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
Cache and Infinity Fabric
If it hasn’t been hammered in already, the big change in the cache is the L1 instruction cache which has been reduced from 64 KB to 32 KB, but the associativity has increased from 4-way to 8-way. This change enabled AMD to increase the size of the micro-op cache from 2K entry to 4K entry, and AMD felt that this gave a better performance balance with how modern workloads are evolving.
The L1-D cache is still 32KB 8-way, while the L2 cache is still 512KB 8-way. The L3 cache, which is a non-inclusive cache (compared to the L2 inclusive cache), has now doubled in size to 16 MB per core complex, up from 8 MB. AMD manages its L3 by sharing a 16MB block per CCX, rather than enabling access to any L3 from any core.
Because of the increase in size of the L3, latency has increased slightly. L1 is still 4-cycle, L2 is still 12-cycle, but L3 has increased from ~35 cycle to ~40 cycle (this is a characteristic of larger caches, they end up being slightly slower latency; it’s an interesting trade off to measure). AMD has stated that it has increased the size of the queues handling L1 and L2 misses, although hasn’t elaborated as to how big they now are.
Infinity Fabric
With the move to Zen 2, we also move to the second generation of Infinity Fabric. One of the major updates with IF2 is the support of PCIe 4.0, and thus the increase of the bus width from 256-bit to 512-bit.
Overall efficiency of IF2 has improved 27% according to AMD, leading to a lower power per bit. As we move to more IF links in EPYC, this will become very important as data is transferred from chiplet to IO die.
One of the features of IF2 is that the clock has been decoupled from the main DRAM clock. In Zen and Zen+, the IF frequency was coupled to the DRAM frequency, which led to some interesting scenarios where the memory could go a lot faster but the limitations in the IF meant that they were both limited by the lock-step nature of the clock. For Zen 2, AMD has introduced ratios to the IF2, enabling a 1:1 normal ratio or a 2:1 ratio that reduces the IF2 clock in half.
This ratio should automatically come into play around DDR4-3600 or DDR4-3800, but it does mean that IF2 clock does reduce in half, which has a knock on effect with respect to bandwidth. It should be noted that even if the DRAM frequency is high, having a slower IF frequency will likely limit the raw performance gain from that faster memory. AMD recommends keeping the ratio at a 1:1 around DDR4-3600, and instead optimizing sub-timings at that speed.
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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....