Resolved iMac 27" (5K, Late 2015) correct RAM configuration

[Edgecliff]

This is discussed many times before, but without definitive answer.
What is the correct way to arrange 16GB of additional memory?

By default, the iMac has got 8GB of memory installed in a 4/0/4/0 way, or:

BANK0/DIMM0 4GB
BANK0/DIMM1 Empty slot
BANK1/DIMM0 4GB
BANK1/DIMM1 Empty slot​

Now, some people advice to add 16GB in the empty slots, making it 4/8/4/8, or:

BANK0/DIMM0 4GB
BANK0/DIMM1 8GB
BANK1/DIMM0 4GB
BANK1/DIMM1 8GB​

Others say the banks must have equal memory in their DIMMs, making it 4/4/8/8, or:

BANK0/DIMM0 4GB
BANK0/DIMM1 4GB
BANK1/DIMM0 8GB
BANK1/DIMM1 8GB​

Does the memory have to be equal in the banks or in the DIMMs to run in dual channel mode?

Also, some say it's better to have the higher capacities first, making it 4 possible combinations:

4/8/4/8
4/4/8/8
8/4/8/4
8/8/4/4​

Which of these is the best configuration? And why?

 

[redheeler]

Arrangement doesn't affect performance as long as the RAM is installed in matched pairs, which the factory RAM should be. Easiest to just use the empty slots.

 

[Edgecliff]

Arrangement doesn't affect performance as long as the RAM is installed in matched pairs, which the factory RAM should be. Easiest to just use the empty slots.

So you're saying slot 1 matches 3 and slot 2 matches 4, making a 4/8/4/8 configuration preferable?

Anyone who can confirm this?

 

[loekf]

Should be:

BANK0/DIMM0 4GB
BANK0/DIMM1 8GB
BANK1/DIMM0 4GB
BANK1/DIMM1 8GB

A DIMM in a slot in bank 0 ... should have the same DIMM in bank 1, same slot. Otherwise dual-channel
can't work. In order words... dual channel means the memory controllers accesses data in two banks
at the same time. So not to have timing issues, you should the same DIMMs.

 

[Edgecliff]

A DIMM in a slot in bank 0 ... should have the same DIMM in bank 1, same slot. Otherwise dual-channel can't work. In order words... dual channel means the memory controllers accesses data in two banks at the same time. So not to have timing issues, you should the same DIMMs.

This is exactly what I was looking for, thanks!

From another thread:

I can only speculate that one of the paradigms at play here is a carry-over from the olden days of placing the higher capacity memory modules in the memory slot(s) closest to the CPU socket on the motherboard.

Whether or not that survives today's contemporary circuit architecture? I have no idea. But there's no reason not to pair the larger capacity SODIMM's in the DIMM0 slots and to pair the lower capacity SODIMM's in the DIMM1 slots

Combined with the input from loekf, I assume 8/4/8/4 is the way to go then. Correct me if I'm wrong.

 

[tonywalker23]

Did you figure out the best configuration? I've been using the same macbook pro since 2011. I just bought a new 2015 27" imac that macrumors linked to last week. I bought an extra 16gb of ram from owc but I cant figure out the best way to put the sticks in.

 

[vistokid]

I'd like to know this as well please!