Question Performance of 7950X with 4-DIMM 128GB ram?

BenHe

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Oct 22, 2022
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Planning on building a workstation with 128GB DDR5, can't decide whether to go Intel, or AMD, or wait for Zen4 v-cache. Please advice:

Q1: I am a bit confused with 7950x's memory limitation. According to the spec, in the 2x2R configuration, max memory speed reduces from 5200MT/s to 3600MT/s. Does this mean that when 4 DIMMs are used, the total bandwidth is 4(DIMM)x3600MT/s, or does this actually mean 2(Channel)x3600MT/s?
Q2: Is there a possibility that this memory limitation can be alleviated with future firmware updates?
Q3: Will there be 64GB DDR5 sticks?

Any help would be much appreciated,
Cheers

Edit: Haven't bought anything yet, fully open to any alternative setup suggestions.
 
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Tech Junky

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1 - 2 slots / dual rank. If you go with 4 slots they typically drop the speed further.

2 - maybe if it's not a CPU limitation

3 - probably as time progresses but, DDR5 is a different tech than DDR4. It might not happen though as laptops even top out at 64hb total.
 

BenHe

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Oct 22, 2022
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Thanks you for the reply.

About 1), I understand that when using 4 DIMMs reduces speed, what I am confused about is whether the speed specification is per-DIMM or per-Channel. If the spec is per-DIMM, then the total transfer rate is 4x3600; as oppose to per-Channel 2x3600.
 

Tech Junky

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I haven't bothered to dig too deep into DDR5 because right now it's more of a gimmick IMO. The timings are on par or worse for their DDR4 equivalent. Back when it launched with ADL on the Intel side which was about a year ago it was a huge price difference and didn't make sense. It's come down to reality though lately but, it's still a big ask for such a little boost in performance.


For instance back then I picked up 16GB for $60 where DDR5 was at MSRP $135 bit, inventory got scalped and nothing was available to purchase unless you were stuck with a DDR5 board and no ram being forced to pay ~$500 or more.

Comparing 5 to 4 though is complicated by the new dual channel / single stick and different bus sizes. Basically taking whatever they're selling and divide by 2. Anyway... It seems like a dunch of hype considering it's not the 90's anymore and the slightest improvement isn't really noticeable.

I wouldn't worry about it too much personally. Just get what you think you need and it should be fine. The only things that come to mind that use a huge amount of RAM is a VM which allocates based on per image that's spun up. Comparatively the OS makes a difference as well as Windows will give itself a base of ~10% on boot to the desktop and Linux ~20% but, as soon as you launch anything on Windows sit bloats to ~25% and Linux just stays at 20%. Start doing anything in the least demanding in Windows and boom you're at 50%+ pretty easily. Gaming isn't as much of an impact as most of it's offloaded to the GPU anyway.

So, it comes down more to what you're using it for.
 
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igor_kavinski

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For Zen 4, it is a bit too early to be using it for 128GB RAM. AMD needs a lot of refinement and experience with DDR5 to work out all the kinks in their memory controller to better support DDR5. You may want to consider 5950X or a 13900K with DDR4 if you want better stability with higher speed 128GB RAM. With a 13900K, you might actually have a decent chance of running the whole 128GB at above 6000 MT/s if you go DDR5.

Does this mean that when 4 DIMMs are used, the total bandwidth is 4(DIMM)x3600MT/s, or does this actually mean 2(Channel)x3600MT/s?
Consumer mobos cannot do more than dual channel. You have to get a Threadripper or Xeon for quad channel memory.
 
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Leeea

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Q1: I am a bit confused with 7950x's memory limitation. According to the spec, in the 2x2R configuration, max memory speed reduces from 5200MT/s to 3600MT/s. Does this mean that when 4 DIMMs are used, the total bandwidth is 4(DIMM)x3600MT/s, or does this actually mean 2(Channel)x3600MT/s?
2x3600 MT/s
 

Tech Junky

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@igor_kavinski we're on the same page as usual here. I don't know if OP has the AMD in hand already or not but, pointing to DDR4 hints at going Intel. I still don't see what the need is for 128GB as most top end builders are happy with 64GB and most prosumer's go for 32GB.
 
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Leeea

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It could be 5950X too which is still relevant for multithreaded workloads. But yeah, if the OP wants something modern, 13900K it is, with DDR4.
If he is going to go consumer intel, and he cares about memory speed, ddr5 seems a no brainer.

I did not see any limitations on the intel side for per channel memory speed.


On the flip side, as you already mentioned, if memory speed is very important threadripper is waiting in the wings with quicker memory speeds then anything consumer.
 

Tech Junky

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It could be 5950X too which is still relevant for multithreaded workloads. But yeah, if the OP wants something modern, 13900K it is, with DDR4.
Well, since they mentioned 7950X that's an automatic DDR5. 5950X would be DDR4. So, yeah, of they have the 7950X in hand already it's a moot point. If they haven't bought anything yet then it becomes relevant to deciding on the RAM choices based on expense for capacity. The other thing depends on the use whether AMD is the better choice but, the differences between AMD vs Intel are niche and not many. The price difference in building either though can be significant besides those niche options. There's a cost difference on a foundational BOARD / CPU alone is a few hundred dollars not even considering the 2X cost of DDR5.
 

igor_kavinski

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Q3: Will there be 64GB DDR5 sticks?
Maybe on server. For consumers, I don't think Intel and maybe even AMD want people to have a cheap workstation with 256GB RAM built with consumer CPUs.

To be honest, all the prelaunch hype for DDR5 was just that. Hype. I was really excited that we would have DDR5-10000 by the end of 2022 and 64GB DIMMs would become a reality. Nope. Maybe after two more years?

Don't even ask me about laptop DDR5. There we are stuck with slow DDR5-4800. Only way up is with soldered LPDDR5. Honestly, they should have just kept enhancing DDR4.
 
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Shmee

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Wait, are you guys telling me you cannot manually tune DDR5 on the AM5 platform? I know AMD may recommend a supported maximum frequency for different memory configurations, but cannot simply one adjust settings to get higher speeds?
 

repoman0

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Honestly, they should have just kept enhancing DDR4.
That’s exactly what they’re doing with DDR5. The same manufacturers with the same DDR4 expertise using essentially the same memory tech that’s been around for decades. DDR5 is an iteration on DDR4 with a few new features that the engineers decided were necessary for higher frequency and bandwidth. On die ECC is a particularly nice one for reliability at those higher speeds.
 
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Tech Junky

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Wait, are you guys telling me you cannot manually tune DDR5 on the AM5 platform? I know AMD may recommend a supported maximum frequency for different memory configurations, but cannot simply one adjust settings to get higher speeds?
Considering AM5/Zen4 has been out for ~2 minutes it's hard to say. But, from the perspective of ADL using DDR5 as an option for ~1 year now it's still suspect in the actual benefits it's offering for the price premium.

DDR5 is an iteration on DDR4 with a few new features
I wouldn't call them new features but more of a shifting of how it works. Splitting it into a dual controller on chip vs the MOBO or in addition to the MOBO / chipset. It's impressive if you look at the higher CAS but, not factor in how it gets split between the 2 banks on the single stick and the change in the bus size to accommodate it.
 

igor_kavinski

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I guess they wanted less people breaking things when having to remove the keyboard to access the other 2 slots to add more RAM.
On my Thinkpad, it's almost painless and really easy to remove the keyboard and add the RAM. Yes, people with fatter fingers may have a harder time. But at least the 17.3 inch laptops should have all four slots on the bottom. I have a hard time believing that it's just the OEMs not wanting to support 128GB RAM in laptops. I think it also has something to do with Intel and AMD not wanting the extra validation headache for their mobile CPUs.

At least Intel is supporting 128GB RAM with the HX mobile CPUs but the cheapest laptop using that CPU is like $4000. And not all of them have four slots. Fun world we live in.
 
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BenHe

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Oct 22, 2022
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2x3600 MT/s
Appreciate this straight answer.

v-cache is more of a gaming thing, minimal effect on workstation
Very true, the v-cache probably won't have much effect on workstation workloads. I was actually hoping that the v-cache CPUs also conveniently come with upgraded memory controllers that goes beyond 3600MT/s for 4-DIMMs.

However, for the OP, DDR5 on Zen 4 will drop to 3600 MT/s which will definitely be slower than DDR4-3200.
When you say "slower than DDR4-3200", I assume you are only talking about latency? I thought bandwidth-wise 3600MHz is still higher than 3200MHz? Or were you accounting for overclock headroom of DDR4 vs DDR5?

Well, since they mentioned 7950X that's an automatic DDR5. 5950X would be DDR4. So, yeah, of they have the 7950X in hand already it's a moot point. If they haven't bought anything yet then it becomes relevant to deciding on the RAM choices based on expense for capacity. The other thing depends on the use whether AMD is the better choice but, the differences between AMD vs Intel are niche and not many. The price difference in building either though can be significant besides those niche options. There's a cost difference on a foundational BOARD / CPU alone is a few hundred dollars not even considering the 2X cost of DDR5.
I haven't bought anything yet. 13900K with 128GB DDR5 now seems to be a better modern option.
The major downside of 13900K in stock setting is a much worse energy efficiency compared to 7950x. Although efficiency might be improved by setting power limit, and under- volting/clocking (still waiting for more 3rd party test reports on this). If that can be achieved, I think 13900K will be what I ultimately go for.
 
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Tech Junky

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I haven't bought anything yet. 13900K with 128GB DDR5 now seems to be a better modern option.
The major downside of 13900K in stock setting is a much worse energy efficiency compared to 7950x. Although efficiency might be improved by setting power limit, and under- volting/clocking (still waiting for more 3rd party test reports on this). If that can be achieved, I think 13900K will be what I ultimately go for.
First I wouldn't pay up for the "900" option with any CPU because you're paying 25%+ more than a "700" and only getting 10% more performance. It's a gimmick to get more money for the same stuff.

You can UV/UC/etc. on any K CPU to your liking to drop the W's consumed but, to what benefit is it needed? If things are properly cooled it shouldn't matter much unless you're trying to cap the electricity bill by a couple of bucks per year.

Gaming a few hours per day / week won't make a significant impact on the bill if you're powering down between sessions. Now, if you run 24/7 like I do on my systems it might make a bigger difference depending on gaming vs normal use. Laptop is usually about 60W and when the GPU is engaged it hits 160-170W. Server is rated to ~400W at max but, doesn't use that much normally because it's headless and not used for gaming. It might tick upwards for short bursts when processing video files for Plex but, that's 30-60 seconds per file.

So, the whole DDR5 vs 4 is a big issue because it's not a direct comparison speed for speed. The DDR5 option has split the capacity between 2 sets on the single stick. It's not worth the markup at this point for the RAM or the boards needed for it.

Let's put it this way...

12700K + z690 / DDR4 = $530 + RAM
same but DDR5 = $560+ RAM

DDR4 32GB - $88
DDR5 32GB - $150

So, $130 more roughly for what gains?

13900K - $660
Z790 - $280
-- $400= more for extra E cores that most don't/won't use and a slightly higher P rate + DDR5 tax
-- you could potentially use the Z690 board w/ a bios upgrade which would save $100

The perks / advances from 690 >> 790 are minimal. The AMD side is a complete overhaul but, still not all that impressive for the price to get into the game.

There's more to it other than the raw numbers though. It comes down to how you're using it and whether it's worth the upcharge for certain options and whether they're a convenience worth spending on.
 
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igor_kavinski

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When you say "slower than DDR4-3200", I assume you are only talking about latency? I thought bandwidth-wise 3600MHz is still higher than 3200MHz? Or were you accounting for overclock headroom of DDR4 vs DDR5?
In practice, bandwidth and latency both contribute to performance. Higher latency would waste CPU cycles and lower bandwidth would mean less data gets to all the cores which would then be data starved. DDR5-3600 does offer slightly higher bandwidth but the latency is so high that the higher bandwidth advantage is negated.

1666539989377.png

Even if we suppose that DDR5-3600 is running at CL28 (it will likely be higher), your CPU still has to wait 5.5ns more, during which your performance will suffer.

Try the latency calculator here: https://notkyon.moe/ram-latency2.htm

To match the latency of pedestrian DDR4, you need to buy expensive DDR5-6000 CL30 and hope it runs at that speed with 128GB.

1666540297183.png
 

BenHe

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Oct 22, 2022
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First I wouldn't pay up for the "900" option with any CPU because you're paying 25%+ more than a "700" and only getting 10% more performance. It's a gimmick to get more money for the same stuff.

You can UV/UC/etc. on any K CPU to your liking to drop the W's consumed but, to what benefit is it needed? If things are properly cooled it shouldn't matter much unless you're trying to cap the electricity bill by a couple of bucks per year.

Gaming a few hours per day / week won't make a significant impact on the bill if you're powering down between sessions. Now, if you run 24/7 like I do on my systems it might make a bigger difference depending on gaming vs normal use. Laptop is usually about 60W and when the GPU is engaged it hits 160-170W. Server is rated to ~400W at max but, doesn't use that much normally because it's headless and not used for gaming. It might tick upwards for short bursts when processing video files for Plex but, that's 30-60 seconds per file.

So, the whole DDR5 vs 4 is a big issue because it's not a direct comparison speed for speed. The DDR5 option has split the capacity between 2 sets on the single stick. It's not worth the markup at this point for the RAM or the boards needed for it.

Let's put it this way...

12700K + z690 / DDR4 = $530 + RAM
same but DDR5 = $560+ RAM

DDR4 32GB - $88
DDR5 32GB - $150

So, $130 more roughly for what gains?

13900K - $660
Z790 - $280
-- $400= more for extra E cores that most don't/won't use and a slightly higher P rate + DDR5 tax
-- you could potentially use the Z690 board w/ a bios upgrade which would save $100

The perks / advances from 690 >> 790 are minimal. The AMD side is a complete overhaul but, still not all that impressive for the price to get into the game.

There's more to it other than the raw numbers though. It comes down to how you're using it and whether it's worth the upcharge for certain options and whether they're a convenience worth spending on.
Thank you for the kind and detailed explanation. I agree with your assessments. From a financial point of view, saving a few dollars of electricity cost may not worth the trouble of UV/UC, etc.
For gaming and general PC users, 13900K+DDR5 won't bring much FPS gain when compared to 12700K DDR4, and that the later offers a much better value proposition. It makes total sense for gamers to avoid new platform + DDR5 tax.
 

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