Apple Event: May 7th at 7 am PT
Hi. The late 2015 5k iMac has two 4GB RAM (total of 8GB, dual channel). What would happen if you mixed in two 16GB RAM and mix it with those? Would there be a significant performance hit or it'd be normal as if you had the same size modules?
Thank you. God bless. Proverbs 31
iMac, macOS Sierra (10.12.1), iMac (Late 2015)
Applications -> Utilities -> System Information -> Memory
DIMM0's should be equal in size
DIMM1's should be equal in size
That is to say
BANK 0/DIMM0 16GB
BANK 1/DIMM0 16GB
BANK 0/DIMM1 4GB
BANK 1/DIMM1 4GB
Or if you prefer put the 16's in DIMM1 and the 4's in DIMM0.
If you have the DIMM0's matching and the DIMM1's matching you get memory interleaving.
Reading RAM is destructive, so after reading a value, it has to be written back to the memory cells. This re-write takes an extra cycle, during which the DIMM is unavailble.
Interleaving means the CPU alternates DIMMs when reading consecutive memory locations. So while BANK 0 is rewriting the value just read, BANK 1 can be fetching the next memory value. And while BANK 1 is rewriting the value it just fetched, BANK 0 will be ready to read another value.
NOTE: The Mac will still work if you do not properly match RAM sizes, it will just take twice as long to read consecutive memory locations as interleaving will not be used, so the CPU must wait 2 memory cycles for each memory access.
Applications -> Utilities -> System Information -> Memory
DIMM0's should be equal in size
DIMM1's should be equal in size
That is to say
BANK 0/DIMM0 16GB
BANK 1/DIMM0 16GB
BANK 0/DIMM1 4GB
BANK 1/DIMM1 4GB
Or if you prefer put the 16's in DIMM1 and the 4's in DIMM0.
If you have the DIMM0's matching and the DIMM1's matching you get memory interleaving.
Reading RAM is destructive, so after reading a value, it has to be written back to the memory cells. This re-write takes an extra cycle, during which the DIMM is unavailble.
Interleaving means the CPU alternates DIMMs when reading consecutive memory locations. So while BANK 0 is rewriting the value just read, BANK 1 can be fetching the next memory value. And while BANK 1 is rewriting the value it just fetched, BANK 0 will be ready to read another value.
NOTE: The Mac will still work if you do not properly match RAM sizes, it will just take twice as long to read consecutive memory locations as interleaving will not be used, so the CPU must wait 2 memory cycles for each memory access.
You would have 40GB of RAM.
That would be perfectly acceptable. As long as you are inserting DIMMs in pairs of the same size, then you get memory interleaving to improve RAM performance.
I suggest you use a RAM vendor that properly support Mac RAM, as Apple tends to need tight tolerances, an sub-par components may not perform as desired.
Crucial.com and MacSales.com (OWC) are well respected RAM vendors within these forums.
So from slots 1-4 top to bottom?
1. 16GB
2. 4GB
3. 16GB
4. 4GB
What Would Happen if You Mix Two 16GB RAM Modules with the Current Two 4GB?
