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iMac memory configuration

D

Dr. McKay

macrumors 6502a
Original poster
Jan 20, 2010
821
112
Belgium, Europe
I'll be buying a 3.4Ghz 27 inch iMac with a 512Gb SSD. The base memory is 8Gb, occupying two of the four slots with 2 4Gb DIMMS.
I would go for 16gb, but how does Apple configure the memory. Will I have 4 x 4Gb or 2 x 8Gb ? Also, does having only two slots occupied have an influence on memory speed (dual channel memory configuration) ? In other words, which is faster, 4 x 4Gb or 2 x 8 Gb, or does it make no difference at all ?
 
No difference at all as long as memory is installed in pairs of two. Only the Xeon line of processors supports quad-channel memory which would actually make a difference.

Get the base 8 GB from Apple and purchase the RAM yourself. For the money that Apple charges for another 8GB you can get 16GB anywhere else, for Apple's 16GB you can usually get 32GB anywhere else, etc.
 
when i bought mine, i got a great deal on a referb 16gb that came with 2x8gb sticks installed. I ended up moving the original Apple memory over to my mac mini and put in 32gb ram that I bought. if it wasn't for the great deal i would have gotten 8gb and pulled it all to repalce with 32.

As for those who say you don't get any benefit from extra ram. I have watched my Activity monitor and it seems to use just about all my ram and it's not that I am doing all that much intensive stuff. So the iMac will use whatever you have.
 
So if I bought a 16Gb RAM kit from, say, Crucial, with 2 4Gb DIMMS (or even 2 8Gb DIMMS, cheaper than the upgrade from 8 to 16 from Apple), I can combine that without problems with the already installed 2x4gb DIMMs, even though it wouldn't be the exact same RAM (same specs, different brand, maybe different CAS latencies...).

I'm asking that question because apparently, getting third party RAM can be a bit of hit & miss sometimes...
 
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Yes you can.

But the one thing that I like to do is actually test the ram before you install it. Get something like memtest86 booting from a USB. For the best and only reliable way to test memory is to do it single stick of memory at a time in the first slot. (ie you remove the ram that Apple installed and just test a single stick of the newly bought one). This way any memory errors that will be hidden by dual channel writing will be found. I have had in the past memory that fails single stick but past fine if done in pairs.
 
The 2017 27" iMac has four slots and ordering 16GB fills two of them, so that's the answer to your first question. I ordered my iMac that way. It turned out that 16GB is plenty for me so far, but it's nice to have the option to add memory down the road.

Here's a clip from the Apple site on compatible memory:

Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:
  • PC4-2400 (19200)
  • Unbuffered
  • Nonparity
  • 260-pin
  • 2400MHz DDR4 SDRAM
source: https://support.apple.com/en-us/HT201191#27inch2017​

The factory installed DIMMS in my machine are installed as follows:

Bank 0/DIMM0 8 GB
Bank 0/DIMM1 Empty
Bank 1/DIMM0 8 GB
Bank 1/DIMM1 Empty

Most discussion elsewhere assumes that modules are added in pairs and that iMacs use dual-channel memory. I don't think you'd see a difference between 4x4GB and 2x8GB.
 
I've just purchased a 27 k iMac 3.4ghz with 8gb ram installed ,bought 2no 8gb sticks from crucial its now running 24gb ram took me all of 5 mins to install.
 
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VinOrdinaire said:
Most discussion elsewhere assumes that modules are added in pairs and that iMacs use dual-channel memory. I don't think you'd see a difference between 4x4GB and 2x8GB.
Click to expand...
Nobody will be able to see a difference simply because there is no difference. The Core i5/i7 CPUs Apple uses in its iMac lineup do not support quad-channel memory. If you want to benefit from four memory modules you have to go the Xeon route, aka iMac Pro. A Xeon CPU is actually faster with four modules than two because it does support quad-channel memory access.

And by the way: for the amount of money you paid Apple for your 2x 8GB sticks as replacement for the original 2x 4GB sticks you could have purchased 2x 8GB on Amazon/Newegg/eBay/Best Buy/NameYourFavoriteRetailerHere, add them in addition to the 2x 4GB sticks for a total of 24GB instead of 16GB, and saved some money as well. Ordering RAM from Apple for the 27" iMac is a complete waste of money.

Which is why I am 100% convinced that Mr. Cook and his money-grabbing minions are actively working on taking user upgradeability away in the next iteration of the iMac like they did with the iMac Pro. It's just a matter of time.
 
Dr. McKay said:
So if I bought a 16Gb RAM kit from, say, Crucial, with 2 4Gb DIMMS (or even 2 8Gb DIMMS, cheaper than the upgrade from 8 to 16 from Apple), I can combine that without problems with the already installed 2x4gb DIMMs, even though it wouldn't be the exact same RAM (same specs, different brand, maybe different CAS latencies...).
Click to expand...

Yes and, in fact, if you use the Crucial RAM selector tool to get their recommended kit for the iMac it probably will be the exact same RAM that Apple use. (My 2017 iMac came with Micron RAM and Crucial are basically the retail arm of Micron).

Getting a 2x8GB kit to give you 24GB total is probably the sweet spot, unless you're doing something seriously RAM intensive.
 
Geordie bry said:
I've just purchased a 27 k iMac 3.4ghz with 8gb ram installed ,bought 2no 8gb sticks from crucial its now running 24gb ram took me all of 5 mins to install.
Click to expand...

Nice update and it shows the value of ordering an iMac that has two empty memory slots.

Dr. McKay also plans to buy an iMac with a 512GB SSD, and the SSD is a worthwhile buy because it makes a huge positive impact on the perceived responsiveness of the machine.
 
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theluggage said:
Getting a 2x8GB kit to give you 24GB total is probably the sweet spot, unless you're doing something seriously RAM intensive.
Click to expand...

Then that's what I'll do, buy the iMac with the base 8Gb and order a 16gb 2x8 kit from Crucial...

VinOrdinaire said:
Dr. McKay also plans to buy an iMac with a 512GB SSD, and the SSD is a worthwhile buy because it makes a huge positive impact on the perceived responsiveness of the machine.
Click to expand...

Yes, the SSD is indispensable, and I've gone straight for 512Gb because 256Gb is a bit tight, even if I will be using a 2Tb USB3 external drive to store most of my stuff...
In any case, thanks for the feedback, everyone !

Edit : I live in the EU, so used the EU site, obviously...
http://eu.crucial.com/eur/en/imac-(...4ghz-intel-core-i5,-2017)-imac18,3/CT10559913

http://eu.crucial.com/ProductDispla...p_category=&parent_category_rn=&storeId=10152

Could be me, but where does it state that these upgrade kits actually consist of 2 modules, either 2 x 4Gb or 2 x 8Gb ?
 
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Dr. McKay said:
Then that's what I'll do, buy the iMac with the base 8Gb and order a 16gb 2x8 kit from Crucial...



Yes, the SSD is indispensable, and I've gone straight for 512Gb because 256Gb is a bit tight, even if I will be using a 2Tb USB3 external drive to store most of my stuff...
In any case, thanks for the feedback, everyone !

Edit : I live in the EU, so used the EU site, obviously...
http://eu.crucial.com/eur/en/imac-(retina-5k,-27-inch-3*4ghz-intel-core-i5,-2017)-imac18,3/CT10559913

http://eu.crucial.com/ProductDispla...p_category=&parent_category_rn=&storeId=10152

Could be me, but where does it state that these upgrade kits actually consist of 2 modules, either 2 x 4Gb or 2 x 8Gb ?
Click to expand...

In the US they sell both single parts and pairs. On the website they list both a single 8 gb part, and one las a 16 gb kit (x x 8) at around double the price of the single stick.
 
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There's a retailer who does a promotion on one of the iMacs in stock (new, boxed not a demo model) : it's a 27 inch inch 3.4Ghz 2017 iMac, 512Gb SSD and 16Gb of RAM, just what the doctor ordered. Normal price in Belgium for this config from Apple is 2699 euros, now down to 2599 euro (that's the equivalent of 114 dollars off).
Apple charges 240 euros for the upgrade from 8Gb to 16gb.

It would still be cheaper to buy the base config with 8Gb and 512Gb SSD from Apple directly, but that would get me 2 4Gb SODIMM's with room for two more, taking it to 24Gb with an extra 2 x 8Gb. Upgrading later to, say, 32Gb would mean wasting 2 4Gb RAM sticks.
I don't need more than 16Gb at the moment, so this config with 2 x 8Gb would suit me better in the long run. If I should ever need more than 16Gb of RAM, I could upgrade to 32Gb later on (or just spend the 170 euros now and bump it to 32Gb straight away).

Initially, I would pay 2459 euros for the 3.4ghz 27 inch iMac with 512Gb SSD and 8Gb of RAM (2x4). Add 177 euros for an extra 16Gb (2x8) for a total of 24Gb, which brings the total cost to 2636 euros (2699 from Apple with 16Gb of RAM).
Buying this discounted iMac, would cost me 2599 euros for 512Gb SSD and 16Gb of RAM (2x8), leaving two slots empty for a future upgrade to 32 gb...
 
OP:

Just a bit of advice (that you didn't ask for):

Try the iMac with 8gb for 2-4 weeks before you buy more RAM.
You may not need "more RAM".

It may run just fine with 8gb.
Why "buy more" if you don't really have the need for it?
 
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Single rank = memory chips on one side of the PCB
Dual rank = memory chips on both sides of the PCB

Between the two you've posted the single rank modules have 8GB worth of memory chips on one side of the PCB whereas the other modules have 2x 8GB worth of memory chips on both sides of the PCB.

It doesn't matter which ones you add - single vs. dual rank does not make a difference. I highly doubt you'll finde 16GB single-rank modules anyway due to lack of chips. Pick a size and go with it, you're massively overthinking this...
 
mj_ said:
It doesn't matter which ones you add - single vs. dual rank does not make a difference. I highly doubt you'll finde 16GB single-rank modules anyway due to lack of chips. Pick a size and go with it, you're massively overthinking this...
Click to expand...

I'm not a techie, so I was thinking along the lines of single and dual 'channel' RAM, which, obviously, is something completely different.
Theyre 16Gb sets, each with two 8Gb sticks. Guess I'll go with the cheapest one then...
 
Dr. McKay said:
I'm not a techie, so I was thinking along the lines of single and dual 'channel' RAM, which, obviously, is something completely different.
Theyre 16Gb sets, each with two 8Gb sticks. Guess I'll go with the cheapest one then...
Click to expand...

For some reason, Crucial offer this part for the 2017 3.4GHz i5 iMac 5k :

http://uk.crucial.com/gbr/en/imac-(...4ghz-intel-core-i5,-2017)-imac18,3/CT10559913

...and this part for the other 2017 iMac 5ks:

http://uk.crucial.com/gbr/en/imac-(retina-5k,-27-inch,-2017)-imac18,3/CT10559860

It is not clear what the difference is - they seem to have the same specs - but they do specifically note on the second link to get the appropriate RAM for the 5k 3.4.

Personally, I'd pay slightly more to get the RAM direct from Crucial - since they explicitly state that it is compatible with a given machine there should be no quibbling if there is a problem.

The availability of the "set of two" kits just seems to vary according to stock...
 
Dr. McKay said:
Theyre 16Gb sets, each with two 8Gb sticks. Guess I'll go with the cheapest one then...
Click to expand...
Nope, wrong again :D

One of them is a 16GB kit with 2x 8GB (first Amazon link).
The other is a 32GB kit with 2x 16GB (second Amazon link).

Which is why one of them is twice as expensive as the other. And also why one is called Crucial CT2K16G4SFD824A 32Go Kit (16Gox2) while the other is called Crucial CT2K8G4SFS824A 16Go Kit (8Gox2). And why for one of them the dropdown box on Amazon reads 16Go Kit while for the other it reads 32Go Kit.
 
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mj_ said:
Nope, wrong again :D

One of them is a 16GB kit with 2x 8GB (first Amazon link).
The other is a 32GB kit with 2x 16GB (second Amazon link).

Which is why one of them is twice as expensive as the other. And also why one is called Crucial CT2K16G4SFD824A 32Go Kit (16Gox2) while the other is called Crucial CT2K8G4SFS824A 16Go Kit (8Gox2). And why for one of them the dropdown box on Amazon reads 16Go Kit while for the other it reads 32Go Kit.
Click to expand...

Whoops, my bad, must have posted the wrong URL's...
 
theluggage said:
Yes and, in fact, if you use the Crucial RAM selector tool to get their recommended kit for the iMac it probably will be the exact same RAM that Apple use. (My 2017 iMac came with Micron RAM and Crucial are basically the retail arm of Micron).

Getting a 2x8GB kit to give you 24GB total is probably the sweet spot, unless you're doing something seriously RAM intensive.
Click to expand...

Depends on workload but I agree, for me 24gb or more is the sweetspot. At 24gb using Gimp and Divinci Resolve at the same time will start to increase memory pressure and I’ll start seeing swap and a lot more compressed memory. I’ll start feeling it slow down a bit.

With an SSD the differences aren’t bad but with an HDD I would recommend maxing out the ram to try to keep programs and commonly used files buffered in memory. This also applies if using external storage that are network bound.

Prior to me putting an SSD in my iMac it was agonizing to use with less than 16gb of RAM although tolerable with 24gb since my of my programs would stay cached in RAM. Conversely if you are just a casual user you might not be able to tell a difference with 8gb and 32gb just depends on specific workload and what the bottleneck are in those workloads.
 
Is there any difference ?
http://eu.crucial.com/eur/en/imac-(retina-5k,-27-inch-3*4ghz-intel-core-i5,-2017)-imac18,3/CT10559878
Directly from Crucial, 190 euros, an additional 18 euros for delivery on September 11th...

And then this :
https://www.coolblue.be/nl/product/803123/crucial-16gb-ddr4-sodimm-2400-mhz-kit-2x8gb.html
Next day FREE delivery...

On Crucial's website, they say this memory is for the 3.4Ghz model specifically. Is the RAM for the mid-range and high-end 2017 27inch imac any different,then ? I get confused...
 
Installed the extra 16Gb, so I now have 32Gb in total (4x8Gb).
And I must say, the whole system just feels that bit more snappy, on the whole. And I'm not imagining things.
Also, system monitor reports lower memory usage with 32gb than when I only had 16Gb.

With 16Gb, I was using almost 12Gb of RAM with the following apps running : Chrome (8 tabs open), Spotify, Word, Indesign CC, App Store, Messages, Photos. Now, with 32Gb of RAM installed, running those same apps (same tabs in Chrome), I'm only using about 10Gb...
 
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