Used iMac with weird memory config

[imrazor]

Wanting to grab a piece of history before they fade from memory (and the market) I bought a used 2017 iMac 27" 5K. It had a rather odd memory load out of 56GB. Ran Apple diagnostics at the seller's house, and everything checked out so I paid up and brought it home. About this Mac shows a rather odd memory config:

Apple's specs state that the 2017 iMac takes a maximum SODIMM capacity of 16GB, so that's one oddity. I'm also concerned that it may be running in single channel mode (i.e., non-interleaved.) Can anyone comment on either issue?

 

[wilberforce]

A firmware update enabled the 2017 iMac to support 128GB max instead of 64GB max.

Your memory will operate in "flex mode," where 4x8 = 32GB will operate in dual channel mode, and the remaining 24GB will operate in single channel mode. It is not possible to control which part of memory will be used by a particular app.

The impact on overall computer speed performance by operating in single channel mode is not huge (e.g., like 10% or 20% drop, not 50% drop)

Unless you use a lot of memory, personally I would replace the 32GB with 8GB, which is pretty cheap, to optimize the speed. If you use a lot of memory, stick with what you have (or upgrade all the slots), because single channel RAM is still much faster than swapping to disk.

 

[Fishrrman]

Does the iMac run ok?

No apparent "memory-related" problems? Performance seems fast enough?

If so, then I'd just USE IT, and not worry about such things.

RE the question about non-interleaved RAM...
I'm thinking that with the extra RAM capacity you have, the OS won't need to "hit the drive" much (VM disk swapping).
And, because of that, the performance will be "snappier", even WITHOUT "interleaved" RAM...

 

[imrazor]

Does the iMac run ok?

No apparent "memory-related" problems? Performance seems fast enough?

If so, then I'd just USE IT, and not worry about such things.

RE the question about non-interleaved RAM...
I'm thinking that with the extra RAM capacity you have, the OS won't need to "hit the drive" much (VM disk swapping).
And, because of that, the performance will be "snappier", even WITHOUT "interleaved" RAM...

One of the reasons I grabbed this particular iMac was that it included an 8GB RX 580 GPU. Theoretically, that should it make it usable for gaming, and hence I would like to get as much performance as possible out of it. And yes, slow RAM can cause reduced frame rates in games.

 

[wilberforce]

Hmm, wouldn't the gaming frame rate be more dependent on the GPU's 8GB VRAM, than the RAM? Could try taking out some RAM (say leaving 8GB in each of slots 1 and 3, for dual channel) and see if it makes much difference to the frame rate

 

[imrazor]

You're not wrong about that. But since the iMac is so old it's going to need every advantage it can get to run games in a playable manner. Thus the interest in making sure it runs as fast as possible, even if the difference is only 10% or so.

 

[theorist9]

Why not just test it on some of your games, as @wilberforce suggested, comparing your current config with one in which you have with 8 GB sticks in slots 1 and 3 (and with the other other two slots empty)? The latter would give you a dual-channel performance reference.

You could also do synthetic testing using Amorphous Memory Mark (free download from the App Store).

Memory replacement is pretty easy. Just clear a coffee table or kitchen table, and place a clean folded bath towel where you'll be laying down your iMac's screen.

 

[leifp]

You could replace one of the 8GB sticks with another 32GB stick. Make sure of two things:
1. That the speed of all of the sticks of RAM are identical - impact of ignoring this is reduced speed, otherwise nothing
2. That both 32GB sticks are in the matched sockets (you can check that through Apple or a galaxy of search results) - impact of ignoring this is not fixing your original concern… you end up adding more single channel memory access but do not improve speeds

 

[imrazor]

Sorry I haven't responded in a few days; it turns out that it is likely that the 2TB Fusion drive this came with is defective. I'm trying to sort that out before trying testing and experimenting with different memory configurations.

Towards that end I've broken the Fusion drive and installed MacOS to the SSD, which *seems* fine so far. Disk First Aid insists that both drives (the 120GB SSD and 2TB hard drive) are absolutely fine, but the hard drive was reporting hundreds of 'unallocated sectors' to Windows (under Boot Camp of course) via SMART which is very bad news. First Aid reports that SMART is disabled or not available - is that normal for a Fusion drive?

 

[Fishrrman]

OP:

If you don't have issues "managing" the two drives inside as a 128gb "boot SSD" AND a 2tb "extra-data" HDD, I suggest you just keep using it that way for now.

The 128gb SSD will run "at its best" as a stand-alone drive.

You WILL have to pay some attention as to how you manage your data.
I would endeavour to keep the SSD as "lean and clean" as I could.

It should contain
OS
Apps
"Basic" accounts
... and not much more.

By "basic" accounts, I mean that the large libraries that would normally be inside your home folder are moved to the HDD. They can exist there fine, and the slower speeds shouldn't affect them.

 

[imrazor]

https://www.youtube.com/watch?v=QpjvslNTUZc

The 128gb SSD will run "at its best" as a stand-alone drive.

This seems true enough, as booting off the Windows partition seems noticeably faster than when it was part of the Fusion drive. While the onboard SSD has no room for Bootcamp, I worked around this issue by installing a new install of OS X on the hard drive, then setting up Boot Camp. Unfortunately it seems impossible to set up Boot Camp on anything other than the startup drive, which seems...well, stupid.

Now that I've clawed my way back to where I was, the hard drive now seems fine. SMART reports zero problems; no pending sectors, no uncorrectable sectors, and *zero* reallocated sectors. This seems really strange; how did all those hundreds of bad sectors just disappear? I did zero out the disk before reformatting it by using Secure Erase, so maybe writing all those zeros forced the drive to reassess the 'bad' sectors. But I have trouble believing that out of hundreds of bad sectors, they were all suddenly just fine.

Since all that's over (maybe?) I'll move on to doing some tests with memory latency and gaming...

 

[TheSufle]

A firmware update enabled the 2017 iMac to support 128GB max instead of 64GB max.

Your memory will operate in "flex mode," where 4x8 = 32GB will operate in dual channel mode, and the remaining 24GB will operate in single channel mode. It is not possible to control which part of memory will be used by a particular app.

The impact on overall computer speed performance by operating in single channel mode is not huge (e.g., like 10% or 20% drop, not 50% drop)

Unless you use a lot of memory, personally I would replace the 32GB with 8GB, which is pretty cheap, to optimize the speed. If you use a lot of memory, stick with what you have (or upgrade all the slots), because single channel RAM is still much faster than swapping to disk.

iMac 27 2017 he can't use 128 GB

 

[imrazor]

@TheSufle Well, we know it will take at least one 32GB DIMM. Why not 4x32GB?

 

[wilberforce]

iMac 27 2017 he can't use 128 GB

2017, 27-inch Retina 5K iMac - 128GB Ram Upgrade

I have successfully upgraded my 2017, 27-inch Retina 5K iMac with 128GB of Samsung DD4 2666MHz SODIMM ram. (4) Samsung 32GB DDR4 2666MHz sodimm of ram (M471A4G43MB1-CTD) Model Name: iMac Model Identifier: iMac18,3 Processor Name: Intel Core i7 Processor Speed: 3.6 GHz Number of Processors: 1...

forums.macrumors.com

Quote: "128GB works in the 2017 as long as you have updated to at least 10.14.5. There was a firmware update that happened that then allowed 128GB. We have tested it here and it works well!"

 

[imrazor]

2017, 27-inch Retina 5K iMac - 128GB Ram Upgrade

I have successfully upgraded my 2017, 27-inch Retina 5K iMac with 128GB of Samsung DD4 2666MHz SODIMM ram. (4) Samsung 32GB DDR4 2666MHz sodimm of ram (M471A4G43MB1-CTD) Model Name: iMac Model Identifier: iMac18,3 Processor Name: Intel Core i7 Processor Speed: 3.6 GHz Number of Processors: 1...

forums.macrumors.com

Quote: "128GB works in the 2017 as long as you have updated to at least 10.14.5. There was a firmware update that happened that then allowed 128GB. We have tested it here and it works well!"

It's running Sonoma, so that should qualify. And the fact that it already recognizes one 32GB DIMM is also a positive sign.

 

[TheSufle]

I received the 128GB RAM today, installed and the Mac can see the 128GB RAM but cannot use it just as I suspected. Both Bootcamp (Win10) and the very latest Catalina update today does not allow more than 64GB to be used on this 2017 5k 27inch i7-7700k 4.2 turbo 4.5 Mac.

If you are wanting/thinking of doing this upgrade do not waste your time, effort or money (if not returnable). If someone says it does they are wrong. OWC did a 128GB install on a 2019 Mac and showed proof that not only did that Mac see but also could use the full 128GB installed. The Wired Memory info in Activity Monitor is the proof along with Atomic software RAM test.

My wired memory is 67.78GB. That is all the proof you need to see that your Mac cannot use all the Physical Memory installed. In my case 128GB. I can only use 64GB RAM not the Physical of 128GB which was installed. I knew going in that Kaby Lake i7-7700k shows only 64GB supported but others in this forum said it can use 128GB because it worked for them. There is no possible way it did work unless I'm missing something which is unlikely.

I would love for these to work, that is why I bought them but for others to say they tested extensively and works 100% is not true. I welcome anyone to show proof of this particular Mac to actually work with 128GB and not just show it can hold 128 GB. Like I said, I would keep them if in fact they worked but sadly they do not and are being returned.

Most likely I hear crickets when it comes to anyone showing proof they work. That is why OWC doesn't sell 128GB RAM upgrade for this model. If it could work they definitely would be selling it. I knew this going in but was willing to try installing them since others have said it worked 100% and they tested extensively. I hope I'm wrong but unlikely at this point.

See pics below.

 

[TheSufle]

you can use 3200mhz memory

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