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
New Instructions
Cache and Memory Bandwidth QoS Control
As with most new x86 microarchitectures, there is a drive to increase performance through new instructions, but also try for parity between different vendors in what instructions are supported. For Zen 2, while AMD is not catering to some of the more exotic instruction sets that Intel might do, it is adding in new instructions in three different areas.
The first one, CLWB, has been seen before from Intel processors in relation to non-volatile memory. This instruction allows the program to push data back into the non-volatile memory, just in case the system receives a halting command and data might be lost. There are other instructions associated with securing data to non-volatile memory systems, although this wasn’t explicitly commented on by AMD. It could be an indication that AMD is looking to better support non-volatile memory hardware and structures in future designs, particularly in its EPYC processors.
The second cache instruction, WBNOINVD, is an AMD-only command, but builds on other similar commands such as WBINVD. This command is designed to predict when particular parts of the cache might be needed in the future, and clears them up ready in order to accelerate future calculations. In the event that the cache line needed isn’t ready, a flush command would be processed in advance of the needed operation, increasing latency – by running a cache line flush in advance while the latency-critical instruction is still coming down the pipe helps accelerate its ultimate execution.
The final set of instructions, filed under QoS, actually relates to how cache and memory priorities are assigned.
When a cloud CPU is split into different containers or VMs for different customers, the level of performance is not always consistent as performance could be limited based on what another VM is doing on the system. This is known as the ‘noisy neighbor’ issue: if someone else is eating all the core-to-memory bandwidth, or L3 cache, it can be very difficult for another VM on the system to have access to what it needs. As a result of that noisy neighbor, the other VM will have a highly variable latency on how it can process its workload. Alternatively, if a mission critical VM is on a system and another VM keeps asking for resources, the mission critical one might end up missing its targets as it doesn’t have all the resources it needs access to.
Dealing with noisy neighbors, beyond ensuring full access to the hardware as a single user, is difficult. Most cloud providers and operations won’t even tell you if you have any neighbors, and in the event of live VM migration, those neighbors might change very frequently, so there is no guarantee of sustained performance at any time. This is where a set of dedicated QoS (Quality of Service) instructions come in.
As with Intel’s implementation, when a series of VMs is allocated onto a system on top of a hypervisor, the hypervisor can control how much memory bandwidth and cache that each VM has access to. If a mission critical 8-core VM requires access to 64 MB of L3 and at least 30 GB/s of memory bandwidth, the hypervisor can control that the priority VM will always have access to that amount, and either eliminate it entirely from the pool for other VMs, or intelligently restrict the requirements as the mission critical VM bursts into full access.
Intel only enables this feature on its Xeon Scalable processors, however AMD will enable it up and down its Zen 2 processor family range, for consumers and enterprise users.
The immediate issue I had with this feature is on the consumer side. Imagine if a video game demands access to all the cache and all the memory bandwidth, while some streaming software would get access to none – it could cause havoc on the system. AMD explained that while technically individual programs can request a certain level of QoS, however it will be up to the OS or the hypervisor to control if those requests are both valid and suitable. They see this feature more as an enterprise feature used when hypervisors are in play, rather than bare metal installations on consumer systems.
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wurizen - Friday, June 14, 2019 - link
flex^^^wurizen - Friday, June 14, 2019 - link
OMFG! I. Am. Not. Talking. About. Intel. Mesh.I. Am. Talking. About. Infinity. Fabric. High. Memory. Latency!
Now that I got that off my chest, let's proceed shall we...
OMFG!
L3 Cache? WTF!
Do you think you're so clever to talk about L3 cache to show off your knowledge as if to convince ppl here you know something? Nah, man!
WTF are you talking about L3 cache, dude? Come on, dude, get with the program.
The program is "Cross-CCX-High-Memory-Latency" with Infinity Fabric 1.0
And, games (BO3, BF1, BF4 from my testing) are what is affected by this high latency penalty in real-time. Imagine playing a game of BO3 while throughout the game, the game is "micro-pausing" "Micro-slow-motioning" repeatedly throughout the match? Yep, you got it, it makes it unplayeable.
In productive work like video editing, I would not see the high latency as an issue unless it affects "timeline editing" causing it to lag, as well.
I have heard some complain issues with it in audio editing with audio work. But I don't do that so I can't say.
As for "compute-intensive applications (y'know, real work)" --delatFx2
....
.....
......
You duh man, bruh! a real compute-intensive, man!
"This article mentions a Windows 10 patch to ensure that threads get assigned to the same CCX before going to the adjacent one." --deltaFx2
Uhhh... that won't fix it. Only AMD can fix it in Infinity Fabric 2.0 (Ryzen 2), if, indeed, AMD has fixed it. By making it faster! And/or, reducing that ~110ns latency to around 69ns.
Now, my question is, and you (deltaFx2) hasn't mentioned it in your wise-response to my comments is that SLIDE of "Raw Memory Performance" showing 69ns latency at 3200 Mhz RAM. Is that Raw memory performance Intra-CCX-Memory-Performance or Inter-core-Memory-Performance? Bada-boom, bish!
wurizen - Friday, June 14, 2019 - link
it's a problem ppl are having, if you search enough....Alistair - Wednesday, June 12, 2019 - link
those kinds of micro stutters are usually motherboard or most likely your windows installation causing it, reinstall windows, then try a different motherboard maybewurizen - Wednesday, June 12, 2019 - link
Wow, really? Re-install windows?I just wanna know (cough, cough Anand) what the Cross-CCX-Latency is for Ryzen 2 and Infinity Fabric 2.0.
If, it is still ~110ns like before.... well, guess what? 110 nano-effin-seconds is not fast enough. It's too HIGH a latency!
You can't update bios/motherboard or re-install windows, or get 6000 Mhz RAM (the price for that, tjo?) to fix it. (As shown in the graph for whatever "Raw Memory Latency" is for that 3200 Mhz RAM to 3600 Mhz stays at 69 ns and only at 37333 Mhz RAM does it drop to 67ns?).... This is the same result PCPER did with Ryzen IF 1.0 showing that getting Faster RAM at 3200 Mhz did not improve the Cross-CCX-Memory-Latency....
supdawgwtfd - Thursday, June 13, 2019 - link
O don't get any stutters with my 1600.As above. It's nothing to do with the CPU directly.
Something else is causing the problem.
deltaFx2 - Thursday, June 13, 2019 - link
How so you know for sure that the microstutter or whatever it is you think you are facing is due to the inter-ccx latency? Did you actually pin threads to CCXs to confirm this theory? Do you know when inter-ccx latency even comes into play? Inter-ccx latency ONLY matters for shared memory being modified by different threads; this should be a tiny fraction of your execution time, otherwise you are not much better going multithreaded. Moreover, Each CCX runs 8 threads so are you saying your game uses more than 8? That would be an interesting game indeed, given that intel's mainstream gaming CPUs don't have a problem on 4c8t.To me, you've just jumped the the gun and gone from "I have got some microstutter issues" to "I know PCPer ran some microbenchmark to find out the latency" to "that must be the problem". It does not follow.
FreckledTrout - Thursday, June 13, 2019 - link
I agree. If micro stutter from CCX latency was really occurring this would be a huge issue. These issues really have to be something unrelated.wurizen - Friday, June 14, 2019 - link
Another thing that was weird was GPU usage drop from 98% to like 0% in-game, midst-action, while I was playing... constantly, in a repeated pattern throughout the game... this is not a server or games hitching. we understand as gamers that a game will "hitch" once in a while. this is like "slow-motion" "micro-pause" thing happening through out the game. happens in single player (BF1) so I ruled out server-side. It's like the game goes in "slow-motion" for a second... not once or twice in a match, per se. But, throughout and in a repeated constant fashion... along with seeing GPU usage to accompany the effect dropping from 98% or so (normal) to 0% for split seconds (again, not once or twice in a match; but a constant, repeated pattern throughout the match)And, there are people having head-scratching issues similar to me with Ryzen CPU.
No one (cough, cough Anand; nor youtube tech tubers will address it) seems to address it tho.
But, I think that Ryzen 2 is coming out and if Cross-CCX-High-LAtency-Issue is the same, then we're bound to hear more. I'm sure.
I am thinking tech sites are giving AMD a chance... but not sure... doesn't matter tho. I got a 7700k (I wanted the 8-core thing when 1700x Ryzen came out) but its fine. Im not a fanboy. Just a techboy.... if anything...
wurizen - Friday, June 14, 2019 - link
The "micro-stutter" or "micro-pausing" is not once or twice (I get those with Intel, as well) but, a repeated, constant pattern throughout the match and round of game. The "micro-stutter" and "micro-pause" also "FEELS" different than what I felt with my prior 3700K CPU and current 7700K CPU. It's like a "micro-slow-motion." I am not making this up. I am not crazy!