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logas69

macrumors newbie
Original poster
Jul 5, 2010
16
8
Hello there,
I am planning to buy the iMac 5k with Radeon Pro 580. As you all know, it comes with two 4gb ram sticks. I would like to add 16gb more. I know it has 4 slots in total and I know that for the best performance it should go in couples. But I want to add another 16gb when I can afford it later this year.
So my question is, will it make much of a difference if I buy one 16gb stick instead of two 8gb sticks? So the total can be 4+4+16 for now instead of 4+4+8+8.
And in the future i will add a second 16gb stick.
Thank you in advance
 

SecuritySteve

macrumors demi-god
Jul 6, 2017
911
1,020
California
Hello there,
I am planning to buy the iMac 5k with Radeon Pro 580. As you all know, it comes with two 4gb ram sticks. I would like to add 16gb more. I know it has 4 slots in total and I know that for the best performance it should go in couples. But I want to add another 16gb when I can afford it later this year.
So my question is, will it make much of a difference if I buy one 16gb stick instead of two 8gb sticks? So the total can be 4+4+16 for now instead of 4+4+8+8.
And in the future i will add a second 16gb stick.
Thank you in advance
Mixing RAM sticks of different sizes is a bad idea. The sticks will either not work, or they will be limited in size to the size of the smallest in the slot. You will need to replace to two sticks of 4 if you want to exceed 16 GB RAM.
 
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Mike Boreham

macrumors 68020
Aug 10, 2006
2,489
787
UK
Mixing RAM sticks of different sizes is a bad idea. The sticks will either not work, or they will be limited in size to the size of the smallest in the slot. You will need to replace to two sticks of 4 if you want to exceed 16 GB RAM.

Incorrect.

Read this thread about RAM for the 2017 iMac 5K

https://forums.macrumors.com/threads/what-ram-to-buy-to-update-new-imac-launched-today.2048694/

Lots of people have added 2 x 8GB to the Apple 2 x 4GB to get 24GB with no problem. The only exception seems to be one the Kingston types which downclock the RAM frequency from 2400GHZ if mixed with the Apple RAM, but operates at 2400 if all are Kingston. Its all in that thread.
 

SecuritySteve

macrumors demi-god
Jul 6, 2017
911
1,020
California
Incorrect.

Read this thread about RAM for the 2017 iMac 5K

https://forums.macrumors.com/threads/what-ram-to-buy-to-update-new-imac-launched-today.2048694/

Lots of people have added 2 x 8GB to the Apple 2 x 4GB to get 24GB with no problem. The only exception seems to be one the Kingston types which downclock the RAM frequency from 2400GHZ if mixed with the Apple RAM, but operates at 2400 if all are Kingston. Its all in that thread.
Huh, I stand corrected. I was only speaking from my own experience. I've never had success mixing RAM from different manufacturers / sizes.
 

logas69

macrumors newbie
Original poster
Jul 5, 2010
16
8
Incorrect.

Read this thread about RAM for the 2017 iMac 5K

https://forums.macrumors.com/threads/what-ram-to-buy-to-update-new-imac-launched-today.2048694/

Lots of people have added 2 x 8GB to the Apple 2 x 4GB to get 24GB with no problem. The only exception seems to be one the Kingston types which downclock the RAM frequency from 2400GHZ if mixed with the Apple RAM, but operates at 2400 if all are Kingston. Its all in that thread.

I understand about the couples. 4+4 and 8+8.
I am asking if there will be any performance issue if the placement is 4+4 and a single 16gb stick instead of 8+8 until I buy a second 16gb. I do not want the originals to go wasted.
 

Phil77354

Contributor
Jun 22, 2014
1,117
965
Pacific Northwest, U.S.
I understand about the couples. 4+4 and 8+8.
I am asking if there will be any performance issue if the placement is 4+4 and a single 16gb stick instead of 8+8 until I buy a second 16gb. I do not want the originals to go wasted.

As long as the added RAM satisfies the specifications for your model, see:

Install memory in an iMac: https://support.apple.com/en-us/HT201191

I do not see anything in these instructions that suggest your plan will have any difficulty.
 

Mike Boreham

macrumors 68020
Aug 10, 2006
2,489
787
UK
Yes, I have already checked that and it didn’t specify my question. That’s why I wanted a second opinion. Thank you!

If you go on Crucial.com and follow through to iMac 18,3 compatible RAM you get the attached screenshot, which shows a single stick of 16GB as a compatible option. I think you will be fine, and if you buy it from Crucial you can send it back if it isnt.
 

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logas69

macrumors newbie
Original poster
Jul 5, 2010
16
8
A friend will try it in a couple of days on his machine and we’ll know then.
Thank you all!
 

iamchrisstone

macrumors newbie
Feb 18, 2018
25
3
I know this doesn't answer your question, but I ordered mine with the minimum amount of ram and at the same time ordered 64GB from Crucial. I'm glad I did. I should never have to upgrade until I upgrade my entire machine.
 

logas69

macrumors newbie
Original poster
Jul 5, 2010
16
8
I know this doesn't answer your question, but I ordered mine with the minimum amount of ram and at the same time ordered 64GB from Crucial. I'm glad I did. I should never have to upgrade until I upgrade my entire machine.

That is exactly why I want to buy now one 16gb stick instead of two 8gb. So I can add one by one whenever I can afford it until I reach 64gb in the end.
 
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theluggage

macrumors 603
Jul 29, 2011
5,450
4,562
That is exactly why I want to buy now one 16gb stick instead of two 8gb. So I can add one by one whenever I can afford it until I reach 64gb in the end.

Unfortunately, there is a complete lack of convincing/reliable information on the best way to install RAM in an iMac.

I'm not saying it won't work, but unless someone convincingly posts here saying that they have 2x4G and 1x16G installed and working, I'd avoid it. You certainly won't get dual-channel mode with a single stick, though. However, it looks like the performance impact of not having dual-channel is fairly small.

Plenty of people (including myself) have (e.g.) 2x4gb and 2x8gb installed and working fine. However, whether we're getting dual-channel is anybody's guess unless someone can come up with a way to discover if dual-channel is enabled.

The problem is whether you put the matching pair of modules in the same bank (either Bank 0 or 1) or stagger them across two banks (e.g. one in Bank0/DIMM0 and the matching one in Bank1/DIMM0). I've got mine staggered, and am not noticing any problems.

They do have to be on different channels but is "Bank0/1"=="Channel 0/1" or is "DIMM0/1"=="Channel0/1"?

...unfortunately, I've seen authoritative-sounding arguments for both arrangements so, with all due respect folks, don't bother replying unless you're citing Apple or some more authoritative source than a blog/wiki/forums.

Complicating issue is that my iMac came with the two 4GB sticks in different banks and one would assume that Apple put them in the correct place for dual-channel.
 

logas69

macrumors newbie
Original poster
Jul 5, 2010
16
8
Unfortunately, there is a complete lack of convincing/reliable information on the best way to install RAM in an iMac.

I'm not saying it won't work, but unless someone convincingly posts here saying that they have 2x4G and 1x16G installed and working, I'd avoid it. You certainly won't get dual-channel mode with a single stick, though. However, it looks like the performance impact of not having dual-channel is fairly small.

Plenty of people (including myself) have (e.g.) 2x4gb and 2x8gb installed and working fine. However, whether we're getting dual-channel is anybody's guess unless someone can come up with a way to discover if dual-channel is enabled.

The problem is whether you put the matching pair of modules in the same bank (either Bank 0 or 1) or stagger them across two banks (e.g. one in Bank0/DIMM0 and the matching one in Bank1/DIMM0). I've got mine staggered, and am not noticing any problems.

They do have to be on different channels but is "Bank0/1"=="Channel 0/1" or is "DIMM0/1"=="Channel0/1"?

...unfortunately, I've seen authoritative-sounding arguments for both arrangements so, with all due respect folks, don't bother replying unless you're citing Apple or some more authoritative source than a blog/wiki/forums.

Complicating issue is that my iMac came with the two 4GB sticks in different banks and one would assume that Apple put them in the correct place for dual-channel.

It is usually slot 1 & 3 occupied and then slot 2 & 4 free in all motherboards
 

Phil77354

Contributor
Jun 22, 2014
1,117
965
Pacific Northwest, U.S.
Unfortunately, there is a complete lack of convincing/reliable information on the best way to install RAM in an iMac.

I'm not saying it won't work, but unless someone convincingly posts here saying that they have 2x4G and 1x16G installed and working, I'd avoid it. You certainly won't get dual-channel mode with a single stick, though. However, it looks like the performance impact of not having dual-channel is fairly small.

Plenty of people (including myself) have (e.g.) 2x4gb and 2x8gb installed and working fine. However, whether we're getting dual-channel is anybody's guess unless someone can come up with a way to discover if dual-channel is enabled.

The problem is whether you put the matching pair of modules in the same bank (either Bank 0 or 1) or stagger them across two banks (e.g. one in Bank0/DIMM0 and the matching one in Bank1/DIMM0). I've got mine staggered, and am not noticing any problems.

They do have to be on different channels but is "Bank0/1"=="Channel 0/1" or is "DIMM0/1"=="Channel0/1"?

...unfortunately, I've seen authoritative-sounding arguments for both arrangements so, with all due respect folks, don't bother replying unless you're citing Apple or some more authoritative source than a blog/wiki/forums.

Complicating issue is that my iMac came with the two 4GB sticks in different banks and one would assume that Apple put them in the correct place for dual-channel.

It sounds like you're knowledgeable on this. For most people it would seem sufficient that we simply follow Apple's instructions which are considerably less complicated. The Apple information previously cited does not get into differences in performance that might result from putting different capacity RAM modules in our computers, as long as they satisfy the specs that are stated. It doesn't surprise me that there is more to it, although if it was going to make a significant difference in performance, I would think that Apple would make their specs sufficiently detailed to help customers do it correctly. After all, they make it easy for the customer to do their own RAM installation, and give clear instructions on how to do it, so it is not like this is something that they expect you to take your computer to the shop for.
 

wardie

macrumors 6502a
Aug 18, 2008
538
173
Why would Crucial offer the single stick of 16GB as a compatible option if it wasn't?

Maybe because people may need individual replacements / spares? I can’t comment on the technical spec in detail but my iMac came with 1 & 3 occupied and 2 & 4 spare. No rational reason why Apple would not put them in the fastest configuration.
 

theluggage

macrumors 603
Jul 29, 2011
5,450
4,562
For most people it would seem sufficient that we simply follow Apple's instructions which are considerably less complicated.

Unfortunately, there's a difference between "less complicated" and "dumbed down" and supplying memory fitting instructions without saying if/how DIMMs should be paired is definitely "dumbed down". Its not too hard to say "if you have two RAM modules of the same size put them in adjacent/alternate slots" or "you do/do not/optionally need to install RAM in matching pairs" which would solve all the mysteries.

They do warn us not to burn our poor fingers on the hot RAM chips though, which is nice of them.
 

SaSaSushi

macrumors 601
Aug 8, 2007
4,147
533
Takamatsu, Japan
I'm not saying it won't work, but unless someone convincingly posts here saying that they have 2x4G and 1x16G installed and working, I'd avoid it. You certainly won't get dual-channel mode with a single stick, though. However, it looks like the performance impact of not having dual-channel is fairly small.

I am running 40GB (2x4GB and 2x16GB) and in testing have successfully booted the machine with 24GB (2x8GB and 1x16GB)

They do have to be on different channels but is "Bank0/1"=="Channel 0/1" or is "DIMM0/1"=="Channel0/1"?

You and I have had this discussion before and I informed you last time that Bank 0 and Bank 1 should have matching RAM for each DIMM channel in the iMac.

With mismatched RAM in DIMM0 and DIMM1 my Geekbench scores were 4000 points lower.

...unfortunately, I've seen authoritative-sounding arguments for both arrangements so, with all due respect folks, don't bother replying unless you're citing Apple or some more authoritative source than a blog/wiki/forums.

See above. The benchmarks clearly show the proper configuration.

Complicating issue is that my iMac came with the two 4GB sticks in different banks and one would assume that Apple put them in the correct place for dual-channel.

That's something else I pointed out the last time we discussed this: Apple ships the RAM in the proper configuration. Why wouldn't they?!
[doublepost=1519000223][/doublepost]Just to clarify, once again:

The correct RAM configuration is to put matching DIMMs in DIMM0 and DIMM1 exactly as Apple ships it. It will run with mismatched RAM in DIMM0 and DIMM1 but not in dual channel and at a considerable loss in performance.

Always put matching RAM in alternating slots in the machine for best performance.
 
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logas69

macrumors newbie
Original poster
Jul 5, 2010
16
8
I am running 40GB (2x4GB and 2x16GB) and in testing have successfully booted the machine with 24GB (2x8GB and 1x16GB)



You and I have had this discussion before and I informed you last time that Bank 0 and Bank 1 should have matching RAM for each DIMM channel in the iMac.

With mismatched RAM in DIMM0 and DIMM1 my Geekbench scores were 4000 points lower.



See above. The benchmarks clearly show the proper configuration.



That's something else I pointed out the last time we discussed this: Apple ships the RAM in the proper configuration. Why wouldn't they?!
[doublepost=1519000223][/doublepost]Just to clarify, once again:

The correct RAM configuration is to put matching DIMMs in DIMM0 and DIMM1 exactly as Apple ships it. It will run with mismatched RAM in DIMM0 and DIMM1 but not in dual channel and at a considerable loss in performance.

Always put matching RAM in alternating slots in the machine for best performance.

Perfect, thank you for the experiment!
Case closed.
 
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SaSaSushi

macrumors 601
Aug 8, 2007
4,147
533
Takamatsu, Japan
I forgot to mention one more fact proving that the way Apple ships their RAM with matching modules in DIMM0 and DIMM1 is correct which I did mention in passing in the thread linked to above:

In BootCamp Windows with RAM in the proper configuration CPU-Z shows dual channel mode active but not with it mismatched.
 
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theluggage

macrumors 603
Jul 29, 2011
5,450
4,562
I am running 40GB (2x4GB and 2x16GB) and in testing have successfully booted the machine with 24GB (2x8GB and 1x16GB)

That's useful to know (my comment about not believing it until someone confirms it works was meant literally - lots of people speculating on why it should work).

You and I have had this discussion before and I informed you last time that Bank 0 and Bank 1 should have matching RAM for each DIMM channel in the iMac.

Yup, and I'm sure that you're right (I admit that I'd forgotten the bit about bootcamp indicating dual channel mode which, together with the fact that Apple ships it that way pretty much clinches it - benchmarks, not so much, unless you swapped a couple of times and did repeated tests with each configuration to eliminate any other factors). But with so much misinformation out there its really frustrating that there's no documentation from the horse's mouth confirming it - something you'd easily find in the manual for any PC motherboard - that could be used to definitively shoot down the incorrect advice.

For entertainment here's the moderator on the Crucial forum apparently getting it wrong and here is Wikipedia getting into a knot over "banks" and "channels". However, I've just followed up the reference Wikipedia cite:

https://web.archive.org/web/20110929024052/http://www.kingston.com/newtech/MKF_520DDRwhitepaper.pdf - see page 8.

"For the best dual-channel memory performance on motherboards with the Intel dual- channel DDR chipsets, you must use identically paired memory modules in DIMM sockets 0 of channel A and B. Identically paired memory modules must also be used when populating DIMM sockets 1 of channel A and B. One can, for example, plug in matching 256MB DIMMs in both DIMM 0 slots, and plug in matching 512MB DIMMs in both DIMM 1 slots."

So, finally, an (albeit elderly and buried in the wayback machine) article written by Infineon (a bone fide RAM manufacturer) that actually makes sense... a clear description without turbid statements such as "install matching DIMMs in matching banks" backed up by an example to remove any doubt. Hurrah!

(See what I mean about needing references when there is uncertainty? The wikipedia article is... unhelpful, but the reference clears it all up).
 

Phil77354

Contributor
Jun 22, 2014
1,117
965
Pacific Northwest, U.S.
This discussion is piquing my interest and I'm wondering if I can upgrade the RAM in my iMac beyond what Apple originally offered as maximum capacity for this model.

I have an iMac (Retina 5K, 27-inch, Late 2014), with processor 3.5 GHz Intel Core i5 and currently total memory of 16 GB 1600 MHz DDR3 (2 x 8GB memory cards).

Configuration in my iMac is currently:

BANK 0/DIMM0 8 GB DDR3 1600 MHz
BANK 1/DIMM0 8 GB DDR3 1600 MHz
BANK 0/DIMM1 Empty
BANK 1/DIMM1 Empty

The Crucial website gives me this recommendation:

- 8 GB DDR3L-1600 SODIMM RAM
- Crucial part number: CT6313732
- Specs: DDR3 PC3-12800 • CL=11 • Unbuffered • NON-ECC • DDR3-1600 • 1.35V • 1024Meg x 64 • lead free • halogen free • for Mac.

I can get two of those 8 GB RAM sticks and upgrade my iMac to a total of 32 GB RAM, and there is no doubt it will perform correctly since I'll have essentially the same spec 8 GB RAM sticks in all four slots.

However Crucial also offers 16 GB RAM cards with compatible specs (to be clear, Crucial is not suggesting this for my model, I found this RAM by making a more general search of their site to see what various cards were available):

- 32 GB (2 x 16 GB) DDR3L-1600 SODIMM RAM
- Crucial part number: CT2KIT204864BF160B
- Specs: DDR3 PC3-12800 • CL=11 • Unbuffered • NON-ECC • DDR3-1600 • 1.35V • 2048Meg x 64 •

Question: Could I install this 32 GB kit in my iMac, into the two empty slots as identified above, and therefore upgrade my total memory to 48 GB RAM??

This is beyond what Apple says is the maximum memory for this model, which is 32 GB total.

(Similarly, could someone theoretically install 4 x 16 GB into this model iMac for a total of 64 GB RAM?)
 
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