iMac memory configuration

[carbontune]

The 2017 27" iMac has four slots and ordering 16GB fills two of them, so that's the answer to your first question. I ordered my iMac that way. It turned out that 16GB is plenty for me so far, but it's nice to have the option to add memory down the road.

Here's a clip from the Apple site on compatible memory:

Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

• PC4-2400 (19200)
• Unbuffered
• Nonparity
• 260-pin
• 2400MHz DDR4 SDRAM

source: https://support.apple.com/en-us/HT201191#27inch2017​

The factory installed DIMMS in my machine are installed as follows:

Bank 0/DIMM0 8 GB
Bank 0/DIMM1 Empty
Bank 1/DIMM0 8 GB
Bank 1/DIMM1 Empty

Most discussion elsewhere assumes that modules are added in pairs and that iMacs use dual-channel memory. I don't think you'd see a difference between 4x4GB and 2x8GB.

If that's how the slots look visually in terms of Bank numbers, which is different to a typical PC motherboard, then should the DIMMs be moved so that matching pairs are in the same bank? This is what I would normally do when I build PCs.

As an example, say we are upgrading the above 2x8GB configuration to 24GB by adding 2 x 4GB dual-channel DIMMs. What I would do on a new PC build is this (from a Bank layout point of view):

Bank0 / DIMM0 - 8GB
Bank0 / DIMM1 - 8GB
Bank1 / DIMM0 - 4GB
Bank1 / DIMM1 - 4GB

This would translate to this from a visual/physical layout point of view on a typical PC motherboard:

Bank0 / DIMM0 - 8GB
Bank1 / DIMM0 - 4GB
Bank0 / DIMM1 - 8GB
Bank1 / DIMM1 - 4GB

This ensures that matching dual-channel DIMMs are working in pairs.

This would mean that adding the two 4GB DIMMs would require rearranging the factory installed DIMMs on an iMac. Just by adding two DIMMs to the factory configuration would give you this (from a Bank point of view):

Bank0 / DIMM0 - 8GB
Bank0 / DIMM1 - 4GB
Bank1 / DIMM0 - 8GB
Bank1 / DIMM1 - 4GB

Would this not result in all of the installed DIMMs operating in single-channel mode, resulting in slower performance?

I would have expected the iMac DIMM slots to be physically arranged like this (just like on a PC motherboard) as the factory installed DIMMs are installed in alternating slots and not next to each other:

Bank0 / DIMM0
Bank1 / DIMM0
Bank0 / DIMM1
Bank1 / DIMM1

Does anybody have a definitive answer to the visual/physical Bank arrangement of the four DIMM slots (I don't have my iMac yet to check)?

 

[carbontune]

I've spoken to Apple over the last couple of days about the memory slot configuration on a 2017 iMac 27-inch. From what I've been told, the DIMM number represents the channel pairing so the paired DIMMs should be in the same DIMM numbered slot to run in dual-channel mode.

As an example, if you have 40GB made up of 2 x 4GB (stock RAM) and 2 x 16GB DIMMs (aftermarket RAM), then the slot configuration should be as follows:

Slot 1 (Bank0/DIMM0): 16GB
Slot 2 (Bank0/DIMM1): 4GB
Slot 3 (Bank1/DIMM0): 16GB
Slot 4 (Bank1/DIMM1): 4GB

I was also told that you should not mix memory sizes as it may cause BIOS errors, and that adding more than 32G isn't really necessary for most people.

 

[Brian Y]

I've spoken to Apple over the last couple of days about the memory slot configuration on a 2017 iMac 27-inch. From what I've been told, the DIMM number represents the channel pairing so the paired DIMMs should be in the same DIMM numbered slot to run in dual-channel mode.

As an example, if you have 40GB made up of 2 x 4GB (stock RAM) and 2 x 16GB DIMMs (aftermarket RAM), then the slot configuration should be as follows:

Slot 1 (Bank0/DIMM0): 16GB
Slot 2 (Bank0/DIMM1): 4GB
Slot 3 (Bank1/DIMM0): 16GB
Slot 4 (Bank1/DIMM1): 4GB

I was also told that you should not mix memory sizes as it may cause BIOS errors, and that adding more than 32G isn't really necessary for most people.

You can mix memory sizes absolutely fine.

 

[carbontune]

Just tried what Apple suggest on my brand new iMac and their suggestion didn't work for me!

I had to put the factory 2 x 4GB DIMMs in adjacent slots and my Crucial 2 x 16GB DIMMs in adjacent slots for the iMac to boot. This is what I suspected all along that I'd have to do.

 

[marekkurlmann]

I'm ordering a 2017 27" iMac with 8 GB of RAM. I want to add an extra 16 GB. Is it kosher to add a single stick of 16 GB (for a total of 3 slots occupied) or is it better to go 2 x 8 GB so all 4 slots are filled?

My preference is 1 x 16 GB to allow for future upgradability, but that may not be advisable.

 

[SaSaSushi]

Just tried what Apple suggest on my brand new iMac and their suggestion didn't work for me!

I had to put the factory 2 x 4GB DIMMs in adjacent slots and my Crucial 2 x 16GB DIMMs in adjacent slots for the iMac to boot. This is what I suspected all along that I'd have to do.

Matching pairs of different sized modules is no problem. The speed and CAS latency needs to match. I also recommend using the same maker.

I have been using the stock 8GB that came with my 2017 iMac (2 x 4GB Apple stock Micron) along with 2 x 16GB Crucial Ballistix I added myself for 40GB with zero issues for the past year and a half.

As mentioned above, matching pairs should be installed in alternating DIMM slots for best performance (dual-channel).

 

[Fishrrman]

marekkurlmann wrote in #30:
"I'm ordering a 2017 27" iMac with 8 GB of RAM. I want to add an extra 16 GB. Is it kosher to add a single stick of 16 GB (for a total of 3 slots occupied) or is it better to go 2 x 8 GB so all 4 slots are filled?
My preference is 1 x 16 GB to allow for future upgradability"

This should work.

Yes, there is a speed increase when RAM is upgraded "in pairs".
But it should still work with "one extra DIMM".
I really doubt that you'll notice any "slowdown".

I've been running my 2012 Mini this way for a few years now (2 DIMM slots, with a 2gb DIMM in one slot and an 8gb DIMM in the other).
Runs fine.

Granted, it's a Mini, not an iMac, but the basic principles should be the same...

 

[BigBoy2018]

Simply because I didn't want to stress out about mismatching pairs (even though I'm sure it's much ado about nothing) when I got my 2017 iMac a couple weeks ago I bought 4 matching 8gb Corsair Vengence sticks. I removed the 2 stock apple 4 gig sticks and set them aside.

I have 32 gigs, which is plenty for me, and know I'm solid with the whole matching pair calculus.

I did consider just getting two 16gig sticks and keeping the stock apple ram in for 40 gigs total, but for my uses I'm sure the extra 8 gigs wouldn't make a difference and I prefer the peace of mind.

 

[LTMstudios_Andy]

I tried mix matching 3rd party ram sizes made by exact same company with same mhz (pair of 8gb in slots 1-3 and pair of 16gb in 2-4) and proper spec ram for 2017 iMac (all ram was Corsair Vengeance ddr4 2400mhz PC-19200 260pin Sodimm) Installed in proper dimm slots and I get kernal panics from time to time performing certain tasks. It will boot up fine and work but crashes once in a while. Tried swapping dimm slots still being in matching pairs and still had same issues. So I tested using apples hardware test and it detected no issues but when I tested with Rember it would fail to test. So I ran back to back tests on all of my pairs of ram separately to see if I got faulty ram and they all passed the tests. Spent nearly the whole day testing. Noticed mix matching sizes was the culprit. Runs flawlessly when I use 4x 8gb all same brand sticks compared to mix matching size sticks even if the same brand. I don’t know if there’s any issues with other brands of ram while mix matching but If you’re using Corsair Vengeance I’d advise to use all same capacity size ram in all slots. I ran my iMac for past few months with 32gb (4x 8gb sticks) with no issues at all.

 

[SaSaSushi]

I tried mix matching 3rd party ram sizes made by exact same company with same mhz (pair of 8gb in slots 1-3 and pair of 16gb in 2-4) and proper spec ram for 2017 iMac (all ram was Corsair Vengeance ddr4 2400mhz 260pin Sodimm) Installed in proper dimm slots and I get kernal panics from time to time performing certain tasks.

Going with known working brands is also highly recommended. See this exhaustive thread.

 

[nambuccaheadsau]

For my 2016 iMac had to go 2x matched pairs. 4x8GB or 4x16GB and same answer to this problem in another thread today.