iMac (Retina 5K, 27-inch, Late 2015) optimal RAM layout

[Zmaran]

So, just received my Crucial 16GB Kit (8GBx2) DDR3 1866 MT/s (PC3-14900) SODIMM 204-Pin Memory and upon doing the upgrade, I replaced the included 4GB ram DIMMs with the new 8GB ones and placed the 4GB ones in the empty slots. Now, this is what it looks like :



I tried searching around but couldn't find a definitive answer. I just wanted to inquire from the more informed if this placement is optimal or not? Would it make any discernible difference to plug the 8GB DIMMs to Bank 0s and 4GB DIMMs to Bank 1s? Most of what I could gather from the internet is that it doesn't make a difference. :/

 

[April Knight]

I am also interested in this question. I have the exact same scenario as you (ordered iMac with stock 8GB RAM, picked up the Crucial 16GB Kit). I also put them in the same layout as you and have not noticed any poor results, but I want to confirm that that's the best configuration.

 

[AlexisV]

Yep, that's the best config.

Stock: 4 - 0 - 4 - 0
Yours: 8 - 4 - 8 - 4

 

[ivoruest]

Mine's a 2014 with 4x4GB setup. Is this in anyway better or worse than the one you proposed before?

I mean, I have had some issues with my iMac shutting down due to unexpected problems plus some graphical issues. Is this because of the config or has to due directly with bad memory? (if memory was the case) The Apple Technician claims the computer is fine and passed all of the tests correctly. And couldn't reproduce the graphics issues on the days he had the machine. He explained to me that the issues I was getting seemed more like software related and/or memory.

 

[sabester24]

Yep, that's the best config.

Stock: 4 - 0 - 4 - 0
Yours: 8 - 4 - 8 - 4

I'm confused...why does this make a difference? I put my 3rd party 16 GB Crucial RAM kit in the open slots and thought that would be fine.

 

[AlexisV]

The same DIMMS should be in opposite slots so they run in dual channel mode. Not that it would make that much of a difference if you didn't.

Alternately referred to as Multi-channel memory, Dual-channel memory is a DDR, DDR2, or DDR3 chipset on the motherboard providing memory with two memory channels instead of one. The dual-channel platform helps alleviate bottlenecks that occur. Dual-channel lets one controller handle the reading and writing while the other controller prepares the next access thus preventing a delay.

To take advantage of a dual-channel platform when purchasing memory make sure to buy memory in identical pairs. For example, if you wanted to upgrade to an additional 2GB of memory. Buy two 1GB memory modules that are exactly identical. While the speed of each memory chip does not need to be the same as the others, it is best to install memory with matching speeds. If the speeds differ, the motherboard runs the memory chips at the speed of the slowest chip.

Finally, if you are only installing two memory modules at a time make sure the memory is installed into the proper memory slots. Almost always the memory slots will be color coded to indicate the memory channel, as shown in the picture below. The first channel is often slots one and two, and the second channel is three and four. When installing memory in pairs make sure to install them into the same colored slot to take advantage of the dual-channel platform.

 

[sabester24]

The same DIMMS should be in opposite slots so they run in dual channel mode. Not that it would make that much of a difference if you didn't.

So it doesn't matter which RAM goes into slots 1 & 3 vs 2 & 4?

Yep, that's the best config.

Stock: 4 - 0 - 4 - 0
Yours: 8 - 4 - 8 - 4

Mine is: Stock 4GB, Crucial 8GB, Stock 4GB, Crucial 8GB.

I'm just trying to figure out why one would rearrange their stock RAM and not just put any additional RAM in the open slots.

 

[FreemanW]

Never having been a code monkey or a circuit designing electrical engineer or physicist . . . . 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

The other consideration remains utterly true and valid, that being, you want your memory modules paired so as to facilitate the provision for "Dual Channel" operation.

/that is all ;-)

 

[sabester24]

Never having been a code monkey or a circuit designing electrical engineer or physicist . . . . 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

The other consideration remains utterly true and valid, that being, you want your memory modules paired so as to facilitate the provision for "Dual Channel" operation.

/that is all ;-)

Interesting...I'd like to know if this still applies. While it's not a huge deal to rearrange the RAM, I'd rather not if it makes no difference nowadays.

 

[Zmaran]

Yep, that's the best config.

Stock: 4 - 0 - 4 - 0
Yours: 8 - 4 - 8 - 4

Thanks.

 

[Zmaran]

I am also interested in this question. I have the exact same scenario as you (ordered iMac with stock 8GB RAM, picked up the Crucial 16GB Kit). I also put them in the same layout as you and have not noticed any poor results, but I want to confirm that that's the best configuration.

I see you've been a member since 2011 as well, and it's quite interesting that you too have an iMac 5k now. Looks like we're on the same boat for a while. I was using a MBP mid 2010 all this while.

 

[seetheforest]

I don't know if I should make a separate post for this, but I'm looking at the same configuration and have a question about it. If I'm getting the Crucial Ballistix 16GB kit with a CL of 9, and the stock ram has a CL of 13, is it better for me to just run the Ballistix and remove the stock RAM, or is it better for me to use all the sticks that I have.

Which will be better for my overall computing experience? Is the difference in latency in the RAM not a big deal, of not, then why spring for the Crucial Ballistix?

No Stock: 8 - 0 - 8 - 0
Stock: 8 - 4 - 8 - 4

 

[FreemanW]

Presuming both pairs run concurrently without issue, you would require a software diagnostic and clocking clocking routine to tell the difference. Taking the speed of electricity and applying that speed to distance traveled in "T", we are discussing nano-millimeters here.

 

[mtbdudex]

Yep, that's the best config.

Stock: 4 - 0 - 4 - 0
Yours: 8 - 4 - 8 - 4

So I bought these
http://www.amazon.com/gp/product/B00DSGLMSM

Crucial Ballistix Sport 16GB Kit (8GBx2) DDR3 1866 (PC3-14900) 204-Pin SODIMM Memory BLS2K8G3N18AES4 / BLS2C8G3N18AES4

went from this

to this

and the memory instructions here never said to put the bigger ones like you stated
https://support.apple.com/en-us/HT201191

I'm open to doing that, just wish Apple had include such tid-bits in their formal instructions

 

[FreemanW]

Apple likely does not address the issue, because, well, it likely doesn't matter a lick.

 

[tonywalker23]

thanks for this thread! I just bought a new 2015 27" imac with 8gb of ram and i bought an extra 16gb to upgrade to a total of 24. ive been googling the last 2 hours trying to figure this out.

 

[RaXha]

Apple likely does not address the issue, because, well, it likely doesn't matter a lick.

Just found this thread because i was running som diagnostics on a 27" 2015 iMac, and what's interesting is that while Apple doesn't mention memory pairing for the iMac on the memory upgrade support page, they do in the diagnostic software used during troubleshooting. It doesn't, however, indicate how to pair them correctly.

 

[FreemanW]

Just found this thread because i was running som diagnostics on a 27" 2015 iMac, and what's interesting is that while Apple doesn't mention memory pairing for the iMac on the memory upgrade support page, they do in the diagnostic software used during troubleshooting. It doesn't, however, indicate how to pair them correctly.

I would pair in alternating slots.

 

[RaXha]

I would pair in alternating slots.

Yea I tried that, no dice. When I first received it they were paired 8-8-4-4 and I changed it to 8-4-8-4 and also 4-8-4-8 with the same warninf being shown...

 

[FreemanW]

Do any of the memory modules work when individually installed? Do the pairs work?
Does this memory conundrum have as it's catalyst new memory? If yes? The new memory may be faulty.