AMD Zen 2 Microarchitecture Analysis: Ryzen 3000 and EPYC Rome
by Dr. Ian Cutress on June 10, 2019 7:22 PM EST- Posted in
- CPUs
- AMD
- Ryzen
- EPYC
- Infinity Fabric
- PCIe 4.0
- Zen 2
- Rome
- Ryzen 3000
- Ryzen 3rd Gen
Decode
For the decode stage, the main uptick here is the micro-op cache. By doubling in size from 2K entry to 4K entry, it will hold more decoded operations than before, which means it should experience a lot of reuse. In order to facilitate that use, AMD has increased the dispatch rate from the micro-op cache into the buffers up to 8 fused instructions. Assuming that AMD can bypass its decoders often, this should be a very efficient block of silicon.
What makes the 4K entry more impressive is when we compare it to the competition. In Intel’s Skylake family, the micro-op cache in those cores are only 1.5K entry. Intel increased the size by 50% for Ice Lake to 2.25K, but that core is coming to mobile platforms later this year and perhaps to servers next year. By comparison AMD’s Zen 2 core will cover the gamut from consumer to enterprise. Also at this time we can compare it to Arm’s A77 CPU micro-op cache, which is 1.5K entry, however that cache is Arm’s first micro-op cache design for a core.
The decoders in Zen 2 stay the same, we still have access to four complex decoders (compared to Intel’s 1 complex + 4 simple decoders), and decoded instructions are cached into the micro-op cache as well as dispatched into the micro-op queue.
AMD has also stated that it has improved its micro-op fusion algorithm, although did not go into detail as to how this affects performance. Current micro-op fusion conversion is already pretty good, so it would be interesting to see what AMD have done here. Compared to Zen and Zen+, based on the support for AVX2, it does mean that the decoder doesn’t need to crack an AVX2 instruction into two micro-ops: AVX2 is now a single micro-op through the pipeline.
Going beyond the decoders, the micro-op queue and dispatch can feed six micro-ops per cycle into the schedulers. This is slightly imbalanced however, as AMD has independent integer and floating point schedulers: the integer scheduler can accept six micro-ops per cycle, whereas the floating point scheduler can only accept four. The dispatch can simultaneously send micro-ops to both at the same time however.
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fmcjw - Tuesday, June 11, 2019 - link
All good and fine, but I want Zen 2 and 7nm on my laptop. If they aren't announcing it today, products aren't gonna ship by holiday 2019, and most consumers will end up buying 10nm Intel devices. Missed chance.mode_13h - Tuesday, June 11, 2019 - link
Eh, they have perfectly good 12 nm laptop SoCs. 7 nm would've been nice, but it's hard to do everything at once.levizx - Tuesday, June 11, 2019 - link
Nope, those 12nm APUs have worse battery life (than current 8th Gen) and no TB3/USB4 support. I can't think of a reason where I would choose Ryzen 3xxxU over Ice Lakemode_13h - Tuesday, June 11, 2019 - link
Do price & availability count?Xyler94 - Tuesday, June 11, 2019 - link
Misleading remarks. Huawei was able to make a Ryzen APU have better battery life than an 8th gen processor. TB3 and USB4 aren't readily used mainstream yet. Heck USB-C hasn't even caught on yet.Currently laptop makers aren't optimizing AMD's CPU, that's just the fact.
Cooe - Wednesday, June 12, 2019 - link
This is mostly nonsense. Performance AND battery life for Ryzen Mobile 2nd Gen is extremely close to Intel's current 8th & 9th gen 4-core parts. And until Ice Lake is a real thing that you can actually buy, Ryzen still has a major value advantage + far better iGPU performance. Ice Lake also isn't really any faster CPU wise than Whiskey Lake, because despite increasing IPC by +18%, clock-speeds were dropped from 4.8 to 4.1GHz, or about -16%, erasing nearly all those gains.fmcjw - Tuesday, June 11, 2019 - link
Yeah, I get that they still need time to get the GPU down to 7nm, so they pushed it back to focus on the CPU for desktop (where performance per watt matters much less than server or mobile). But the silence is not reassuring, and mobile-wise, Zen is still inferior to Intel, maybe not performance-wise as Huawei demonstrates with its Matebook, but definitely battery-wise because of the more powerful GPU.scineram - Tuesday, June 11, 2019 - link
Nobody is going to buy Shintel vaporware. Or only very few.The_Assimilator - Tuesday, June 11, 2019 - link
Please edit the table on page 1 to combine the rows with identical values into a single row (e.g. the RAM speed). Also edit the 3950X price to have a ? after it as it's not yet confirmed.jfmonty2 - Tuesday, June 11, 2019 - link
The 3950X price is most definitely confirmed; Lisa Su said it loud and clear (and showed it on the slide) in AMD's E3 presentation yesterday: https://www.youtube.com/watch?v=yxPBXNuX6Xs&t=...