Have I lost dual channel?

[Luis Ortega]

I have the 2020 intel iMac that came with 2x 4gb ram and I bought 2x 16gb ram of the correct specs from OWC and added it so that the modules are arranged like this.

If I move them so each dimm 0 and dimm 1 have the same ram size, the speed drops to 2133.

With my arrangement, have I lost dual channel mode?
Would it be better to just use the 2x 16gb modules rather than mix them with the apple ram?

 

[chacaito]

Hi, I have the same setup as yours. It has been working no problem for 6 months with 2x16. Then, as you, I added 2x4 with same speed you get, 2667. Run it like it for 1 month, then it stop starting, just dead. Remove the RAM, it restart. Readd the Ram, works for 2-3 days..

So finally, I gave up and I keep only 2x16 (to avoid to break anything).

You should be fine, but if you got a no boot one day, you will know what to do, if you get the same issue I got.

 

[mfram]

Given the descriptions, I would think you'd want matched memory sticks in slots 1+3 and 2+4. In other words, in slots 1, 2, 3, 4 you should have 4, 16, 4, 16 instead of 4, 4, 16, 16.

 

[Luis Ortega]

Given the descriptions, I would think you'd want matched memory sticks in slots 1+3 and 2+4. In other words, in slots 1, 2, 3, 4 you should have 4, 16, 4, 16 instead of 4, 4, 16, 16.

When I do that, the speed drops to 2133 from 2667.

 

[velocityg4]

From what I can find. For dual channel you want to match A1 with B1 and A2 with B2. (16, 4, 16, 4) or (4, 16, 4, 16). Which is consistent with most other motherboards. You commonly interleave for dual channel.

The issue you are describing is normal. When modules in Channel A and Channel B aren't matched sizes. I don't know why. It's just the way it is with the iMac. Apple even says to minimize the difference between modules in Channel A and the modules in Channel B.

Install memory in an iMac - Apple Support

Get memory specifications and learn how to install memory in iMac computers.

support.apple.com

You'll just need to benchmark. With 2133Mhz Dual Channel and 2666 Mhz single channel. Then decide which is best for you. You can also take out the 2x4GB. Which many people opt to do. In most scenarios. You aren't likely to notice the difference between 32GB and 40GB.

There's some conjecture. This problem occurs because 4GB modules are single rank memory and 16GB modules are dual rank memory. But that's not confirmed.

 

[mfram]

That might be true, but it won't be operating in dual-channel mode if the DIMMs don't match (DIMM0, DIMM1). I know I had an older PC motherboard manual that said the memory would be slower if all slots were used vs. just one pair being used due to electrical issues.

Have you looked at this: Installing RAM in an iMac

 

[chscag]

When I do that, the speed drops to 2133 from 2667.

As has been recommended, pull the 2 x 4 GB modules and only use the OWC modules. It's not going to matter. Or, just buy two more 16 GB modules from OWC.

 

[casperes1996]

I have the 2020 intel iMac that came with 2x 4gb ram and I bought 2x 16gb ram of the correct specs from OWC and added it so that the modules are arranged like this.
View attachment 1796169
If I move them so each dimm 0 and dimm 1 have the same ram size, the speed drops to 2133.

With my arrangement, have I lost dual channel mode?
Would it be better to just use the 2x 16gb modules rather than mix them with the apple ram?

I believe modern memory controllers are quite sophisticated, so that you're actually running dual channel for the first 16GB of memory, as it can interleave the 2*4GB plus 2*4GB (out of the 16. But then the remaining 24GB will be single-channel.

The reason your frequency drops when properly interleaving all the memory for dual channel, is that running mixed size modules like that is harder for the CPU's memory controller, so Intel's SPD specification there lowers the supported frequency

 

[Luis Ortega]

That doesn’t sound very efficient or elegant.
I guess my concern is whether having 40 gb ram running at odd speeds and/or in dual channel or not is faster or better than having 32 gb running in dual channel and top speed for the majority of daily use which isn’t very demanding for me.

 

[casperes1996]

That doesn’t sound very efficient or elegant.
I guess my concern is whether having 40 gb ram running at odd speeds and/or in dual channel or not is faster or better than having 32 gb running in dual channel and top speed for the majority of daily use which isn’t very demanding for me.

As long as you actively use less than 32GB, running just the 32GB kit will run faster. If you actively do use 40GB, the frequency drop won't matter nearly as much as going to swap would performance wise - So there isn't a single answer, but since you say "daily use", then that's not hitting the 40GB mark, so there'd likely be a minimal performance boost to only using the 32GB kit there

 

[chown33]

You can find out whether 32GB is working well or not by using the Activity Monitor app. It's part of the standard OS, and resides in the Utilities folder.

Here's some info from the User Guide about interpreting the Memory Pressure data:

View memory usage in Activity Monitor on Mac

In Activity Monitor, view the amount of system memory being used on your Mac, including physical, compressed, wired, and app memory.

support.apple.com

At the bottom of that page is a link to another page that discusses using the data to help determine if more RAM would be helpful to you. If the data from running with 32GB isn't consistently red, i.e. it's mainly yellow or green, then you can reasonably conclude that 32GB RAM is sufficient.