Intel Skylake-X i7-7820X & Kabylake-X i7-7740X review

Intel new HEDT platform is here and the first members to hit our office are Core i7-7820X, a Skylake-X CPU with 8 cores / 16 threads, the replacement of i7-6900K, and Core i7-7740X, the KabyLake-X flagship.

The jump from Broadwell-E to Skylake-X brought some new and interesting changes, starting with the architecture up to the motherboard socket. So, it is a new platform all around. The good news is that LGA2066 supports many types of Intel Core series, so you can choose from i5 to i9 and they all run on the same motherboard.

7820X-7740X-set

Skylake-X family contains i5, i7 and i9 CPUS, while Broadwell-E only fits i7s. All new CPUs have the X suffix, so K is missing from this series.

Intel chose to include the 10-core 7900X in i9 series, not in i7, as for previous generation. So, the new CPUs are:

 

Base Clock (GHz)

Turbo 2.0 (GHz)

Turbo Boost 3.0 (GHz)

Cores / threads

L3-cache

PCIe 3.0

Memory

RAM channels

TDP

MSRP

Core i9 7980XE

2.6

4.2

4.4

18/36

24.75 MB

44

DDR4-2666

Quad

165

$1999

Core i9 7960X

2.8

4.2

4.4

16/32

22 MB

44

DDR4-2666

Quad

165

$1699

Core i9 7940X

3.1

4.3

4.4

14/28

19.25 MB

44

DDR4-2666

Quad

165

$1399

Core i9 7920X

2.9

4.3

4.4

12/24

16.5 MB

44

DDR4-2666

Quad

140

$1199

Core i9 7900X

3.3

4.3

4.5

10/20

13.75 MB

44

DDR4-2666

Quad

140

$999

Core i7 7820X

3.6

4.3

4.5

8/16

11 MB

28

DDR4-2666

Quad

140

$599

Core i7 7800X

3.5

4.0

-

6/12

8.25 MB

28

DDR4-2666

Quad

140

$389

Core i7 7740X

4.3

4.5

-

4/8

8 MB

16

DDR4-2666

Dual

112

$339

Core i5 7640X

4.0

4.2

-

4/4

6 MB

16

DDR4-2666

Dual

112

$242

 

Skylake-X vs Broadwell-E

Similarities are at TDP values and quad-channel support, but then the differences start to step in. First of all, it is the improved architecture that brings new capabilities and features.

The new socket LGA2066 contains the number of pins the name implies. The good news is that your LGA2011 cooler is perfectly compatible with X299 platform, so you can keep it.

Skylake-X has different cache architecture. In details, L2 cache was increased from 256KB to 1MB, so each core benefits from larger amount of data in its private stash, which means faster access to data, albeit higher performance. On the other hand, L3 shared cache was decreased from 2.5MB/core (Broadwell-E) to 1.375MB/core. Aside from capacity, Skylake-X L3 is not inclusive as for Broadwell-E (inclusive means that all data from L2 caches is also copy in L3), but exclusive, so data from one core’s cache will not be found that fast by other core, but Intel said they implemented new caching algorithms, so overall performance is better. On the other hand, larger L2 cache means higher single-threaded performance.

DDR4-2666 is certainly better than 2400, but the CPU IMC supports 4133+MHz which is far beyond the 3400MHz that we managed to reach on X99 platform.

Intel chose to implement fewer PCI-E lanes for Skylake-X i7 series, using only 28 of them for these CPUs. Only i9 has 44 lanes, so for multi-GPU setups, you have to spend some more cash.

Turbo Boost Max 3.0 was launched with previous generation, but it supported highest clock on 1 core, now being able to raise 2 cores at maximum frequency. In our case, i7-7820X has a top clock of 4.5GHz which is 900MHz higher than default 3.6GHz, so single/dual core capable apps will run at this frequency. Even better, each Skylake-X has its 2 best cores determined and will move any single/dual-threaded request to those cores for highest performance possible. The technology is supported on Windows 10 Anniversary as well as on Linux kernel from 2017.

AVX-512 is a new feature brought by Skylake-X family and not present in the previous desktop CPUs. Its usage is limited as capable software is not implemented yet. And even so, there a small group of users who will need it.

Last, but not least, the new chipset, X299, is basically a Z270 with small changes.

 

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