When Intel launched the Skylake-X platform, it communicated to reviewers the chips would have a bifurcated AVX-512 strategy. The Core i9-7800X and all the multicore chips above that point supposedly had two AVX-512 units, while the Core i7 CPUs in the Skylake-X family only had one. Kaby Lake-X CPUs didn’t have any, but we’re not sure if anyone ever actually bought a Kaby Lake-X CPU, and then the much-more-appealing Core i7-8700K replaced it as an overall recommendation.
At any rate, the point was clear: If you wanted full AVX-512 capabilities, you had to buy a 10-core or higher CPU. Except Intel is now telling people that’s not true. All Skylake-X CPUs have access to two full AVX-512 units.
Intel has since updated its own Ark.Intel.com website to confirm this. Tech Report notes hardware testers who bought Skylake-X CPUs after they were released last fall had posted figures that seemed to indicate a full two AVX-512 units were operational. I’m a bit embarrassed to admit it, but I actually saw the same threads when I was working on my Core i9-7980XE review. Saw them — and decided that they must either be mistaken, or that some other issue was goosing the results. One thing we discussed in the 7980XE review was that we had trouble getting our CPU to behave in the manner Intel specified. Given that AVX-512 imposes a pretty hefty clock penalty, it seemed reasonable to think a CPU without that penalty might turn in much higher numbers, even if it had two AVX-512 units. I didn’t have time to dig deeper into the situation in the hectic “This thing has to be published in six hours and the only thing I’ve written is a string of curse words” that often defines reviewing, but I do apologize for the mistake.
There’s two ways to look at this situation. If you’ve wanted to program AVX-512 and were concerned about only having one AVX-512 unit to work with, you can now buy a lower-end Skylake-X product with no concern on that front. On the other hand, anyone who bought a more expensive CPU specifically for AVX-512 is unlikely to be pleased to discover there was no reason to spend the extra cash.
Intel may bring AVX-512 to its ordinary desktop CPUs with Cannon Lake later this year, but it’s not clear how important the feature will be to consumer workloads. Intel’s AVX improvements over the years have typically been aimed at the HPC market. While there are consumer applications that take advantage of them, we haven’t seen the kind of general speedup that informed the shift from x87 FPU code to SSE2. The clock penalty that AVX-512 equipped CPUs take when using the SIMD instruction set could also create scenarios where it’s more expensive to use AVX-512 than conventional AVX. AMD’s Ryzen has proven stronger than we thought when it comes to FPU performance, despite the fact that Ryzen only uses 128-bit registers for AVX compared with Intel’s 256-bit registers.
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