Discussion Speculation: Zen 4 (EPYC 4 "Genoa", Ryzen 7000, etc.)

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Vattila

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Oct 22, 2004
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Except for the details about the improvements in the microarchitecture, we now know pretty well what to expect with Zen 3.

The leaked presentation by AMD Senior Manager Martin Hilgeman shows that EPYC 3 "Milan" will, as promised and expected, reuse the current platform (SP3), and the system architecture and packaging looks to be the same, with the same 9-die chiplet design and the same maximum core and thread-count (no SMT-4, contrary to rumour). The biggest change revealed so far is the enlargement of the compute complex from 4 cores to 8 cores, all sharing a larger L3 cache ("32+ MB", likely to double to 64 MB, I think).

Hilgeman's slides did also show that EPYC 4 "Genoa" is in the definition phase (or was at the time of the presentation in September, at least), and will come with a new platform (SP5), with new memory support (likely DDR5).

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What else do you think we will see with Zen 4? PCI-Express 5 support? Increased core-count? 4-way SMT? New packaging (interposer, 2.5D, 3D)? Integrated memory on package (HBM)?

Vote in the poll and share your thoughts! :)
 
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eek2121

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Ugh, hope that's an early sample :(. That, or there are allot of tricks in Zen4 to increase throughput within the cores (aka more IPC, which is a misnomer). Moving more towards an Apple A (or M) series CPU architecture??

High performance chips rarely stick to optimal clocks. I seriously doubt Zen 4 will have lower frequencies than Zen 3.
 
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mikk

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No PCIe 5 for AM5 and Zen4 is a big bummer for a late 2022 generation, a real next gen platform should support both DDR5&PCIe5. I wonder if it means no PCIe5 until Zen5 in late 2024.
 

uzzi38

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No PCIe 5 for AM5 and Zen4 is a big bummer for a late 2022 generation, a real next gen platform should support both DDR5&PCIe5. I wonder if it means no PCIe5 until Zen5 in late 2024.
PCIe 5 is pretty much irrelevant for consumers right now and will remain that way for many years. All it does is increase motherboard costs. The only thing it possibly could affect is storage, because GPUs aren't going to come close to saturating a PCI4x16 link any time soon.
 

Thibsie

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Apr 25, 2017
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Even for storage, we can't even find acceptably priced consumer PCIe4 SSDs yet.
Let's look at what PCIe4 can bring (the real drives low end / mid end, not the expensive 'for the show' drives).
 
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IntelUser2000

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Oct 14, 2003
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Ugh, hope that's an early sample :(. That, or there are allot of tricks in Zen4 to increase throughput within the cores (aka more IPC, which is a misnomer). Moving more towards an Apple A (or M) series CPU architecture??

Moving to Apple or ARM like architecture would mean cutting pipeline stages from current 18-20 to 10-12, which will result in 4GHz or lower clocks.

If you mean by increasing width, sure that's what'll happen. Rumors put Zen 4 being 30% faster per clock which is substantial. Even at 4.5GHz it'll clobber Zen 3.
 

DisEnchantment

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Mar 3, 2017
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PCIe 5 is pretty much irrelevant for consumers right now and will remain that way for many years. All it does is increase motherboard costs. The only thing it possibly could affect is storage, because GPUs aren't going to come close to saturating a PCI4x16 link any time soon.
PCIe 5.0 is used as the physical layer for CXL and is supposed to bring in a new era of cache coherent accelerators.

I hope this is not the case.
Otherwise it means buying some Zen4 based TR Pro to try out some CXL based accelerators.
Only option on AM5 would be to go A+A, which is kind of against AMD open and inclusive philosophy. Issue with this is that AMD only got GPUs at the moment, so if you wanna try out FPGA based CXL capable accelerators like the ones from Xilinx you would be out of luck on AM5
It makes no sense to not have it, if they already have the PCIe 5.0 IP on Genoa. They could just have the support in the IOD and the chipset and let the Board OEMs take the cost on the high end boards.
Either that or this generation has no CXL support which is going to be an issue with developers wanting to try out CXL based accelerators.

Not happy with this.
 
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DisEnchantment

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Will AMD platforms even support CXL?
CCIX is effectively dead and with the MoU between CXL consortium and GenZ consortium in place, CXL is basically the de facto protocol for in chassis and GenZ outside.
AMD is a major partner in CCIX/GenZ/CXL consortium (in the steering committee and board of director for CXL actually)

For the next wave of server processors having no CXL is a major handicap.
AMD had a number of patents for memory expansion and coherent FPGA Engine based on CCIX, but as of today I doubt they would go with CCIX. But since that changed in 2019 we would not be seeing any new patents on CXL based systems yet.
Azure in particular is very interested in CXL and to some extent GenZ. You could argue that for the cloud providers having something like CXL is more interesting than GenZ in the short term. While HPC deployments for sure would be interested in GenZ
CXL based devices according to most folks following the server industry is the next big thing. I keep hearing this also internally from some of our folks in the office who are maintaining our Compute farm

AMD's McNamara even chimed in on the CXL Memory expansion kit

Update:
The need for 57 bit VA in upcoming CPUs also stems from the fact that there could be a system wide gigantic expandable memory pool which is beyond what could be handled with today's 48 bit VA
 
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dnavas

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Feb 25, 2017
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Xillinx is too in the CXL consortium and has already products supporting PCIe 5.0 and CXL

It was my assumption that the combination was going to leverage this, specifically. If AM5/pcie5 ends up being compatible at the socket layer with AM5/pcie4, I don't think having an initial release with pcie4 desktop parts is such a negative -- pcie4 already costs a pretty penny, and I'm not sure a chipset (ed: or motherboard!) for pcie5 is going to be particularly quiet, cheap, or energy efficient. Would make sense in TR, though. Although who knows, maybe they're going to differentiate the Pro line this way. That'd be disappointing, but it's not insensible.
 

DisEnchantment

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I see some folks saying that some Zen3+ CPU will be the first CPU on the AM5 chipset and it has PCIe 4. Actual Zen4 CPU will have the PCIe 5 is the supposition.
Lets see how it pans out in reality
 

jamescox

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Nov 11, 2009
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It isn’t the same interface. DDR5 alone will require changes, and if it also has PCIE 5? Yeah, no way. The new IOD will need a lot more bandwidth.
It has generally been the rule, not the exception, to have a bandwidth constrained link in desktop systems. You had to pay extra for the server version with a lot more IO bandwidth. The link between the cpu and IO die is very high bandwidth and can reach higher utilization that the DDR interface. I am not saying that they would make a Zen 3 on on AM5 with a Zen 4 IO die, but I think it is definitely technologically possible. I was not talking about using the old IO die, just the older CPU. There may not be any need for them to do that though unless they are severely constrained on 5 nm or something like that.

I doubt that we will see pci-express 5 on desktop systems soon. There just isn’t any need for it at all. It will almost certainly be on Epyc as soon as they can get it there though.
 

soresu

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Dec 19, 2014
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It makes no sense to not have it, if they already have the PCIe 5.0 IP on Genoa.
If just supporting v4 rather than v5 makes early AM5 cheaper (and quieter) then it makes every kind of sense.

I would not expect the majority of AM5 users to miss v5 bandwidth or CXL capabilities just yet, and it doesn't rule out a later revision granting v5 support ala x570 based AM4.

I suspect that they want to wait this time to avoid needing a motherboard fan again with v5 in the mainstream as they did with v4 on x570 when they get around to supporting it.
 

mikk

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May 15, 2012
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PCIe 5 is pretty much irrelevant for consumers right now and will remain that way for many years. All it does is increase motherboard costs. The only thing it possibly could affect is storage, because GPUs aren't going to come close to saturating a PCI4x16 link any time soon.


Right now doesn't matter. The thing is Zen 3 isn't only for (late) 2022, it's AMDs generation for 2023 and 2024 as well. Not everyone is changing their CPU every year. If someone buys Zen 3 in 2023 he will be limited to PCIe4 over the course of 2024, 2025...
 

jpiniero

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Oct 1, 2010
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I doubt that we will see pci-express 5 on desktop systems soon. There just isn’t any need for it at all. It will almost certainly be on Epyc as soon as they can get it there though.

It is definitely not needed any time soon, but since Intel is doing it AMD will have to too. SSDs might want to move to 4.0x8 soon though so 5.0x4 as an option might be beneficial.
 

Gideon

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Nov 27, 2007
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Right now doesn't matter. The thing is Zen 3 isn't only for (late) 2022, it's AMDs generation for 2023 and 2024 as well. Not everyone is changing their CPU every year. If someone buys Zen 3 in 2023 he will be limited to PCIe4 over the course of 2024, 2025...
What would have been your PC platform recommendation for people between August 2019 and March 2021, when they absolutely needed to buy A Desktop PC right then and weren't exclusively gamers?

I agree that having PCIe5 for a later upgrade is a non-trivial plus that might be relevant to some people but I'm 100% sure not having PCIe 4.0 on an Intel platform never swayed your recommended platform choice for 1 millisecond.

The biggest flaw for Alder Lake I see is that it doesn't support PCIe 5.0 on the M.2 slot, making the difference largely academic. I really wish it did, could be a noticable difference in Direct Storage bandwidth, once games implement it. As it is now it doesn't really matter, because half of the pipeline, SSDs, will still be bound by PCIe 4.0.

All in all I really hope AMD adds support for at the latest a gen after Zen 4 (like they did with Pcie 4.0 on an existing platform). Hopefully this rumor is false and It's actually supported on highest-end mobos
 
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uzzi38

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Right now doesn't matter. The thing is Zen 3 isn't only for (late) 2022, it's AMDs generation for 2023 and 2024 as well. Not everyone is changing their CPU every year. If someone buys Zen 3 in 2023 he will be limited to PCIe4 over the course of 2024, 2025...
Except I don't see it as being relevant to 90%+ of consumers out until 2025, if not later as well. GPUs certainly don't need PCIe Gen 5 any time soon unless 1080p 1800fps or something else stupid like that picks up traction, so really only SSDs are left. But I'd rather see more lanes if anyone's going to tackle this issue, be those additional lanes from the chipset or from the CPU (and the chipset link being moved to 8 lanes as well would be nice).
 
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May 17, 2020
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If just supporting v4 rather than v5 makes early AM5 cheaper (and quieter) then it makes every kind of sense.

I would not expect the majority of AM5 users to miss v5 bandwidth or CXL capabilities just yet, and it doesn't rule out a later revision granting v5 support ala x570 based AM4.
When the CPU supports CXL in combinaison of PCIe 5.0, so CXL is only needed for people that can't afford TR or EPYC platform to work with it else for others people it not needed
 

LightningZ71

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Except I don't see it as being relevant to 90%+ of consumers out until 2025, if not later as well. GPUs certainly don't need PCIe Gen 5 any time soon unless 1080p 1800fps or something else stupid like that picks up traction, so really only SSDs are left. But I'd rather see more lanes if anyone's going to tackle this issue, be those additional lanes from the chipset or from the CPU (and the chipset link being moved to 8 lanes as well would be nice).

I agree about the chipset link. I don't particularly care about PCIe 5 on the consumer platform for the next few years. I DO care about the chipset getting enough bandwidth. An 8 lane PCIe 4.0 link would last for a while. X570 us a nice chipset with lots of PCIe 4.0 lanes, but that upstream link is a bit of a limit in some cases. Doubling it's throughput shouldn't cause too much of an increase in board costs. I realize that this will require a new IOD, but, I suspect that we're getting one anyway with the move to DDR5 support soon.
 

Ajay

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Right now doesn't matter. The thing is Zen 3 isn't only for (late) 2022, it's AMDs generation for 2023 and 2024 as well. Not everyone is changing their CPU every year. If someone buys Zen 3 in 2023 he will be limited to PCIe4 over the course of 2024, 2025...
Do you mean Zen4???
 
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soresu

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GPUs certainly don't need PCIe Gen 5 any time soon unless 1080p 1800fps or something else stupid like that picks up traction, so really only SSDs are left.
I could see it playing a role in DirectStorage access from M2 SSD's to the GPU when UE5 facilitates more and more insane bandwidth requirements for virtualised geometry and textures in future games.

What's comfortable for a highly optimised platform like PS5 and XSX/S could require more BW for heavier scenes on Windows 10 PCs.
 
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