2019 iMac and DDR4 2400 memory

Lethcoeb

macrumors member
Original poster
Jun 18, 2011
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Houston
Question for the group - I happen to have 4x16GB DDR 4 2400 sticks and I have a 2019 27” iMac on the way. I do not expect a problem, but any one have experience with using slower memory in newer iMacs?
Thanks!
 

Lethcoeb

macrumors member
Original poster
Jun 18, 2011
34
8
Houston
Let me rephrase - not specifically the 2019 iMac, but any newer Mac with replaceable memory, any issue with using slower memory with new Macs? I’ll post my experience when mine is received.
 

Lethcoeb

macrumors member
Original poster
Jun 18, 2011
34
8
Houston
Apart from hobbling them with the slower memory ~ no.

But why on earth would you do this?
I happen to have relatively new sticks (~ 2 months old), and I'm curious if 1) there's going to be an issue and 2) if the slightly slower speed really makes that much of a difference.
 
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Lethcoeb

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Original poster
Jun 18, 2011
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I understand I can sell and get new sticks. I will probably do this. I'm just asking if anyone has used "slower" memory with newer Macs. That's all.
 

Rockadile

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Jun 11, 2012
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You won't tell the difference between 2400 and 2666 RAM. Just use it and avoid the selling hassle unless you're doing multiple VM, coding, and such. For gaming, around 5% average FPS loss.
 

Lethcoeb

macrumors member
Original poster
Jun 18, 2011
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8
Houston
You won't tell the difference between 2400 and 2666 RAM. Just use it and avoid the selling hassle unless you're doing multiple VM, coding, and such. For gaming, around 5% average FPS loss.
Thanks - no gaming, standard office apps (and one VM). I figure replacing 8GB of 2666 with 64GB of 2400 would have, at worst, a minor performance hit, but I would think that going from 8 to 64GB would overcome it. I’ll upgrade to 2666 later when RAM prices come down, but I figure it’d be a waste to not try with the relatively new 2400 I have. Not sure I’d get a decent price for my 2400 if I try to sell it.
 

Rockadile

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Jun 11, 2012
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Thanks - no gaming, standard office apps (and one VM). I figure replacing 8GB of 2666 with 64GB of 2400 would have, at worst, a minor performance hit, but I would think that going from 8 to 64GB would overcome it. I’ll upgrade to 2666 later when RAM prices come down, but I figure it’d be a waste to not try with the relatively new 2400 I have. Not sure I’d get a decent price for my 2400 if I try to sell it.
Yes, you'll do fine.

I'm seeing some people here planning to get 8GB 2666 iMac and buy 2x16GB for 40GB total.
It won't utilize quad-channel and with mismatched RAM sticks, I reckon that would be a bigger performance hit than 4x16 2400.
 
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Lethcoeb

macrumors member
Original poster
Jun 18, 2011
34
8
Houston
T
Yes, you'll do fine.

I'm seeing some people here planning to get 8GB 2666 iMac and buy 2x16GB for 40GB total.
It won't utilize quad-channel and with mismatched RAM sticks, I reckon that would be a bigger performance hit than 4x16 2400.
Thanks for the vote of confidence. I’ll report back in a couple of weeks how things turn out!
 

macduke

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Jun 27, 2007
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Yes, you'll do fine.

I'm seeing some people here planning to get 8GB 2666 iMac and buy 2x16GB for 40GB total.
It won't utilize quad-channel and with mismatched RAM sticks, I reckon that would be a bigger performance hit than 4x16 2400.
Is that a thing? I’ve always heard it was matched sets of two for dual channel. So as long as I have 16x2 and 4x2 in the proper slots it should be full speed. They usually ship many configs with two sticks, I thought, for this reason.

Edit: Ok I went and searched and couldn’t find anything right away, but Apple doesn’t mention it on the iMac page but goes out of their way to mention it on the iMac Pro page. I also saw a tear down of the iMac Pro that was talking about it like it was a big deal. So I doubt the regular iMac is quad channel, but I’d love for anyone to prove me wrong as I’m for extra performance even if it means a loss of 8GB for a few years until I add another 32GB.
 
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Rockadile

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Jun 11, 2012
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Is that a thing? I’ve always heard it was matched sets of two for dual channel. So as long as I have 16x2 and 4x2 in the proper slots it should be full speed. They usually ship many configs with two sticks, I thought, for this reason.

Edit: Ok I went and searched and couldn’t find anything right away, but Apple doesn’t mention it on the iMac page but goes out of their way to mention it on the iMac Pro page. I also saw a tear down of the iMac Pro that was talking about it like it was a big deal. So I doubt the regular iMac is quad channel, but I’d love for anyone to prove me wrong as I’m for extra performance even if it means a loss of 8GB for a few years until I add another 32GB.
I meant to say dual-channel. Had a brain fart moment after reading about the iMac and iMac Pro for the past few days.

Don't know if a 40GB setup would go down to single channel. The 8GB iMacs come with two, 4GB? I assumed it was one, 8GB.
 

macduke

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I meant to say dual-channel. Had a brain fart moment after reading about the iMac and iMac Pro for the past few days.

Don't know if a 40GB setup would go down to single channel. The 8GB iMacs come with two, 4GB? I assumed it was one, 8GB.
Yeah from searching past machines they did that and everymac.com was reporting that is standard for 2019, likely for dual-channel reasons. From what I’ve read, the slots each have a matched pair, so as long as the two match for each pair, it will be full speed dual channel combined. At least people have done that successfully in the past, according to my searches.
 
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Rockadile

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Jun 11, 2012
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Yeah from searching past machines they did that and everymac.com was reporting that is standard for 2019, likely for dual-channel reasons. From what I’ve read, the slots each have a matched pair, so as long as the two match for each pair, it will be full speed dual channel combined. At least people have done that successfully in the past, according to my searches.
I thought 4GB RAM sticks were obsolete these days like all my old 4GB memory cards and USB drives so I assumed 1x8GB. If it's 2x4GB than you've been set all along. I would match the specs down to its CL latency to mitigate any problems.
 

kaintxu

macrumors regular
Jul 9, 2018
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Edinburgh
Why if you had 1 8GB on one of the slots and adding the another 2 16GB for a total of 40 would be worse than having 2x 4GB and adding the same?
 

Rockadile

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Jun 11, 2012
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Why if you had 1 8GB on one of the slots and adding the another 2 16GB for a total of 40 would be worse than having 2x 4GB and adding the same?
What's certain is that 2x16GB and 2x4GB setup will run in dual-channel.
I don't know if the one, 8GB RAM stick would downgrade the system to single-channel.
There has to be at least one RAM stick in channel A slot and one in channel B slot to run dual-channel.

I suppose I'll try a simplified, road analogy. Someone correct me if I'm wrong :oops:

Single-channel = one lane road
Dual-channel = two-lane road
Quad-channel = four-lane road

Channel A & B = north and south direction

RAM memory size = width of the road

RAM speed = speed of vehicles

RAM latency = the amount of time vehicles have to wait before getting on the road
 

mikehalloran

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Oct 14, 2018
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Why if you had 1 8GB on one of the slots and adding the another 2 16GB for a total of 40 would be worse than having 2x 4GB and adding the same?
Your iMac might only recognize the matched pair. Some won’t boot up at all giving the high pitched sound of incompatible RAM. It depends on the iMac.

2 + 2 matched pairs works fine and is how Apple specifies. Many iMacs ship with only one matched pair. Don’t know if the 8GB version of the 2019 iMac ships that way but the 2017s do.

FWIW, the highest performance in a Mac Pro 6.1 is achieved with matched trios of RAM, 48 or 96GB. I was surprised to learn this.
 

kaintxu

macrumors regular
Jul 9, 2018
167
73
Edinburgh
What's certain is that 2x16GB and 2x4GB setup will run in dual-channel.
I don't know if the one, 8GB RAM stick would downgrade the system to single-channel.
There has to be at least one RAM stick in channel A slot and one in channel B slot to run dual-channel.

I suppose I'll try a simplified, road analogy. Someone correct me if I'm wrong :oops:

Single-channel = one lane road
Dual-channel = two-lane road
Quad-channel = four-lane road

Channel A & B = north and south direction

RAM memory size = width of the road

RAM speed = speed of vehicles

RAM latency = the amount of time vehicles have to wait before getting on the road

So if you match teh original 2x 4GB and add 2x 16 GB, for a total of 40, will they be matches 4+4 and 16+16, or would it do a 4+16 and 4+16?

Also, do you guys think 20GB would be enough to keeps costs down and just get 2x 8GB sticks?
 

Rockadile

macrumors 6502
Jun 11, 2012
458
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So if you match teh original 2x 4GB and add 2x 16 GB, for a total of 40, will they be matches 4+4 and 16+16, or would it do a 4+16 and 4+16?

Also, do you guys think 20GB would be enough to keeps costs down and just get 2x 8GB sticks?
I don't understand what you mean but put the RAM sticks as 16-4-16-4 instead of 16-16-4-4. Just don't put the same size sticks next to each other unless all are the same size.

20GB will be plenty for most folks here.
If you have multiple VM or do demanding photo/video edits than you'll need more than 20GB. There's also those people with over 100 internet browser tabs open :confused:
 
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kaintxu

macrumors regular
Jul 9, 2018
167
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Edinburgh
I don't understand what you mean but put the RAM sticks as 16-4-16-4 instead of 16-16-4-4. Just don't put the same size sticks next to each other unless all are the same size.

20GB will be plenty for most folks here.
If you have multiple VM or do demanding photo/video edits than you'll need more than 20GB. There's also those people with over 100 internet browser tabs open :confused:
Yes, but what does 16-4-16-4 mean, the tiny space where you stick them in, they are all next to each other.

Quick question, when you are trying to use the dual channel and you pair them up 16-4-16-4 those that mean that each 4 goes with a 16 to make 20 per channel or that the 2 16's use one channel and the 2 4's use the other channel?

Thanks
 

Chancha

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Mar 19, 2014
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The confusion comes from this: the 4 slots are arranged such that (from top to bottom) slot 1 & slot 3 is wired as one pair, slot 2 & slot 4 is wired as another pair.

And then normally the "best" practice is to match RAMs with identical specs as one pair for dual channel.

So for anyone who wishes to keep the stock 4+4GB, then add your own 3rd party 16+16GB, you want to arrange the sticks from top to bottom 16-4-16-4 or 4-16-4-16 (doesn't matter which way).
 

jeboles

macrumors newbie
Dec 7, 2011
29
12
What's certain is that 2x16GB and 2x4GB setup will run in dual-channel.
I don't know if the one, 8GB RAM stick would downgrade the system to single-channel.
There has to be at least one RAM stick in channel A slot and one in channel B slot to run dual-channel.

I suppose I'll try a simplified, road analogy. Someone correct me if I'm wrong :oops:

Single-channel = one lane road
Dual-channel = two-lane road
Quad-channel = four-lane road

Channel A & B = north and south direction

RAM memory size = width of the road

RAM speed = speed of vehicles

RAM latency = the amount of time vehicles have to wait before getting on the road
Best analogy I have seen in a long time. Different RAM sizes in modern Intel motherboards still run in Dual Channel using Flex Memory Mode, making matching sticks a thing of the past. On topic, 2400 MHz RAM will work as the processor determines support. Usually more RAM is better than slightly faster RAM.
 
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kaintxu

macrumors regular
Jul 9, 2018
167
73
Edinburgh
The confusion comes from this: the 4 slots are arranged such that (from top to bottom) slot 1 & slot 3 is wired as one pair, slot 2 & slot 4 is wired as another pair.

And then normally the "best" practice is to match RAMs with identical specs as one pair for dual channel.

So for anyone who wishes to keep the stock 4+4GB, then add your own 3rd party 16+16GB, you want to arrange the sticks from top to bottom 16-4-16-4 or 4-16-4-16 (doesn't matter which way).
First of all thanks. My last RAM upgrade was on the 2011 model, which had the RAM socket under the chin and were arranged in a "square" (2x2) which is why 16-4-16-4 did not make sense. Now I know on the newer ones they are top to bottom, and the stock 8 GB are on sockets 2 and 4, so it's just a case on inserting the new ones on 1 and 3.

My last doubt is that if we stick them like that, are we not working with 32 GB on one channel and just 8 on the other? I know about matching sticks, but would it not be better to have the same amount of memory per channel?

I don't understand what you mean but put the RAM sticks as 16-4-16-4 instead of 16-16-4-4. Just don't put the same size sticks next to each other unless all are the same size.

20GB will be plenty for most folks here.
If you have multiple VM or do demanding photo/video edits than you'll need more than 20GB. There's also those people with over 100 internet browser tabs open :confused:
Thanks for this too and it is now all clear, really helpfull.

Do browser tabs use that much RAM? I don't have 100 open, but I Can easily have 6-8 at a time depending what I'm doing
 

Rockadile

macrumors 6502
Jun 11, 2012
458
178
First of all thanks. My last RAM upgrade was on the 2011 model, which had the RAM socket under the chin and were arranged in a "square" (2x2) which is why 16-4-16-4 did not make sense. Now I know on the newer ones they are top to bottom, and the stock 8 GB are on sockets 2 and 4, so it's just a case on inserting the new ones on 1 and 3.

My last doubt is that if we stick them like that, are we not working with 32 GB on one channel and just 8 on the other? I know about matching sticks, but would it not be better to have the same amount of memory per channel?



Thanks for this too and it is now all clear, really helpfull.

Do browser tabs use that much RAM? I don't have 100 open, but I Can easily have 6-8 at a time depending what I'm doing
I'll try continuing with the analogy again.
Since the slots are wired as channel A-A-B-B. It will make sense to keep the width of the road the same for each direction so we put it as 16-4-16-4.
If Mac logic boards can also support Flex Memory Mode as jeboles mentioned than this way of thinking would seem obsolete.

It looks like you'll do fine with 20GB.