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Dr Donald Kinghorn (Scientific Computing Advisor )

Intel Skylake-X vs Skylake-W

Written on December 11, 2017 by Dr Donald Kinghorn
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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

  • 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.

  • 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!).

  • Skylake-W -- These are the single socket "Workstation" version of the new Xeon Skylake architecture (on a smaller die).

Core i(3,5,7)

  • 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 )

  • 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

  • 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 i9-7900XXeon 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:

  • 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.

  • 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.

  • 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.

  • 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!

  • The Skylake-W has 4 more PCIe lanes, which is always nice to have.

  • 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.

  • 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 GFLOPSXeon 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

Tags: Skylake-X, Skylake-W, Intel Xeon, Processors
Boyuan Ning

Hi Donald,

It is really helpful and joyful to visit your HPC blog. I do not know if there is an chance for you to run some tests of the latest AMD Ryzen serises and comparisons with Intel CPUs? Because AMD products seem more suitable for saving budget. Besides, will the computational performace of a GPU be different on Intel or AMD platform?

Still, really enjoy your blog.

Bo-Yuan

Posted on 2017-12-24 02:13:27
Donald Kinghorn

Hi Boyuan,
I just saw your comment ... I wont be testing Ryzen but I will be testing EPYC. In fact I'm really looking forward to that!
Also, I really want to do some GPU compute work on AMD cards this year. I'll be looking art ROCm but I'm not sure what cards I'll look at yet.
Thanks --Don

Posted on 2018-01-11 04:51:38
A

Hi Donald, Love the articles. Great to see a shop that knows significantly more than I do. Do you have any idea when (or if) there will be an Xeon-W refresh similar to the Skylake-X refresh announced today?

I'm on Broadwell-E and I'd like more cores, AVX-512 and looking for an upgrade. I have 128 GB of UDIMMs which would be nice to use for now, but it would also be nice to have the possibility of 256GB in the future and I'm not sure that's possible, need a board that supports UDIMMs and RDIMMs (I've seen rumours the Asus WS SAGE x299 does but it might stop with a future BIOS upgrade!).

So I could get an i9-9980 and a SAGE x299 board and hope this continues to support 256GB RDIMM, or I could get an existing W-2195 and a C422 motherboard, or wait for a possible W-2195 update like the Skylake refresh (and lose my current RAM either way).

What to do? Leaning more to the Xeon-W side ...wait for a refresh or order today? Thank you!!

Posted on 2018-10-09 07:57:28
Donald Kinghorn

I was wondering the same thing myself ... just a few hours ago :-) Intel will have the new "X series" CPU's soon (we are waiting on samples) Those might be a nice way to go on something like that ASUS Sage. Both the x299 and c422 versions seem to be really good. But as you are aware the BIOS "extra features" can go away and there will surely be an update for the new X series procs ... who knows!

I really like Xeon-W but I'm really curious about the new i9 X series. Just comparing info on the Intel site it looks like it is a minor revision. Just a bump in clock freqs. I don't know for sure but maybe they have hardware fixes for Spectre and Meltdown ??? It will probably be a worthy update but not a huge performance boost.

We may not see an update to Xeon-W until the new Xeon cores are finalized. They may do the same kind of incremental update as they are doing for X series. I'm only guessing! My personal system now is a Xeon-W 14 core 2175 with 128GB mem on an ASUS Pro SE board. It's been really good! I have a 1080Ti and a Titan V in it right now too :-)

I totally understand your dilemma! One thing I know for sure, Skylake-X or Xeon-W would be a significant update to your Broadwell sys. I moved from Haswell to the Xeon-W and was stunned by the performance. It's a tough call between Skylake-X or new X-series or Xeon-W. If you can wait to see what pricing and performance are like for the new X procs that could be a good way to go. However, the Xeon-W is just a great processor!

Posted on 2018-10-09 21:48:35
Jorge Oliveira

Great article! How about the number of memory controllers? In case of skylake-sp there are 6 channels (with 2 memory controllers). How many memory controllers do skylake-x and skylake-w have? It is well known that they mostly come with 4 channel memory, but I couldn't find anywhere how many imc's they have.

Posted on 2019-04-06 22:13:41
Donald Kinghorn

That's a good question but I really don't know. I would guess that there are 2 dual controllers?? I did try to find some docs but didn't come up with anything good. There is the ARK stuff which is useful but not detailed https://ark.intel.com/conte... There is good tech documentation for their data-center stuff like Skylake-SP "scalable" but it is hard to find good detail specs on anything else.

I have some links to some of those tech doc areas on the Intel site in the original post I wrote about the "Purley" release. You might enjoy that post! I did a fair amount of analysis on those processors trying to make recommendation on what was worth considering for different use cases. (there's a couple of good links in the last section) https://www.pugetsystems.co...

Posted on 2019-04-08 17:11:37