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
Fetch/Prefetch
Starting with the front end of the processor, the prefetchers.
AMD’s primary advertised improvement here is the use of a TAGE predictor, although it is only used for non-L1 fetches. This might not sound too impressive: AMD is still using a hashed perceptron prefetch engine for L1 fetches, which is going to be as many fetches as possible, but the TAGE L2 branch predictor uses additional tagging to enable longer branch histories for better prediction pathways. This becomes more important for the L2 prefetches and beyond, with the hashed perceptron preferred for short prefetches in the L1 based on power.
In the front end we also get larger BTBs, to help keep track of instruction branches and cache requests. The L1 BTB has doubled in size from 256 entry to 512 entry, and the L2 is almost doubled to 7K from 4K. The L0 BTB stays at 16 entries, but the Indirect target array goes up to 1K entries. Overall, these changes according to AMD affords a 30% lower mispredict rate, saving power.
One other major change is the L1 instruction cache. We noted that it is smaller for Zen 2: only 32 KB rather than 64 KB, however the associativity has doubled, from 4-way to 8-way. Given the way a cache works, these two effects ultimately don’t cancel each other out, however the 32 KB L1-I cache should be more power efficient, and experience higher utilization. The L1-I cache hasn’t just decreased in isolation – one of the benefits of reducing the size of the I-cache is that it has allowed AMD to double the size of the micro-op cache. These two structures are next to each other inside the core, and so even at 7nm we have an instance of space limitations causing a trade-off between structures within a core. AMD stated that this configuration, the smaller L1 with the larger micro-op cache, ended up being better in more of the scenarios it tested.
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Ratman6161 - Friday, June 14, 2019 - link
Better yet, why even bother talking about it? I read these architecture articles and find them interesting, but I'll spend my money based on real world performance.Notmyusualid - Sunday, July 7, 2019 - link
@ Ratman - aye, I give this all passing attention too. Hoping one day another 'Conroe' moment lands at our feet.RedGreenBlue - Tuesday, June 11, 2019 - link
The immediate value at these price points is the multithreading. Even ignoring the CPU cost, the motherboard costs of Zen 2 on AM4 can be substantially cheaper than the threadripper platform. Also, keep in mind what AMD did soon after the Zen 1000 series launch, and, I think, Zen 2 launch to a degree. They knocked down the prices pretty substantially. The initial pricing is for early adopters with less price sensitivity and who have been holding off upgrading as long as possible and are ready to spring for something. 3 months or so from launch these prices may be reduced officially, if not unofficially by 3rd parties.RedGreenBlue - Tuesday, June 11, 2019 - link
*Meant to say Z+ launch, not Zen 2.Spoelie - Wednesday, June 12, 2019 - link
To be fair, those price drops were also partially instigated by CPU launches from Intel - companies typically don't lower prices automatically, usually it is from competitive pressure or low sales.just4U - Thursday, June 13, 2019 - link
I don't believe that's true at all S. Pricing was already lower than the 8th gen Intels and the 9th while adding cores wasn't competing against the Ryzens any more than the older series..sing_electric - Friday, June 14, 2019 - link
That's true, but by most indications, if you want the "full" AM4 experience, you'll be paying more than you did previously because the 500-series motherboards will cost significantly more - I'm sure that TR boards will see an increase, too, but I think, proportionately, it might be smaller (because the cost increase for say, PCIe 4.0 is probably a fixed dollar amount, give or take).mode_13h - Tuesday, June 11, 2019 - link
Huh? There've been lots of Intel generations that did not generate those kinds of performance gains, and Intel has not introduced a newer product at a lower price point, since at least the Core i-series. So, I have no idea where you get this 10-15% perf per dollar figure.Irata - Tuesday, June 11, 2019 - link
So who does innovate in your humble opinion ?Looking at your posts, you seem to confuse / jumble quite a lot of things.
Example TSMC: So yes, they are giving AMD a better manufacturing that allows them to offer more transistors per area or lower power use at the same clock speed.
But better perf/ $ ? Not sure - that all depends on the price per good die, i.e. yields, price etc. all play a role and I assume you do not know any of this data.
Moores law - Alx already covered that...
As for the 16 core - what would the ideal price be for you ? $199 ? What do the alternatives cost (CPU + HSF and total platform cost).
If you want to look a price - yes, it did go up compared to the 2xxx series, but compared to the first Ryzen (2017), you do get quite a lot more than you did with the original Ryzen.
1800x 8C/16T 3,6 Ghz base / 4 Ghz boost for $499
3900x 12C/24T 3.8 Ghz base / 4,6 Ghz boost for $499
Now the 2700x was only $329, but its counterpart the 3700x has the same price, roughly the same frequency but a lower power consumption and supposedly better performance in just the range you mention.
Spunjji - Tuesday, June 11, 2019 - link
Nice comprehensive summary there!