intel Core i series, Tom's Hardware made an article about finding the most efficient clock speed for Core i5 750 http://www.tomshardware.com/reviews/core-i5-750-efficiency,2500.html and Core i7 920 http://www.tomshardware.com/reviews/overclock-core-i7,2268.html. I got curious as to what is the most efficient clock speed for my Core i7 4790K.
[CPU] Intel Core i7 4790K
[GPU] Gigabyte GTX 970 mini-ITX
[MOB] Gigabyte Z97-D3H (F7 bios)
[RAM] G. Skill Trident X 4GBx2 DDR3 @ 2400 MHz 10-12-12-31 2T
[HSF] Noctua NH-U12S
Windows 7 64-bit with Service Pack 1
For productivity testing, I only used HandBrake. The short movie (about 10 minutes long) Big Buck Bunny was encoded from AVI to MP4 using High Profile preset. Video resolution is 1080p. Big Buck Bunny is an open movie which was created using open software. You can download the movie here https://peach.blender.org/about/. To save time, I only encoded the first 60 seconds of the movie.
To know the energy used for encoding the video, I measured the time it took to complete the task then multiplied it by the power draw during the task. The clock speed and voltage with the lowest energy used will be the most efficient.
Turbo Boost was disabled.
The results below are the average of 3 runs
Overclocking Haswell reduces energy efficiency. Because every chip will differ in the required voltage for a given clock speed, it's hard to give a fixed value for the most efficient clock speed for Core i7 4790K. But based on my test, the most efficient clock speed for Core i7 4790K is the one that you can achieve with a core voltage of ~0.900 V.
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