Table of Contents
What’s the difference between Intel Skylake-X core-i9 and core-i7 "enthusiast extreme" "X" processors and Skylake-W (Workstation) Xeon single socket processors? They seem nearly identical. I’ll discuss the differences and make some recommendations on which to use.
First, I really like these processors! I can hardly believe that you can get a CPU for a simple desktop workstation that will perform over 1000 GFLOP/s in double precision. And, you can get that performance for under $2000.
If you are impatient I’ll tell you that these processors are basically the same. To see that you can scroll down to the Linpack results to see for yourself.
Muddy "Lakes"
What about Intel core-i7 Coffee Lake? Isn’t that a "newer" "Lake" processor. Shouldn’t that be better? Nope, it’s not "better" and it’s not "newer". I looked at Coffee Lake for compute. I compared the i7-8700K 6-core against the i7-7900X (Skylake-X). The Skylake-X processors bested the Coffee Lake processor by around 40% for compute. Coffee Lake is great for a lot of use cases since it has very high core clock frequencies. For a "gamer" or "office", (or "normal"?) user it may be a better choice than Skylake-X or -W.
Intel uses code names for processor families during development and the press and industry then often persist in using those names going forward. It can be confusing. The following lists may make things a little clearer.
Xeon
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Sandy Bridge, Ivy Bridge, Haswell, Broadwell — These were v1, v2, v3, v4 respectively, i.e. 2600v1 etc.. These share mostly the same architecture but Haswell made some nice improvements.
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Skylake-SP — These are the new Xeon Scalable Processors. This is a a completely new architecture and unfortunately, new naming scheme. (See my "Xeon-Scalable" Buyers Guide to weed through that mess!).
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Skylake-W — These are the single socket "Workstation" version of the new Xeon Skylake architecture (on a smaller die).
Core i(3,5,7)
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Haswell, Broadwell, Skylake, Kabby Lake, Coffee Lake — These are all basically the Haswell architecture similar to what was in the Haswell Xeon. They have ID numbers that went 4xxx, 5xxx, 6xxx, 7xxx, 8xxx and were 4th, 5th, 6th, 7th, 8th, generation. Nice! Really they are all pretty much the same but with a progression of adjustments for overall better performance as they progressed.
X Series ( Core i7, i9 )
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Haswell-E, Broadwell-E, Kaby Lake X — These were the "Extreme" versions of the regular core-i7 processors that had high clocks and sometimes extra cores. They were still the same basic architecture. They were numbered 5xxx-E, 6xxx-E, 76xx-X and 77xx-X
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Skylake-X — This is a departure from the other X series processors. Skylake-X is the same core archetecture as in the new Xeon Scalable processors (and Xeon W series). It’s basically the same as Skylake-W Xeon. These processors are numbered 78xx-X and 79xx-X. They are available with from 4 to 18 cores (same as Skylake-W). The 79xx-X numbers are the core-i9 series with from 10 to 18 cores.
Spec comparison of Skylake-X and Skylake-W
The following table lists some of the specifications for the 10-core Skylake-X core-i9 7900X and the 10-core Skylake-W Xeon W-2155. I’m not listing all of the specs that are available on Intel Ark for a couple of reasons — There are too many specs, and many are irrelevant for my purpose. This link is the full comparison list on Intel Ark.
| Core i9-7900X | Xeon W-2155 | |
|---|---|---|
| Vertical Segment | Desktop | Server |
| Recommended Customer Price | $989.00 – $999.00 | $1440.00 |
| # of Cores / Threads | 10 / 20 | 10 / 20 |
| Processor Base Frequency | 3.30 GHz | 3.30 GHz |
| Max Turbo Frequency | 4.30 GHz | 4.50 GHz |
| Cache | 13.75 MB L3 | 13.75 MB |
| Intel® Turbo Boost Max Technology 3.0 Frequency | 4.50 GHz | See Max Turbo |
| TDP | 140 W | 140 W |
| Max Memory Size (dependent on memory type) | 128 GB | 512 GB |
| Memory Types | DDR4-2666 | DDR4 1600/1866/2133/2400/2666 |
| Max # of Memory Channels | 4 | 4 |
| ECC Memory Supported ‡ | No | Yes |
| Max # of PCI Express Lanes | 44 | 48 |
| Intel® Turbo Boost Max Technology 3.0 ‡ | Yes | No |
| Instruction Set Extensions | SSE4.1/4.2, AVX2, AVX-512 | SSE4.2, AVX, AVX2, AVX-512 |
| Intel® vPro™ Technology ‡ | No | Yes |
| # of AVX-512 FMA Units | 2 | 2 |
Notes:
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First thing to notice is that the Skylake-W is significantly more expensive. That price premium for the Xeon W-series holds for all of the different (equivalent to Skylake-X!) models.
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The next thing difference the Max Turbo frequency. However, note that the Skylake-X has Turbo Boost 3.0 which is set to 4.5GHz (that’s only available on Windows and you have to run software to enable it). The Skylake-W "just" has Turbo Boost 2.0 but it’s set to 4.5GHz.
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The third thing to notice is that Skylake-X supports up to 128GB memory and Skylake-W up to 512GB. This is the "official" spec but it is actually motherbord dependent. For example the Gigibyte X299 boards support Registered memory which allows 512GB for Skylake-X.
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Skylake-W does support ECC memory error correction. I personally don’t consider that a necessary feature on a workstation … on a server or cluster, yes!
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The Skylake-W has 4 more PCIe lanes, which is always nice to have.
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Skylake-W supports vPro and you can consider that a feature or a bug depending on your taste for out-of-band remote management capability/vulnerability.
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The most important thing to note if you are interested in performance for mathematical calculations is that they both support the AVX-512 vector unit!
Mathematical compute performance for Skylake-X and Skylake-W
You can argue it about it if you like but I still believe that the standard optimized Linpack benchmark is the best CPU performance measure for serious mathematical computing. This is a "simple" benchmark that is solving a system of linear equations making calls to BLAS routines. (Basic Linear Algebra Subroutines those are core matrix and vector operations). Traditionally the best compute performance for linear algebra based problems is obtained from a vendor optimized BLAS library. For Intel hardware that would be MKL, the Intel Math Kernel Library. On Intel hardware that is about as good as it gets for performance.
I ran the Intel optimized Linkpack benchmark with problem sizes (number of simultaneous equations) ranging from 10,000 to 70,000 using from 1 to 10 cores of the Skylake-X 7900X and Skylake-W 2155 processors. The highest performance numbers I got are listed in the table below. (The best performance was usually at a problem size of 60,000).
I’m only reporting numbers for the two 10-core processors but I have run jobs on most of the available core-counts from 4 to 18 cores. The relative performance comparison holds for all of them.
Intel Core i9 7900X and Xeon W2155 Linpack GFLOPS
| CPU Cores Used (OMP_NUM_THREADS) |
i9 7900X GFLOPS | Xeon W2155 GFLOPS |
|---|---|---|
| 10 | 708.3 | 693.3 |
| 8 | 619.7 | 620.0 |
| 6 | 477.9 | 531.3 |
| 4 | 356.2 | 407.2 |
| 2 | 195.3 | 216.1 |
| 1 | 100.7 | 108.7 |
Those numbers mostly speak for themselves. They are great! You do see that there is some variation in relative performance depending on how many cores are being used. This is because the core clocks are slightly different for these processors at the various core utilization numbers. I was not able to find any technical documentation for these processors that give the per core clocks.
There are 5 different core clocks in effect for these processors! See my post on the Intel Xeon Scalable processors for a discussion of this. Intel Scalable Processors Xeon Skylake-SP (Purley) Buyers Guide
Recommendation and Caveats
The Skylake-X really does offer the best price/performance by a significant amount. The performance is essentially identical to the Skylake-W. These two processor families are also chip-set incompatible! The Skylake-X uses the X299 chip-set and there are lots of motherboards available (mostly "gamer" oriented but that is not necessarily a bad thing!). The Skylake-W is a Xeon part and it uses the C422 chip-set. There are not many motherboards available with this chip-set as of this writing. Also, for the X299 boards there has been some BIOS problems from various manufactures. These are, thankfully, mostly fixed now. There are also some interesting boards that are just not out yet so early 2018 will see some more interesting options for X299.
Recommendation, go with Skylake-X. If you need Xeon then you may want to consider going with a "standard" dual socket Xeon configuration using the new Xeon Scalable processors. Under the hood all of these processors are have the same basic core and they are fantastic!
Happy computing! –dbk