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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Gastec - Wednesday, June 19, 2019 - link
I'm 95% convinced that your micro-stuttering is caused by the GPU/drivers. Disable SLI or Crossfire if that's what you have (you never said what video card you use). And please stop trolling.wurizen - Thursday, June 20, 2019 - link
Really? After all that I said about this... you think that you're 95% sure it's caused by GPU drivers and you want me to disable SLI or Crossfire? Really?Qasar - Thursday, June 20, 2019 - link
have you even mentioned which vid card you are using, or what version the drivers are, or if they are up to date ??Gastec - Wednesday, June 19, 2019 - link
It could also be related to G-sync/FreeSync and your monitor. When debugging the best way is to reduce everything to a minimum.wurizen - Thursday, June 20, 2019 - link
Really, dude? You think it's related to Gsyng and Freesync?Qasar - Thursday, June 20, 2019 - link
it very well could be.. a little while ago.. there was a whole issue with micro stuttering and the fix.. was in new drivers after a certain revision...wurizen - Thursday, June 20, 2019 - link
This is gonna be my last comment regarding my comment about Infinity Fabric High memory latency issue... an objective response would be "It could;" or, "it's quit possible;" or, "110 nanoseconds latency via cross-ccx-memory-performance is nothing to sneeze at or disregard or a non-issue;"instead, i get the replies above; which doesn't need to be repeated since one can just read them. but, just in case, the replies basically say I am trolling such as the most recent from user Gastec; and someone prior I jumped to my conclusion of pointing my scrawny little finger at Infinity Fabric high memory latency; someone plain said I didn't know what I was talking about; etc!
So, I just wanna say that as my one last piece. It's odd no one has aired to the caution of objectivity and just plain responded with "It's possible..."
Instead, we get the usual techligious/fanboyish responses.
Qasar - Thursday, June 20, 2019 - link
it doesnt help, you also havent cited any links or other proof of this other then your own posts... and i quote " And, there are people having head-scratching issues similar to me with Ryzen CPU. " oh.. and where are these other people ?? where are the links and URLs that show this ??? lastly.. IF you have a spare hdd ( ssd or mechanical ) that isnt in use that you could install windows on to, so you wont have to touch the current one you are using, try installing windows on to that, update windows as much as you can via windows update, update all drivers, and do the same things you are doing to get this issue.. and see if you still get it.. if you do.. then it isnt your current install of windows, and it is something else.Carmen00 - Friday, June 21, 2019 - link
Qasar, Gastec et al, I appreciate that you're trying to educate wurizen but when you get responses like "bruh!" and "Really?", I think it's time to call it quits. Like HStewart, feeding wurizen will just encourage him and that makes it difficult to go through the comments and see the important ones. Trust that the majority of Anandtech's readership is indeed savvy enough to know pseudo-technical BS when we encounter it!Qasar - Friday, June 21, 2019 - link
well.. the fact that he didnt cite any one else with this problem, or links to forums/web pages.. kind of showed he was just trolling.. but i figured... was worth a shot to give him some sort of help....