2019 base 27" 3.0 Ghz iMac - 1x16GB stick vs 2x8GB sticks?

[helix21]

So I accidentally got 1x16GB stick Crucial RAM delivered instead of the usual 2x8GB sticks for my base model 3.0 Ghz 8-th Gen Core i5 2019 iMac. Couple of questions:

1. Does it matter between 1x16Gb and 2x8GB? I now total have 24GB RAM with 3 slots used (2X4GB as standard shipped, and 1x16GB). Apparently it can make a difference in gaming, and I intend on doing none. My plan for this machine is mostly photo and some video editing.

2. Apple curiously only lets you buy the base edition iMac 27" with max 32GB RAM, whereas all the other ones on the website can be configured to 64GB. Is this an actual limitation of the machine, or just apple thinking someone going for the base model doesn't ever need more than 32GB RAM?

My thinking is I might as well keep the 1x16GB stick of RAM, and should need be, add more RAM to the remaining RAM slot should I need it.

 

[mikehalloran]

In About This Mac, does it show all 24G? Most iMacs won't if there are only 3 sticks but some models will.

If yes, you're fine. Don't worry about it.

If it shows only 8GB, you're not and you either have to make the exchange or purchase another 16GB.

2. Apple curiously only lets you buy the base edition iMac 27" with max 32GB RAM, whereas all the other ones on the website can be configured to 64GB. Is this an actual limitation of the machine, or just apple thinking someone going for the base model doesn't ever need more than 32GB RAM?

Which iMac? The 2019 can be configured to 128GB per OWC. This was posted after the machines started shipping, BTW.

OWC shows all models of a given size and year and doesn't make an exception for the base model. Your speculation is likely to be correct.
https://eshop.macsales.com/shop/apple/memory/iMac

Apparently it can make a difference in gaming, and I intend on doing none. My plan for this machine is mostly photo and some video editing.

Photo and video editing are two uses that absolutely benefit from more RAM. Depends on the app and size of the files. You wouldn't be wrong to buy that other 16GB stick when the budget allows.

 

[casperes1996]

1. Does it matter between 1x16Gb and 2x8GB? I now total have 24GB RAM with 3 slots used (2X4GB as standard shipped, and 1x16GB). Apparently it can make a difference in gaming, and I intend on doing none. My plan for this machine is mostly photo and some video editing.

So, the CPUs here can run memory in Dual Channel. Which means they can access two sticks of "channels" of RAM at once. Very high end machines, like the Mac Pro has Quad Channel CPUs that can access 4 channels at once. Some of the older Mac Pros had tripple channel.
Something that's often done, is to split each channel up, so you can use more sticks. That's why there are four slots even though it's a dual channel system. By only occupying one or three slots, the CPU can't run the sticks in Dual Channel. It can only access the memory in single channel mode. Meaning that if you're accessing data from two sticks at once, instead of getting the data from them at the same time, you have to first get the data from one stick and then after that the data from the next. This can effectively half your RAM speed in certain circumstances.

The reason you're thinking it's important for gaming, is that GPUs require a lot of memory bandwidth, and especially for integrated GPUs that means faster system memory is crucial. But it's equally important for other tasks that need to access a lot of data from RAM.

Also, there should be no physical limitation in terms of the 32GB limit on Apple's site for the base.
Not from the CPU or chipset anyway. In terms of hardware it'd be more effort for Apple to block it than it would to allow it, so it's with very high probability just an issue of wanting users who demand a lot of RAM to move to a more expensive base. Which generally also makes sense for those users - If you're pushing that much memory, you'll probably want a CPU with more throghput for handling all that data.

 

[helix21]

In About This Mac, does it show all 24G? Most iMacs won't if there are only 3 sticks but some models will.

If yes, you're fine. Don't worry about it.

If it shows only 8GB, you're not and you either have to make the exchange or purchase another 16GB.
Which iMac? The 2019 can be configured to 128GB per OWC. This was posted after the machines started shipping, BTW.

OWC shows all models of a given size and year and doesn't make an exception for the base model. Your speculation is likely to be correct.
https://eshop.macsales.com/shop/apple/memory/iMac
Photo and video editing are two uses that absolutely benefit from more RAM. Depends on the app and size of the files. You wouldn't be wrong to buy that other 16GB stick when the budget allows.

Its showing 24 GB, and performing really well in Lightroom and Photos now compared to before. Absolutely no memory pressure. I can't see the value in putting another 16GB stick in there, as 40GB RAM is well above what I can fathom needing (at least right now...).

I think I am just going to forget about it now. Should I need more (with serious video editing needs), I will buy another 16GB stick. I suppose this way I always have the option to just upgrade.

 

[casperes1996]

Its showing 24 GB, and performing really well in Lightroom and Photos now compared to before. Absolutely no memory pressure. I can't see the value in putting another 16GB stick in there, as 40GB RAM is well above what I can fathom needing (at least right now...).

I think I am just going to forget about it now. Should I need more (with serious video editing needs), I will buy another 16GB stick. I suppose this way I always have the option to just upgrade.

It is of course all up to you, but personally, if you are within the return window, I'd return the 1x16 and get a 2x8 instead. Capacity is one thing, but it'd bother me that the theoretical peak bandwidth was so much lower. It loses you the capacity upgrade path, but as you say, you seriously doubt needing it.

 

[AlaskaMoose]

It is of course all up to you, but personally, if you are within the return window, I'd return the 1x16 and get a 2x8 instead. Capacity is one thing, but it'd bother me that the theoretical peak bandwidth was so much lower. It loses you the capacity upgrade path, but as you say, you seriously doubt needing it.

Although Apple recommends a limit of 32GB RAM for the 2019 27"" 5K 3.0GHz iMac, "machales.com" surpasses the Apple RAM limits. I could be wrong, but I wonder if the OP should leave the 1x16GB RAM stick alone, and just buy another 1x16GB stick of the same RAM for a total of 40GB on the 4 slots?
https://9to5mac.com/2019/04/05/how-to-upgrade-27-inch-5k-imac-2019-128-gb-ram-video-tutorial/

 

[Zdigital2015]

So I accidentally got 1x16GB stick Crucial RAM delivered instead of the usual 2x8GB sticks for my base model 3.0 Ghz 8-th Gen Core i5 2019 iMac. Couple of questions:

1. Does it matter between 1x16Gb and 2x8GB? I now total have 24GB RAM with 3 slots used (2X4GB as standard shipped, and 1x16GB). Apparently it can make a difference in gaming, and I intend on doing none. My plan for this machine is mostly photo and some video editing.

2. Apple curiously only lets you buy the base edition iMac 27" with max 32GB RAM, whereas all the other ones on the website can be configured to 64GB. Is this an actual limitation of the machine, or just apple thinking someone going for the base model doesn't ever need more than 32GB RAM?

My thinking is I might as well keep the 1x16GB stick of RAM, and should need be, add more RAM to the remaining RAM slot should I need it.

The CPU officially supports 128GB of DRAM -https://ark.intel.com/content/www/u...5-8500-processor-9m-cache-up-to-4-10-ghz.html - and so does the iMac, albeit unofficially.

I also agree that having both DIMM channels filled would be preferable to get the benefit of the fastest memory bandwidth in dual-channel mode, but to each his own.

 

[casperes1996]

Although Apple recommends a limit of 32GB RAM for the 2019 27"" 5K 3.0GHz iMac, "machales.com" surpasses the Apple RAM limits. I could be wrong, but I wonder if the OP should leave the 1x16GB RAM stick alone, and just buy another 1x16GB stick of the same RAM for a total of 40GB on the 4 slots?
https://9to5mac.com/2019/04/05/how-to-upgrade-27-inch-5k-imac-2019-128-gb-ram-video-tutorial/

The OP has stated that their use case really doesn't warrant that much memory, so I'd say it's overkill and money better spent elsewhere. But I'd personally still like dual-channel, so returning it for 2x8, at the same capacity and probably roughly the same cost but improved speed.

 

[helix21]

The OP has stated that their use case really doesn't warrant that much memory, so I'd say it's overkill and money better spent elsewhere. But I'd personally still like dual-channel, so returning it for 2x8, at the same capacity and probably roughly the same cost but improved speed.

Yes, I agree, this is what I reckon is the best method. But I'm trying to work out if the difference is worth it or not to go through the whole replacement process and refund.

 

[casperes1996]

Yes, I agree, this is what I reckon is the best method. But I'm trying to work out if the difference is worth it or not to go through the whole replacement process and refund.

Again, I can only speak to my personal opinion, but I'd say yes. If not for the speed performance, for the peace of mind.
If you don't do it, in 6 weeks time and further on in the future, there'll always be that voice in your head that tells you it could've performed better.

 

[Fishrrman]

OP:

If what you have works, well... just use it.

There will be some who say that using only one 16gb RAM DIMM instead of 2 8gb DIMMs will result in a "slowdown".

Technically, they're probably correct. The RAM may run a little faster in matched pairs.

However, the speed differences will almost certainly be un-perceivable in normal usage.

Again, just use it and be happy.

 

[casperes1996]

OP:

If what you have works, well... just use it.

There will be some who say that using only one 16gb RAM DIMM instead of 2 8gb DIMMs will result in a "slowdown".

Technically, they're probably correct. The RAM may run a little faster in matched pairs.

However, the speed differences will almost certainly be un-perceivable in normal usage.

Again, just use it and be happy.

I respectfully disagree.

Not because I think that the OP will really notice the CPU accessing memory in dual channel v. single channel mode 90% of the time. But because the OP will know it is the case, and even if it's an imperceptible difference, will subconsciously think "It could be running faster if I had bothered to go through the refund process and get the proper combination of sticks"

 

[trs0722]

Its showing 24 GB, and performing really well in Lightroom and Photos now compared to before. Absolutely no memory pressure. I can't see the value in putting another 16GB stick in there, as 40GB RAM is well above what I can fathom needing (at least right now...).

I think I am just going to forget about it now. Should I need more (with serious video editing needs), I will buy another 16GB stick. I suppose this way I always have the option to just upgrade.

Thank you all for this thread. I just got myself into this situation too today when my RAM upgrade arrived today and to while I thought it would be 2 x 8 it was 1 x 16. I'm not in a huge hurry so I think I'll return it for the 2 sticks. I can't foresee me needing 40GB by adding another 1 x 16 some day...and I will probably always overthink and beat myself up for potentially not getting full performance!

 

[GEmerson]

I understand the dual channel perspective but does it matter if the 2 RAM are the same size? I had the 2 standard 4GB slots filled but one failed so was going to replace with a crucial 16GB in the second slot, so 1x4GB and 1x16GB with 2 slots free. Thoughts?

 

[wilberforce]

I understand the dual channel perspective but does it matter if the 2 RAM are the same size? I had the 2 standard 4GB slots filled but one failed so was going to replace with a crucial 16GB in the second slot, so 1x4GB and 1x16GB with 2 slots free. Thoughts?

8GB (2x4GB) will run in dual channel mode, remaining 12GB will run in single channel mode (i.e., at the half the effective transfer speed). This is known as flex mode. You will have no control over which is used when.
You would be better off to get 2x8GB sticks and toss the 4GB stick.
Or get 3x4GB sticks to add to your 4GB stick

 

[GEmerson]

8GB (2x4GB) will run in dual channel mode, remaining 12GB will run in single channel mode (i.e., at the half the effective transfer speed). This is known as flex mode. You will have no control over which is used when.
You would be better off to get 2x8GB sticks and toss the 4GB stick.
Or get 3x4GB sticks to add to your 4GB stick

Okay thanks. If I put 2x8GB in slots 1&2, and put the 1x4Gb in slot 3 would that be better than tossing the 4GB?

 

[wilberforce]

Okay thanks. If I put 2x8GB in slots 1&2, and put the 1x4Gb in slot 3 would that be better than tossing the 4GB?

No that is just about the worst arrangement.
You need to match the amount of RAM in Channels A and B as closely as possible.
Channel A (aka Bank A) is slots 1 and 2
Channel B (aka Bank B) is slots 3 and 4

So best would be 8GB in each of slots 1 and 3.
If you really want to keep the 4GB, put it in slot 2. This will give you a little more memory which effectively runs at a slower speed.

 

[GEmerson]

No that is just about the worst arrangement.
You need to match the amount of RAM in Channels A and B as closely as possible.
Channel A (aka Bank A) is slots 1 and 2
Channel B (aka Bank B) is slots 3 and 4

So best would be 8GB in each of slots 1 and 3.
If you really want to keep the 4GB, put it in slot 2. This will give you a little more memory which effectively runs at a slower speed.

Thanks all new to me. Appreciate the explanation.

 

[GEmerson]

Thanks all new to me. Appreciate the explanation.

Sorry to ask another question-why do Apple put 4GBs into slots 1&2? Shouldn’t they use 1&3?

 

[wilberforce]

Sorry to ask another question-why do Apple put 4GBs into slots 1&2? Shouldn’t they use 1&3?

Good question. Yes, they should. Even Apple says "If your DIMMs are mixed in capacity, minimize the capacity difference between Channel A (slots 1 and 2) and Channel B (slots 3 and 4) when possible" here:

Install memory in an iMac - Apple Support

Get memory specifications and learn how to install memory in iMac computers.

support.apple.com

I can only think that someone changed the positions inadvertently?

btw, you can test it yourself. Run Novabench memory transfer speed test with both arrangements, and it should clearly show which is better.