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Brookzy

macrumors 601
May 30, 2010
4,952
5,482
UK
OK, so where did you find out that it was only running at 2333? Trying to sort out all of this info as I troubleshoot my 128gb OWC ram problems.
Also from System Information - my post made several points, only one of which was illustrated.
 
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wilberforce

macrumors 65816
Aug 15, 2020
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SF Bay Area
Based on many tests by myself and others, it may be useful to restate what has been learned about upgrading memory on the 2020 27" iMac:

TWO STICKS:
For two sticks, put identical sticks in slots 2 and 4 (where slot 1 is at top)

FOUR STICKS:
In order to get full RAM speed and performance using 4 sticks of RAM in the 2020 27" iMac, need to satisfy all of the following conditions:

1. All sticks must be the same size (GB)
2. Sticks in Slots 1 and 2 must be identical
(brand and part number)
3. Sticks in Slots 3 and 4 must be identical

(Note this is contrary to the common understanding that slots 1 and 3 must be identical, and slots 2 and 4 must be identical.)

Of course, the most straightforward and sure way to ensure all 3 conditions are met, is to have all 4 sticks identical.
However, it is possible to mix brands, and retain full RAM performance, provided they are same size.

If you do not satisfy all of 1, 2, and 3 above, the machine will still "work" but it will not be optimal - either it will run at 2133MHz or it will lose dual channel (reverts to flex mode).

As to answering: why, how can that be, I don't agree, my iMac runs fine, etc, etc - read the multiple long threads on this topic and try it out yourself, or do whatever seems right to you.

This is only offered to help others who don't have time for all the testing and research and just want to know what to do to have their RAM perform optimally in the 2020 iMac
 
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pldelisle

macrumors 68020
May 4, 2020
2,247
1,496
Montreal, Quebec, Canada
I think you are wrong...

1. All sticks must be the same size (GB)
2. Sticks in Slots 1 and 3 must be identical (brand and part number)
3. Sticks in Slots 2 and 4 must be identical

This way, dual channel is kept because :

Channel_0/DIMM0
Channel_1/DIMM0
Channel_0/DIMM1
Channel_1/DIMM1

But, as we know, even when correctly paired, the iMac falls back to 2133 MHz when sizes aren't identical.

So basically, all four sticks must be identical. And this is the case with many, many professional workstation/server motherboards.
 

wilberforce

macrumors 65816
Aug 15, 2020
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SF Bay Area
I think you are wrong...

1. All sticks must be the same size (GB)
2. Sticks in Slots 1 and 3 must be identical (brand and part number)
3. Sticks in Slots 2 and 4 must be identical

This way, dual channel is kept because :

Channel_0/DIMM0
Channel_1/DIMM0
Channel_0/DIMM1
Channel_1/DIMM1

But, as we know, even when correctly paired, the iMac falls back to 2133 MHz when sizes aren't identical.

So basically, all four sticks must be identical. And this is the case with many, many professional workstation/server motherboards.
No your version of items 1, 2 and 3 are incorrect, sorry. I and others have proven this.
As I stated: "Note this is contrary to the common understanding that slots 1 and 3 must be identical, and slots 2 and 4 must be identical." It is extremely unusual, but that's the way it actually is.
 

pldelisle

macrumors 68020
May 4, 2020
2,247
1,496
Montreal, Quebec, Canada
No your version of items 1, 2 and 3 are incorrect, sorry. I and others have proven this.
As I stated: "Note this is contrary to the common understanding that slots 1 and 3 must be identical, and slots 2 and 4 must be identical." It is extremely unusual, but that's the way it actually is.
ok well... that's weird.
 

pldelisle

macrumors 68020
May 4, 2020
2,247
1,496
Montreal, Quebec, Canada
I agree with that statement! ("weird")
It causes great confusion. Why Apple did it this way completely perplexes me. Maybe there is a good reason, but personally I think it is a bug, that we have to work around

I have never had trouble with any workstation/server I have ever assembled : never mix and match RAMs. It's a simple rule that avoid all problems possibles.
 
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BazzaT

macrumors newbie
Oct 7, 2020
2
0
I have tried installing 4 32gb Samsung sticks in my new iMac 2020 received last week but it can only handle 2 at a time... I can put any pair of them in slots 1 and 3, or any pair of them in slots 2 and 4, and the iMac starts... recognises 64gb at 2667... BUT it will not start with all 4 sticks in at the one time!! Any clues? I have sent the iMac to the local Apple agent and they just said it must be a ram problem and gave the computer back to me today.
 

mikehalloran

macrumors 68020
Oct 14, 2018
2,094
622
The Sillie Con Valley
I have tried installing 4 32gb Samsung sticks in my new iMac 2020 received last week but it can only handle 2 at a time... I can put any pair of them in slots 1 and 3, or any pair of them in slots 2 and 4, and the iMac starts... recognises 64gb at 2667... BUT it will not start with all 4 sticks in at the one time!! Any clues? I have sent the iMac to the local Apple agent and they just said it must be a ram problem and gave the computer back to me today.
While the machine is under warranty, there's no reason to expect any other response.
 

neuralengine

macrumors regular
Jul 13, 2020
116
51
Is not being able to use 4 sticks of RAM a thing? I’m still hoping to add another 2x16 to my 2x16 (waiting on the Crucial RAM to be restocked).

Anyone successfully running 4 sticks of RAM without kernel panics?
 

AZREOSpecialist

Suspended
Mar 15, 2009
2,354
1,279
I have tried installing 4 32gb Samsung sticks in my new iMac 2020 received last week but it can only handle 2 at a time... I can put any pair of them in slots 1 and 3, or any pair of them in slots 2 and 4, and the iMac starts... recognises 64gb at 2667... BUT it will not start with all 4 sticks in at the one time!! Any clues? I have sent the iMac to the local Apple agent and they just said it must be a ram problem and gave the computer back to me today.
How do you know it's not starting? Are you waiting for the iMac to perform it's RAM check at startup? First startup takes longer after installing new RAM because iMac has to check RAM first. This happens while the screen is black.
 
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mikehalloran

macrumors 68020
Oct 14, 2018
2,094
622
The Sillie Con Valley
I have never had trouble with any workstation/server I have ever assembled : never mix and match RAMs. It's a simple rule that avoid all problems possibles.
It may be your simple rule but for the past 34 years, it hasn't really been a problem. Yes, 1986 was when I upgraded my Mac Plus from 1B to 2.5MB RAM with a couple sticks from OWC.
 

Nemanja604

macrumors newbie
Nov 16, 2020
10
0
I think we can establish a rule of thumb here with the 2020 iMac for now.

1. If putting 4 sticks of the same size in, put the matched pairs in the adjacent slots( ie 1 and 2 and 3 and 4).

2. if putting just 2 sticks in the machine, put in either slots 2 and 4 or 1 and 3.

3. Don’t mix different sizes of RAM. But if you must, keeping dual channel is better than restoring full speed. So put the same size sticks in 2 and 4 and the other size in 1 and 3.
What is the scenario where you "MUST" leave the stock 8gb ram? How would it contribute to performance when keeping the stock 2x4 when you add 2x16?

I seen some test, read in posts, that having mixed 40gb ram influences the cpu performance... that its not just about the ram speed.

Thanks!
 

quagmire

macrumors 604
Apr 19, 2004
6,673
1,779
What is the scenario where you "MUST" leave the stock 8gb ram? How would it contribute to performance when keeping the stock 2x4 when you add 2x16?

I seen some test, read in posts, that having mixed 40gb ram influences the cpu performance... that its not just about the ram speed.

Thanks!

If you absolutely must keep the stock 8 GB of RAM installed and adding two 16 GB sticks, best configuration is to leave the stock RAM where it is currently installed and install the 16 GB sticks in slots 2 and 4. This will lower the clock speed of the RAM to 2133 Mhz, but it will retain dual channel. Dual channel has a bigger impact on CPU performance than regaining the 500 Mhz of the RAM speed.
 

MauriceG

macrumors regular
Feb 9, 2013
104
44
If you absolutely must keep the stock 8 GB of RAM installed and adding two 16 GB sticks, best configuration is to leave the stock RAM where it is currently installed and install the 16 GB sticks in slots 2 and 4. This will lower the clock speed of the RAM to 2133 Mhz, but it will retain dual channel. Dual channel has a bigger impact on CPU performance than regaining the 500 Mhz of the RAM speed.
Correct👍🏼
And if you have more RAM, the Mac will do less (or no) compression of memory. Just to make it more complex 🤗
 

Nemanja604

macrumors newbie
Nov 16, 2020
10
0
If you absolutely must keep the stock 8 GB of RAM installed and adding two 16 GB sticks, best configuration is to leave the stock RAM where it is currently installed and install the 16 GB sticks in slots 2 and 4. This will lower the clock speed of the RAM to 2133 Mhz, but it will retain dual channel. Dual channel has a bigger impact on CPU performance than regaining the 500 Mhz of the RAM speed.
Don't need to absolutely keep it. Just wondering why someone would want to do so.

I just want to have best possible performance for my needs - that is why I bought the machine and added the ram, and don't want something to downgrade it, rather upgrade it :)

Bottom line, what I take from your reply - additional 8gigs in dual channel is worth more than higher speeds for best possible performance?
 

JED POLAND

macrumors newbie
Nov 16, 2020
11
6
Hi there.
I've just registered to get some help/knowledge I need after reading this thread (I actually wish I never read it ;) ).

iMac 2020 w/ 5700XT, originally 8GB RAM.

Upon receiving the iMac, I added one 32 GB stick from Samsung.
The system recognized everything OK, stating 40 GB 2667 Mhz.

The Geekbench results were 1245/7310 but what got me was the speed: 1333 Mhz, although I presumed it was OK due the way it is measured (correction needed).

Now, today, on here, I saw people talking about Novabench results, so I decided to give it a go to see how my iMac compares to others.
To my highest surprise, the RAM speed clocked in at 17100 MB/s which is very, very low, right?
After reading this thread I came up to conclusion that it must be those mixed sticks, so I decided to get rid of those OEM 8 GB sticks and to keep one, 32 GB only.
I think the results are pretty interesting, however the main issue remains unsolved - RAM speed.

For a better overview:

40 GB
Score: 2957
CPU: 1570
GPU: 911
Disk: 181
RAM: 295 @17978 MB/s

32 GB
Score: 2994 (!)
CPU: 1612 (!)
GPU: 915
Disk: 186
RAM: 281 @17841 MB/s

Geekbench 5: 1322/7071 without (why?) stated speed.

Question: why the memory speed is so low even with one chip? Because it is only one, working in single channel mode?
I don't need 64 GB but would it be advisable to get another stick to benefit from dual-channel (IF that's the case here)?

Please, can someone evaluate the results as well?
 
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quagmire

macrumors 604
Apr 19, 2004
6,673
1,779
You have two 4 GB sticks mixed with one 32 GB stick. You're down to single channel with that RAM setup. Need to get another 32 GB stick and either go with 64 GB of RAM with the two sticks( same brand) in slot 1 and 3( or 2 and 4). Or keep the 8 GB of RAM for 72 GB with the 4 GB sticks in their present location and the two 32 GB sticks occupying the remaining slots.
 
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wilberforce

macrumors 65816
Aug 15, 2020
1,316
1,238
SF Bay Area
Yes, get another matching 32GB stick and put them in slots 2 and 4 (slots 1 and 3 should also work instead, but will sometimes not boot). This will run at 2667 MHz and dual channel. ("Dual channel" means the memory controller accesses each channel simultaneously thus effectively doubling the RAM speed, so you need matching size sticks in each of channel A and channel B). After doing this the Novabench RAM speed should be 28000+ MB/s.

You can put the 2x4GB sticks in the remaining two slots, but on the 2020 iMac this drops the memory frequency to 2133 Mhz, so it is not worth it. You will find that the Novabench RAM speed will drop below 28000 MB/s.
 
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JED POLAND

macrumors newbie
Nov 16, 2020
11
6
Thanks a lot.
Wish I knew it beforehand as now I am a bit worried about getting the perfect match for the existing stick.
Will see.
 

MauriceG

macrumors regular
Feb 9, 2013
104
44
Thanks a lot.
Wish I knew it beforehand as now I am a bit worried about getting the perfect match for the existing stick.
Will see.
Dual ram: read and write twice the amount of memory per cycle (access 2 modules simultaneously). It only functions when 2 memory modules are (near) identical and are members of 1 memory bank (slots 1+3 is a bank and 2+4 also). The iMac 2020 uses the T2 processor for memory access. The T2 is very picky with regard to matching memory modules.
 
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wilberforce

macrumors 65816
Aug 15, 2020
1,316
1,238
SF Bay Area
Thanks a lot.
Wish I knew it beforehand as now I am a bit worried about getting the perfect match for the existing stick.
Will see.
A perfect match is desirable, but not necessary. I know, because I have used sticks of the same size, but from different manufacturers. I think the essential properties of the sticks need to match, like size, rank, latency, etc
 
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