Update on Xeon W: Turbo Tablesby Ian Cutress on August 7, 2018 9:00 AM EST
- Posted in
- Turbo Boost
Last week we published our Xeon W review - Xeon W is the new name for Intel's Xeon E3-1600 series, but effectively replaces both the E5-1600 and E5-2600 chips that were commonly used in workstations. The new Xeon W line uses Skylake-X equivalent CPUs but enables ECC support for up to 512 GB of DRAM in a system, and while it uses the same LGA2066 socket as Skylake-X, due to product segmentation, Intel requires that the processors be used in a motherboard with an enterprise-grade C422 chipset.
You can read our full review here, where we tested a high, medium, and low-end CPU from the range, as well as the two quad-core parts that are officially 'off-roadmap'. In the review we go into what exactly 'off-roadmap' means.
The Intel Xeon W Review: W-2195, W-2155, W-2123, W-2104 and W-2102 Tested
One of the questions that came out of that review were the per-core turbo values for each processor. Intel has of late had a bifurcated strategy when it comes to disclosing turbo values: on the consumer line it does not disclose any turbo values any more, except single-core turbo, and on the enterprise lines they have fortunately been forthcoming with the data when asked. Although it isn't an automatic process to get the data, we are thankful that it does turn up. The point of this article is to state we finally have the turbo values for Xeon W.
Intel's per-core turbo data for these workstation parts are split up into three sections, due to the instruction sets they have. On the 'hardest' instructions, Intel uses special turbo values for AVX-512, as due to the way these instructions are processed, more heat is generated on chip. The chip has to balance frequency and power draw, so the AVX-512 data comes in at a lower frequency in order to keep the turbo in check.
The first thing to notice with this data is that for most CPUs, when the whole CPU is using AVX-512 instructions, the frequency will drop below the base frequency. For chips like the Xeon W-2123 and W-2133, even single core loading of AVX-512 will drop the frequency below the base frequency. Intel's base frequency does two things: first, it tells you the frequency at which TDP is applicable, and second it is the guaranteed minimum frequency for regular non-AVX instructions.
Behind AVX-512 is AVX2, which is still somewhat of a strain on the processor beyond regular instructions, but not as much. Where AVX-512 requires dedicated die area for support of the vector units, AVX2 is built into the back-end of the standard core design.
For AVX2, the W-2133 and W-2123 still end up below the base frequency of the processor. But for the big ones, like the W-2195, the full 18-core loading of AVX2 is 500 MHz faster than AVX-512. This is just an indication that users that are fine-tuning code should think about how much of the AVX-512 unit they can keep fed - the AVX-512 unit despite the 500 MHz difference is expected to be faster no doubt, but a half-fed AVX-512 might get trumped by a full AVX2.
For the regular instructions, turbo goes a bit like this:
For a number of users, the key metrics here are the all-core turbos, with the 18-core part having an all-core turbo of 3.2 GHz. Interestingly the W-2155 and W-2145 sits well here: for any code that can't reliably go beyond 12-14 threads, having the higher frequency but lower core count part might actually perform better. We saw a bit of this in our review, with the variable threaded loads executing somewhat better on the W-2155 than the W-2195.
We'll add this analysis to our main Xeon W review, but for those that requested the data, here it is! :)
- The Intel Xeon W Review: W-2195, W-2155, W-2123, W-2104 and W-2102 Tested
- The Supermicro X11SRA Motherboard Review: C422 based Workstation for Xeon-W
- HP Z4 Workstations Get Xeon W and Core X, 18-Core
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MrSpadge - Tuesday, August 7, 2018 - linkIn comparison I really AMDs system with Pinnacle Ridge of "keep the power consumption in check and let the CPU work as fast as it can". This nicely avoids those seemingly random steps where the CPU drops by a few 100 MHz due to loading one more core or due to a single AVX512 instruction.
HStewart - Tuesday, August 7, 2018 - linkYou do realize that AVX 512 can be turn off - but if you need it - it probably going significantly faster than AVX 2 (256) probably on a factor of more than twice.
TheFire - Tuesday, August 7, 2018 - linkThis is correct, but the concern is more about people misusing the AVX instructions. If I throw in a couple of AVX intrinsics into my code without really getting a big performance uplift, then the frequency will drop and all of a user's applications running on it will slow down. I'm not really sure how realistic this fear is, though; I would hope that the set of people who know how to use AVX intrinsics is almost entirely contained within the set of people who know how to properly optimize code for performance.
mczak - Tuesday, August 7, 2018 - linkIt shouldn't be too bad anymore with Skylake-SP, since the cores have individual turbo limits. So, a couple random avx-512 instructions thrown in somewhere should only limit one core to the avx-512 limit, whereas all others can still reach the higher avx2 or base turbo limit. Unless of course the complete workload is doing a couple avx-512 instructions every once in a while on all cores...
HStewart - Tuesday, August 7, 2018 - linkIf you worry about slowing it and you don't have AVX 512 application then turn it off. AVX 512 is primary for workstation and servers currently - but we could see in future especially once we see it wide spread used in future consumer CPU. I am very interesting to see what applications use it and how much improvement - I believe it will be significant amount performance - even more than twice.
People including myself had similar worries when CPU's went from 32 bit to 64 bit - and now AVX is transforming between 256 bit or dual 128 bit in AMD's case to 512 bit.
mode_13h - Tuesday, August 7, 2018 - linkDude, stop telling people to turn it off. You should really have some empirical evidence that turning it off *ever* helps, in any case, before advising this.
mode_13h - Tuesday, August 7, 2018 - linkThese instructions have already found their way into common string and math library functions. They're also utilized by image and video codecs, such as libjpeg-turbo (which Firefox uses), not to mention popular image processing programs.
This is good news - you can benefit from them without even having to use specialized scientific or rendering programs.
mode_13h - Tuesday, August 7, 2018 - linkI'm pretty sure a single AVX2 or AVX-512 instruction won't suddenly throw the CPU into the corresponding frequency modes. They're intended for AVX2 or AVX-512-heavy code.
HStewart - Tuesday, August 7, 2018 - linkI would love to see a test with 18 core part with a test application that can run in both AVX-512 and AVX2 mode and compare the differences of performance.
1, 512 AVX
2. AVX 2 with 512 disable
3. AVX 2 with 512 enable
It would also be interesting for application to find out with lower core version how many cores with AVX 2 it takes to be same performance
Yes it is fully understandable that CPU with AVX 512 on will running at lower frequency, but these Workstation CPU and there could be applications that take advantage of AVX 512 and I curious about the impact..
HStewart - Tuesday, August 7, 2018 - linkAnother good test is include AMD AVX 2 in match - last I heard is their AVX 2 system is 2 128bits instead of 256 bit.