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kidunknown

macrumors newbie
Original poster
Jul 10, 2023
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Hi all,

I decided to upgrade the stock RAM (2x4GB) on my iMac 27 2020, and bought an extra 2x8GB.

I installed these in the AABB configuration and it's correctly reporting 24GB RAM at the right speed.

However, on further reading I think that I may lose dual-channel with this config.

So my question is: Am I better off with 24GB as is, or removing the stock 2x4GB and having 16GB dual channel?

Thanks for any help!
 

wilberforce

macrumors 68030
Aug 15, 2020
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SF Bay Area
If you have installed it 4-4-8-8, you have 8GB in Channel A and 16GB in channel B, which means the mismatched portion (8GB) will run in single channel mode which is effectively half the transfer speed. This is the worst combination, IMO.

You have two choices with the 2020 iMac, given the RAM that you have, to run in full dual channel mode:

1. Install it 4-8-4-8 (or 8-4-8-4), in which case you will have 24GB running at 2133 MHz (this a quirk of the 2020 iMac).

2. Install 0-8-0-8, in which case you will have 16GB running at 2667 MHz.

Personally I would opt for Option 1. 50% more RAM will generally be better than 25% faster transfer speed, unless your memory usage is low.

However the best option would be to return the 2x8GB and buy 2x16GB instead, and install 0-16-0-16, which will give 32GB running at 2667MHz

btw, to see what you have in each Channel: About this Mac > System Report... > Memory
 
Last edited:

gilby101

macrumors 68030
Mar 17, 2010
2,686
1,455
Tasmania
Personally I would opt for Option 1. 50% more RAM will generally be better than 25% faster transfer speed, unless your memory usage is low.
However the best option would be to return the 2x8GB and buy 2x16GB instead, and install 0-16-0-16, which will give 32GB running at 2667MHz
+1

I decided to upgrade the stock RAM (2x4GB) on my iMac 27 2020, and bought an extra 2x8GB.
The 2020 is very picky. Back in the day there were multiple threads on this question. Was quite a surprise as previous Macs (like my 2019 iMac) worked fine with different pairs - as you were wanting to do.
With a 2020 iMac it is best/safest to have all modules the same (size, vendor, etc.).
 

Yebubbleman

macrumors 603
May 20, 2010
5,944
2,534
Los Angeles, CA
If you have installed it 4-4-8-8, you have 8GB in Channel A and 16GB in channel B, which means the mismatched portion (8GB) will run in single channel mode which is effectively half the transfer speed. This is the worst combination, IMO.

You have two choices with the 2020 iMac, given the RAM that you have, to run in full dual channel mode:

1. Install it 4-8-4-8 (or 8-4-8-4), in which case you will have 24GB running at 2133 MHz (this a quirk of the 2020 iMac).

2. Install 0-8-0-8, in which case you will have 16GB running at 2667 MHz.

Personally I would opt for Option 1. 50% more RAM will generally be better than 25% faster transfer speed, unless your memory usage is low.

However the best option would be to return the 2x8GB and buy 2x16GB instead, and install 0-16-0-16, which will give 32GB running at 2667MHz

btw, to see what you have in each Channel: About this Mac > System Report... > Memory

+1


The 2020 is very picky. Back in the day there were multiple threads on this question. Was quite a surprise as previous Macs (like my 2019 iMac) worked fine with different pairs - as you were wanting to do.
With a 2020 iMac it is best/safest to have all modules the same (size, vendor, etc.).
So, I missed all this "the 2020 27-inch iMac is quirky with RAM" stuff. I'm not doubting the two of you that it's right; but I'm interested in learning more. Do you have any posts/articles on the topic that you recommend I peruse to learn more?
 

gilby101

macrumors 68030
Mar 17, 2010
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Tasmania
Do you have any posts/articles on the topic that you recommend I peruse to learn more?
The MacRumors threads from 3 years ago:
 
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gilby101

macrumors 68030
Mar 17, 2010
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Tasmania
And from AskDifferent:

And Apple:
https://support.apple.com/en-us/HT201191 which includes this for 2020 iMac: "For optimal memory performance, DIMMs should be the same capacity, speed, and vendor." but not for 2019, etc.
 
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wilberforce

macrumors 68030
Aug 15, 2020
2,894
3,166
SF Bay Area
So, I missed all this "the 2020 27-inch iMac is quirky with RAM" stuff. I'm not doubting the two of you that it's right; but I'm interested in learning more. Do you have any posts/articles on the topic that you recommend I peruse to learn more?
Here are a couple of posts which provide some technical insight (not necessarily answers) about the issue:

#454

#22
 
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Yebubbleman

macrumors 603
May 20, 2010
5,944
2,534
Los Angeles, CA
And from AskDifferent:

And Apple:
https://support.apple.com/en-us/HT201191 which includes this for 2020 iMac: "For optimal memory performance, DIMMs should be the same capacity, speed, and vendor." but not for 2019, etc.

Here are a couple of posts which provide some technical insight (not necessarily answers) about the issue:

#454

#22
Man, I love you guys! This is fascinating stuff! Definitely the sort of information that is useful to me in my side-consulting practice! Many thanks!
 

Yebubbleman

macrumors 603
May 20, 2010
5,944
2,534
Los Angeles, CA
So, stupid question: is this a unique to the 2020 model? Or is this also a factor on 2019 iMacs too? I ask as 2019 seems lumped in with others in one of the linked articles/posts, but all others seem to stress the 2020 model as being the only one with this.

Another stupid question: Is this is a side-effect of the Intel processor and chipset used? Or is this some other random model quirk?
 

gilby101

macrumors 68030
Mar 17, 2010
2,686
1,455
Tasmania
Or is this also a factor on 2019 iMacs too
No, the 2019 iMacs are fine so long as you install in pairs. For a long time I had original Apple (2x4GB) plus OWC (2x16GB). I now have 2x16GB plus 2x32Gb (all from OWC).
I think 2019 and 2020 iMacs get put together because they use the same memory modules. It is just the 2020 which is 'picky' about combinations of modules.
Is this is a side-effect of the Intel processor and chipset used? Or is this some other random model quirk?
AFAIK, this has never been clear.

(It was not a stupid question)
 
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