SK Hynix Reveals DDR5 MCR DIMM, Up to DDR5-8000 Speeds for HPC

Re:SK Hynix Reveals DDR5 MCR DIMM, Up to DDR5-8000 Speeds fo... by moneycantbuy on Monday December 12, 2022 @03:39PM (#33959941)

How noticeably slower is running 128GB DDR5 on AM5? Is it perceptible? I’m building a 7950x rig and am perplexed by the ram. The specs from amd are:

Max Memory Speed 2x1R DDR5-5200 2x2R DDR5-5200 4x1R DDR5-3600 4x2R DDR5-3600

Does anyone know if cas latency (CL) also drops if using 4 sticks? For example, if I use 4x32GB DDR5 5600 CL36-36-36-89 will it drop to 3600 while maintaining CL36 or will CL also slow to like CL 40?

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Re:SK Hynix Reveals DDR5 MCR DIMM, Up to DDR5-8000 Speeds fo... by dinvlad on Monday December 12, 2022 @03:54PM (#33960127)

Basically, on AM5 4x32GB sticks nominally run at only 3,600 (i.e. no better than DDR4, and at probably slightly higher latency). On Level1Techs forums, there’re some folks that got it working as high as 4,800, which seems to be the barrier, but even that is far from guaranteed.

On Intel the situation isn’t really any better. Technically on paper, the stock speed is 4,000 (i.e. +400MT/s compared to AMD), but I’m yet to hear about anyone running 5,200 fully stable in this config.

Also, primary timings like CL36 don’t really matter in DDR5. There’s a whole ActuallyHardwareOverclocking YouTube video on it.

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Re:SK Hynix Reveals DDR5 MCR DIMM, Up to DDR5-8000 Speeds fo... by the_pwner224 on Monday December 12, 2022 @03:56PM (#33960153)

Those are the bare minimum speeds that AMD guarantees. You can find real world info about this on reddit. My memory of this is a bit old, but iirc the Zen 4 memory controller can pretty much always do 4800 MT/s on 4 sticks. And you might be able to do 5000/5200 or maybe even a bit more depending on your motherboard & RAM. The situation has been getting better with newer BIOS versions; speeds were much lower around the initial Zen 4 release.

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Re:SK Hynix Reveals DDR5 MCR DIMM, Up to DDR5-8000 Speeds fo... by the_pwner224 on Monday December 12, 2022 @04:01PM (#33960214)

Found some links from an old notepad. These are close to a month old, so stability at higher speeds should have improved since then.

ASRock Steel Legend runs at 4800(?) https://old.reddit.com/r/ASRock/comments/xtt3ol/x670e_steel_... His initial 5200 was actually unstable, a comment said he went to 4800, not sure whether 4800 is stable

Gigabyte board could run 128gb "stable" at 5200 (need to expand comment thread near bottom for that) https://old.reddit.com/r/gigabyte/comments/y1ns7j/aorus_x670... Expanded comment about 5200 stable, and link to benchmark with it: https://old.reddit.com/r/gigabyte/comments/y1ns7j/aorus_x670...

TUF X670E can only run 128gb at 4400, not 4800 https://old.reddit.com/r/Amd/comments/ya3gxx/psa_dont_buy_an...

On ROG Strix X670, commenter could run 4x16gb at 6000 (6400 was untable). The commenter previously could only do 5200; BIOS update made 6000 work. OP with 4x32gb was stable at 4400, not 5000, didn't say about 4800. https://old.reddit.com/r/overclocking/comments/xzau6z/x670_m...

Many comments here: https://old.reddit.com/r/Amd/comments/xzj69v/is_anyone_runni...

He's running 128gb at 5200 on a Gigabyte Aorus Master 670E https://old.reddit.com/r/Amd/comments/yggpr5/7950x6900xt128g...

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Re:SK Hynix Reveals DDR5 MCR DIMM, Up to DDR5-8000 Speeds fo... by dinvlad on Monday December 12, 2022 @04:05PM (#33960265)

Yeah - I think the key here is 4x32GB specifically, not just 4 sticks. 128GB is much much harder on the memory controller, since those are dual-rank memory modules (or pseudo-4R due to how DDR5 is wired compared to DDR4), and we got 4 of them. So even 4,800 is far from guaranteed in this situation, on any platform atm, and it’s unclear at this point if and when the situation will improve, barring larger memory modules in 2x48Gb and 2x64GB configurations (which would be much easier to run) or the next gen of chips with stronger MCs.

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