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A2115 / 2020 / Processors from 3.1 GHz 6-core i5, up to 3.8 GHz 8-core i7. Released August 4, 2020.

9 Questions View all

Best arrangement of stock and 3rd party RAM?

I bought the 3.8GHz 8-core 10th-generation i7 variety 2020 iMac, with the base config 8GB of 2666MHz DDR4 memory. I’m now trying to install 2x 16GB sticks of HyperX HX426S15IB2K2 (Kingston) and have discovered an impossible optimisation problem.

The stock RAM comes as two 4GB sticks. I tried the following configurations of RAM stick in each slot (remember slots 1 and 3 are DIMM 1 and slots 2 and 4 are DIMM 0):

  1. Empty
  2. 4GB
  3. Empty
  4. 4GB

Result: 8GB @ 2667 MHz

  1. 4GB
  2. 16GB
  3. 4GB
  4. 16GB

Result: 40GB @ 2133 MHz

  1. 16GB
  2. 4GB
  3. 16GB
  4. 4GB

Result: 40GB @ 2133 MHz

  1. 4GB
  2. 4GB
  3. 16GB
  4. 16GB

Result: 40GB @ 2400 MHz

  1. Empty
  2. 16GB
  3. Empty
  4. 16GB

Result: 32GB @ 2667 MHz

Here’s a photo of the last configuration to provide physical orientation. Up to towards the top of the computer.

Block Image

In summary, it seems the only way I can retain the full 2667 MHz speed, is to leave some of the RAM out! Oddly, cross-matching them (putting different size sticks in a single bank) reported a higher speed than matching sizes in a single bank, but I suspect I’d lose out in other ways like parallelisation.

If I want to take full advantage of my RAM bandwidth potential, but would otherwise more RAM than less, what’s my best option? Is there a way to have it all? Is the drop to 2133 MHz real? Is the smaller drop to 2400 MHz a false hope?

In case it’s relevant, the HyperX RAM wasn’t my first choice, but I was steered by stock limitations. It’s not exactly “approved” for use in the iMac by anyone, but matches up spec wise. It has a slightly better CAS Latency of CL15 than the required CL19, but is otherwise much of a muchness. This thread raises interesting points, although nothing hard and fast: What Ram to order with the new 2019 IMac?

Update (08/20/2020)

Well whaddya know, here’s a very detailed thread on why this occurs: iMac 2019 - upgraded w 2 extra ram and the speed dropped to 2400 MHz?

Still nothing conclusive (whether it’s an XMP/overclocking issue, or a single/dual rank issue seems unresolved), and doesn’t really explain why removing one pair restores full speed, but good background nonetheless. Still looking forward to your advice on the best way to proceed (short of buying more RAM).

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Comments:

A configuration is missing in your expertise:

1. 16GB

2. 16GB

3. 4GB

4. 4GB

Result: 40GB @ 2667 MHz

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Even if the stated speed is 2667, don't you suffer from poor performance due to in ability to parallel? See comment here: Best arrangement of stock and 3rd party RAM?

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The tests that I carried out with Geekbench 5 did not show any observable loss of performance with this configuration which is the only one to preserve the advertised speeds of my Crucial bars.

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Based on that I thought it worth a shot! Alas, reported speed drops to 2400, just like in the 4,4,16,16 configuration. I also ran a test with Novabench, just to provide a different data point. It reported 27527MB/s in the 32GB configuration and 24096MB/s in your suggested 40GB configuration. I take these numbers with a grain of salt, but that does line up with the 2400/2667 reported speeds.

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This may be a good point for Crucial.

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2 Answers

I have a mid-model iMac 2020 with 1TB SSD. I bought a pair of OCW 16GB, and it’s work with my pair of 4GB Apple ram. Below is my ram arrangement.

Block Image

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Marco, that's a bad configuration. Run benchmarks like that and then with just the 2 16gb sticks in the same DIMM, and you should see that you are crippling your performance with this config.

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The tests that I carried out with Geekbench 5 did not show any observable loss of performance with this configuration which is the only one to preserve the advertised speeds of my Crucial bars.

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Mix and match+Mac=almost never works. It's a "try at your own risk" issue with Apple, to the point it's NOT recommended unless the RAM is sitting around.

Apple does not play around with their RAM tolerances. Downvoted due to the recommendation to mix and match when it's a known Apple sour spot.

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When it comes to RAM, best to match it all with a matched module set - ESPECIALLY MACS. Apple is particular about this to the point one bit it doesn’t like=POST error. You can’t mix and match with Apple… At all, generally.

Buy a matched 3rd party RAM set from a company like Crucial (MUST be Mac memory to be sure, unless you’re a gambling man), keep your factory RAM for warranty purposes or buy memory from Apple - again, note which ones were added by you and remove them for warranty purposes but keep the stock RAM installed.

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Your comments are interesting and should be taken into consideration.

But I have to say that a lot of Macs have passed through my hands for almost 20 years and I have always used non-Mac modules to increase memory (Apple sells its memory at an outrageous price). I have never encountered a problem.

Obviously, I choose reputable brands that guarantee me a product exactly compatible with the specific model of Mac. And I also rely on the user experience reported on several forums.

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@k2103 The issue is mixed modules moreso then non Mac certified. As long as it's a good module and you're not mixing brands it's fine generally.

Mix and match usually does not work on a Mac

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