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Single vs dual-rank memory, and why it matters for CFD

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Old   June 2, 2020, 21:50
Default Single vs dual-rank memory, and why it matters for CFD
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I've read a couple of explanations of the difference between single and dual-rank memory, but I'm not sure I fully understand it and I definitely don't understand why it matters for CFD.


My (possibly inaccurate / incomplete) understanding is that dual rank memory has two memory ICs addressable simultaneously. Wouldn't that mean it's the same as having two single rank DIMMS connected to the same memory channel? If the latter is correct, then wouldn't it only make a difference for CFD on motherboards with only one DIMM socket per memory channel?


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Old   June 3, 2020, 04:28
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You got it right: one dual-rank DIMM per channel is pretty close performance-wise to two single-rank DIMMs per channel. Rank interleaving is the key benefit here.

There are a few other things to note though:
  • Memory upgrade path: assuming you have a board with two DIMM-slots per channel, filling them all with single-rank DIMMs right from the start makes future memory upgrades more involved. Going for larger dual-rank DIMMs with only one DIMM per channel allows a drop-in memory upgrade later.
  • My motherboard go-to recommendations for performance-oriented CFD workstations only have one DIMM slot per channel: Supermicro H11DSi(-NT) and Asus WS C621E Sage.
  • A motherboard having only one DIMM slot per channel is a performance benefit in itself. Since the topology is simpler and traces can be shorter, it results in slightly lower memory latencies. Slightly related: The top-end overclocking motherboards for any platform only have one DIMM slot per channel for this exact reason.

Edit: measuring memory bandwidth with stream benchmark, two ranks per channel get around 10-15% higher values than one rank per channel on modern platforms. That's where the performance increase of dual-rank DIMMs is coming from.
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Old   June 3, 2020, 09:48
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Makes perfect sense. Thanks very much!
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Old   June 5, 2020, 14:21
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This is a side note and probably not very applicable. However, for non-server setups we usually have the possibility to overclock the memory for almost no increase in cost of the system.


In such cases single rank memory may be better since those memory kits usually overclock a fair bit better.


For more expensive systems this is a non-issue since we are locked to a maximum memory frequency and so we should always go with a rank 2 setup if possible.
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Old   June 5, 2020, 15:26
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As a side-note on this side-note: some -if not all- of the higher possible memory overclocking of one rank per channel gets compensated by the higher memory bandwidth of two ranks per channel.
And then there is Zen2, where transfer speed beyond DDR4-3800 means uncoupling IMC and IF, resulting in a significant performance hit. A memory transfer speed that can still be achieved with two ranks per channel.
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Old   June 5, 2020, 18:55
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On intel platforms many motherboard vendors have a QVL for memories up to 4600 MHz for single rank. Dual rank usually stops at about 3600 MHz.

For Ryzen you probably still wish to use Samsung b-die to get the best timings at 3733 (3800 with some luck) MHz. They are all single rank I think.

With a T-topology setup you might be able to use 4*8 GB with good timings and high overclock, and still reap the benefits of interleaving. Many boards use Daisy chain, but there are a few T-topology available.
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Old   June 5, 2020, 19:15
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Had a quick looksie at two random cheap Z490 motherboards: dual-rank DIMMs rated at DDR4-3800 and 4000 respectively on their QVL. Not that I would deem memory QVL particularly helpful at determining the highest possible memory frequency.

Then there is the price aspect. Memory quickly becomes expensive above DDR4-3600.
And I don't see a lot of people outside of competitive benchmarking running memory transfer speed way beyond DDR4-3600.

Samsung b-die is no longer the be-all and end-all for high memory overclocks on Zen 2. Other IC types can be just as good, without the ridiculous price tags that came with the hype for b-die.
The Samsung b-dies that became popular with Zen 1 could be had in single-rank (8GB DIMMs) and dual-rank (16GB DIMMs) flavor.

And as soon as you put 4 single-rank DIMMs on a dual-channel platform, the potentially higher overclocking potential is gone, regardless of topology. It is the amount of ranks per channel that matters, not the amount of ranks per DIMM. So the whole topic is only relevant if all you need is 16GB of RAM. Or 32GB on a quad-channel platform.
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Old   June 6, 2020, 13:30
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Quote:
Originally Posted by flotus1 View Post
Had a quick looksie at two random cheap Z490 motherboards: dual-rank DIMMs rated at DDR4-3800 and 4000 respectively on their QVL. Not that I would deem memory QVL particularly helpful at determining the highest possible memory frequency.

The OVL gives some indication on the type of topology the manufacturer uses. So if you look at a board with very high values using only 2 sticks then it is most likely a daisy chain setup. Trying to go for 4 single rank sticks in that case will likely not be successful.


Regarding the dual rank capability of high overclocks it seems that it has improved lately. Previously dual rank always performed much worse in terms of overclocking. Also, are you certain that it is dual rank or is it just 16 GB modules you have looked at? ASUS has some highly overclocked 16 GB modules marked as single rank in their QVL.


Quote:
Originally Posted by flotus1 View Post
Then there is the price aspect. Memory quickly becomes expensive above DDR4-3600.
And I don't see a lot of people outside of competitive benchmarking running memory transfer speed way beyond DDR4-3600.

Agreed. But since we are discussing side-notes here I think, from an enthusiast point of view, it is worth bringing up. For Zen 2 there is absolutely no point in going above memory that can handle 3600 CL14 (perhaps CL16 is OK as well).


Quote:
Originally Posted by flotus1 View Post
Samsung b-die is no longer the be-all and end-all for high memory overclocks on Zen 2. Other IC types can be just as good, without the ridiculous price tags that came with the hype for b-die.
The Samsung b-dies that became popular with Zen 1 could be had in single-rank (8GB DIMMs) and dual-rank (16GB DIMMs) flavor.

Based on the Ryzen Dram Calculator (which I have got stellar results with), the b-die is still a top choice. I agree that newer memory designed for zen 2 has improved substantially and may have reached the level of b-die. I have not had the opportunity to test myself.


Quote:
Originally Posted by flotus1 View Post
And as soon as you put 4 single-rank DIMMs on a dual-channel platform, the potentially higher overclocking potential is gone, regardless of topology. It is the amount of ranks per channel that matters, not the amount of ranks per DIMM. So the whole topic is only relevant if all you need is 16GB of RAM. Or 32GB on a quad-channel platform.

I remember that on many overclocking forums this has been discussed, for instance:


https://www.overclockers.com/forums/...-Z370-platform


Sure it is not the most scientific evidence so I will gladly concede if there is a better source. =)


(on a side-side-side note; it is impossible for me to reply and use a Smiley, then I always get a 403 error)
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Old   June 7, 2020, 10:26
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Quote:
Also, are you certain that it is dual rank or is it just 16 GB modules you have looked at?
Pretty sure.

Quote:
I agree that newer memory designed for zen 2 has improved substantially and may have reached the level of b-die. I have not had the opportunity to test myself.
This might be a moot point, but I rather think that the more mature memory controllers of Zen 2 and bios versions made the bigger difference here.
Other memory types that Zen 1 had trouble with for a long time could be used at much higher clock speeds on Intel platforms.

Anyway, to wrap this up on my part: If I were to buy an overclockable platform with the intent of squeezing out the last bit of performance, I personally would use dual-rank DIMMs. Probably along with a motherboard with one DIMM per channel. With 32GB UDIMMs, I could still get some decent amount of memory.
A completely biased and personal opinion on this: the obsession of motherboard manufacturers and overclocking enthusiasts alike with singe-rank DIMMs stems from the draw of high numbers. DDR4-4800 just sounds better than DDR4-4266. I don't think that many of them would take a step back and reconsider if the former is actually faster. Marketing is just easier when you have the higher number. And for our application in particular -CFD, where bandwidth counts- it might actually be the latter, provided it is two ranks per channel. Anyway, the difference will be rather small, and I don't have to stick to small DIMM sizes.
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Old   June 7, 2020, 18:04
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I am not RAM expert

But on my 2066 socket I used single rank 4x16 3666 (overclocked sometimes up to 4400)

Now I have dual rank 4x16 2666

Dual rank is up to 30% faster for my CFD tasks in Ansys CFX

thinking about to change for
https://www.gskill.com/product/165/1...5V64GB-(4x16GB)

BTW:
https://www.youtube.com/watch?v=D_Yt4vSZKVk
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Old   June 7, 2020, 18:16
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Nope nope nope nope nope...

If you want somewhat in-depth information on youtube, I would recommend this guy: https://www.youtube.com/watch?v=tgFXVhsK1ok
The information density of his videos is rather low, but he generally knows what he is talking about.

Quote:
But on my 2066 socket I used single rank 4x16 3666 (overclocked sometimes up to 4400)
I am curious which memory modules you had exactly. Single-rank 16GB UDIMMs are still pretty rare.
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