Mixing Ram capacities in different DIMMs and Banks

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Mixing Ram capacities in different DIMMs and Banks

Good afternoon everyone,

I was curious about something I saw online and I was hoping someone could clarify it for me.

I see that if I mix RAM capacities in my IMAC, for instance keeping the base 4gb in slot 2 and 4 (DiMM 0), and then add 16gb to slot 1 and 3 (DIMM 1), my RAM speed in "System Information" drops from 2667 MHz to 2133 Mhz. But if I mix and match DIM slots, 4gb in slot 1 and 2, and 16gb in slot 3 and 4, then I get the full 2667 MHz showing up in "System Information"

HOWEVER, in practice, it looks online that geek bench tests show that having paired RAM sticks in the same DIMM slot gives twice as much speed as splitting the pairs and having 1 from each pair be both in DIMM 0 and 1.

so I am wondering the following:

Does the drop from 2667 MHz to 2133 MHz shown in "System Information" when having 2 different capacity RAM sticks, but each paired correctly ACTUALLY give you a drop in performance like it shows?
if it does drop in performance for using different capacity RAMs, why does it occur if both RAM sticks are 2667 MHz?
And finally why does "System Information" show the 2667MHz for mismatching the pairs in different DIMM Slots, when in tests this configuration provides half the speed.

From what Im gathering, you get the BEST performance if you only use 2 sticks or 4 sticks of the same capacity. IF you get 2 different capacities, even if you slot the pairs in the same DIMM slot, you will get a reduction in speed. This mixed capacity, but proper pairing in DIMM slots is the second best option, but you get that reduction in speed. and the third and worse option is putting the 1 stick from each pair in both DIMM 0 and 1. Even though "System Information" states its back up to 2667 Mhz, in practice you get a huge reduction in performance for not seating the RAMs in pairs in one DIMM.

Posted on Sep 1, 2020 11:44 AM

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Answer 1

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rkaufmann87

Level 10

140,448 points

Sep 2, 2020 7:44 AM in response to Tymbo

You are way over complicating a very simple thing, install the new RAM as I suggested and boot the computer. It will take it a few moments to "register" the new RAM and then it will be fine. Installing quality RAM in iMacs is critical as they are sensitive to using quality RAM, buying discounted or "value" RAM is almost always a disaster. Most experienced users in these forums will ONLY use Crucial.com or Macsales.com RAM, do not buy Crucial from a reseller such as Amazon as they frequently sell PC RAM that has identical specs but only works in PCs. Critical rules to remember:

Buy from Macsales.com or Crucial.com only.
Install original RAM in slots 1 & 2 and the upgraded RAM in slots 3 & 4.

Its that simple!!!!!

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Answer 2

Sep 2, 2020 7:22 AM in response to Tymbo

Hi, welcome to the Apple Users Community.

Please try this link for help with your issue

https://support.apple.com/en-us/HT201191#27inch2020

It clearly states what rkaufman87 has been telling you. USE 2667MHz! As recommended by Apple. You have posted here, part of Apple's website, under an account tied to your Apple ID, that you have tried to go against the Apple recommended advice and install RAM that Apple does not recommend,

Best of luck, you are going to need it.

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Answer 3

rkaufmann87

Level 10

140,448 points

Sep 1, 2020 8:41 PM in response to Tymbo

I have 2 new 2020 27" iMacs. I purchased both with the base 8GB of RAM and then purchased 2 x 16 GB (32 GB) from www.macsales. In order to get the quickest speed I install the original RAM in banks 1 & 2 and the upgrade RAM in 3 & 4. Below are screenshots showing:

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Answer 4

HWTech

Level 9

55,604 points

Sep 1, 2020 8:03 PM in response to Tymbo

You want to install the memory in the same manner that Apple installed the original RAM at the factory. It is Ok to put the new RAM in the slots originally occupied by the Apple memory from the factory. You want to install the memory in matched pairs. The memory speed is determined by the CPU memory bus and the speed of the slowest memory modules. If the CPU memory bus is slower than the new RAM's speed, then the faster RAM won't have any advantage in speed.

Most users will never notice any slight difference in performance from using faster RAM with most normal workloads.

FYI, Macs are very picky about the RAM used in them so it is best to only use memory from Crucial or OWC using the tools on their respective websites to identify the correct part numbers compatible with your Mac. I'm not saying other brands of memory won't work, but other brands tend to have more compatibility issues.

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Answer 5

Tymbo Author

Level 1

10 points

Sep 1, 2020 8:47 PM in response to rkaufmann87

I haven't done the tests myself because I don't want to be switching things around back and forth over and over again, but it looks from geek bench tests that these people did (https://www.ifixit.com/Answers/View/223429/How+to+couple+RAM+Modules+in+iMac+27) that mixing your ram capacities like that in different DIMM slots will actually give you a massive reduction In performance.

what they didn't test though is: 1. how much performance you get if you only use 2 or 4 sticks of the same capacity of RAM vs using 4 sticks of varying capacity, so that's what I was wondering about. and also why the speed shown in "system information" drops down to 2133 MHz from 2667 MHz when you use varying RAM capacities, but have similar pars in DIMM 0 and DIMM 1

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Answer 6

Tymbo Author

Level 1

10 points

Sep 2, 2020 9:27 AM in response to BORg529

Yes this physically works, but you are reducing your performance, because you are mixing RAM capacities. if you go to the "Memory" tab in "System Information", I believe you will see your speed be 2133 Mhz, while the physical RAM modules you have in there, or you should have at least for IMAC 2020 is 2667 Mhz speed.

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Answer 7

Tymbo Author

Level 1

10 points

Sep 2, 2020 9:33 AM in response to rkaufmann87

Thanks for the info Rkaufmann87; however, I am not wondering about what RAM to purchase, I know I got proper RAM for the 2020 IMAC with OWC.

Secondly, this configuration is clearly wrong as stated throughout the forums, and as how apple ships it to you. Apple seats the RAM in a matching pair in slot 2 and 4, which is DIMM 0, and you should place your other pair in slot 1 and 3, DIMM 1. There are numerous tests done by others showing that having your 16 gb ram split in DIMM 0 and DIMM 1, instead of both in DIMM 0 OR 1, your are slowing down your performance.

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Answer 8

Tymbo Author

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10 points

Sep 2, 2020 9:45 AM in response to Pete-Progressive-Graphics

Hi Pete, and thank you for the reply. Unfortunately it seems you didn't read my inquiry thoroughly. I am aware I need to use 2667 Mhz sticks, its the different possible configurations, and the output performance those configurations offer that I am wondering about. As well as the effect that mixing RAM capacities such as keeping the 4 gb base sticks, and adding 8, 16 and 32 sticks.

Thanks

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Answer 9

rkaufmann87

Level 10

140,448 points

Sep 2, 2020 9:51 AM in response to Tymbo

I told you how it works and proved it to you by showing the graphic. You can follow Apple's suggested installation however the RAM will be slower. I followed what I found on 9:5 where he clearly proves that installing in the method I suggested works and is quicker than the Apple method. I tested his theory and found it to be true. https://www.youtube.com/watch?v=jojJb9I_krI

I'm done on this thread.

Best of luck on your install.

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Answer 10

Tymbo Author

Level 1

10 points

Sep 2, 2020 9:59 AM in response to rkaufmann87

Yes that is where I saw that configuration option as well. Further digging proved that even though it says it’s back up to full 2667 MHz, actual tests show a reduction in speed by half. I’m just trying to help you out so you get the most performance. If you read the article or do actual tests on it you will see the same. Best of luck to you.

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Answer 11

HWTech

Level 9

55,604 points

Sep 2, 2020 10:04 AM in response to Tymbo

Most users will not notice any difference in performance for the majority of workloads even if benchmarks show a reasonable increase.

You want to install the paired memory in the identical DIMM slots. Put the new 16GB modules in DIMM0 slots and the original RAM in DIMM1 slots. I tried looking for something from Apple online on the proper installation of memory in this iMac and I cannot locate any specific information (even most third party sites do not mention any particular order except for the iFixIt article some linked earlier). I believe that information is the proper method from when I researched memory installation for an iMac a few years ago when Apple actually listed the information online (or maybe it was Apple' own memory utility that recommended the configuration suggested by the iFixIt article.

What may be happening is the System Profiler is showing faster speeds for slower memory because that particular memory module may actually be faster than its written rating so when it is paired with a faster module it gets identified with the faster speed. Or it may be a macOS bug on polling information from the RAM. Personally I would stick with the information Apple used to provide for memory upgrades for stability, but it is your system which you can configure as you see fit. You have no idea if the memory paired in the "unsupported" configuration will actually be stable or if it will cause intermittent issues.

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Answer 12

rkaufmann87

Level 10

140,448 points

Sep 1, 2020 8:28 PM in response to Tymbo

It would be helpful if we knew exactly what iMac you are referring to, the year and model. Without these fundamentals we can't really assist you much.

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