5,1 Mac Pro: What is better - 4 x 8Gb or 8 x 4Gb memory modules?

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5,1 Mac Pro: What is better - 4 x 8Gb or 8 x 4Gb memory modules?

I have a single CPU (4 core) 2010 Mac Pro with 32Gb ram (4 x 8Gb modules), & just bought a dual CPU (12 core) with 32Gb ram (8 x 4Gb modules).

I put the (new) 12 core tray in my (old) 2010.

All are 1333mhz modules.

I have the following questions:

1 - do you get the most efficiancy out of the ram to have all the slots filled with the same size smaller modules?

2 - or would I be better off filling 4 of the 8 slots with the higher 8Gb modules?

3 - would it be better to use the 4 x 8Gb modules & 4 x 4Gb modules for a total of 48Gb?

To be honest I dont think I actually even need 32Gb of ram however that's what both machines came with.

Thanks in advance!

Posted on May 14, 2020 6:21 AM

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Answer 1

May 14, 2020 7:50 AM in response to DJstylus

This Mac Pro memory organization is different from many others such as the Mac Pro 2006-2008 or the Mac Pro late 2013.

This particular Mac has triple-channel memory. Slot 3 and 4 on each side are Shared, so populating the last slot can actually SLOW the memory in some cases, and will simply refuse to work in other cases. It can be finicky.

Best performance is from larger modules, and if you can, leave the fourth slot EMPTY on each side. You may also choose to install two @ 8GB on each side for a total of 32GB, and leave the third slot open for easy expansion. The slight loss in performance is not observable in real-world testing, only in artificially-constructed performance tests.

Modules 16GB and larger are always Registered DIMMs, which do not play nice with others. If not careful, some older 8GB modules are Registered DIMMs as well.

Be certain to test new memory in smaller groups of new modules alone before proceeding with any mixing. That way you are working with "known good" modules to start, reducing complexity.

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Answer 2

May 14, 2020 8:27 AM in response to DJstylus

The speed rating is the speed rating -- there is no additional burden for DIMM size.

The cost of an 8GB stick is less than TWICE the cost of two 4GB sticks, so in general 8GB stocks are superior.

But you have hit the Registered DIMM landmine. Registered DIMMs do not play nice with others (unregistered) so you will not be able to install your unregistered modules. If they are slower, they will also slow the overall memory as speed as well.

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Answer 3

DJstylus Author

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May 14, 2020 9:35 AM in response to Grant Bennet-Alder

But you have hit the Registered DIMM landmine. Registered DIMMs do not play nice with others (unregistered) so you will not be able to install your unregistered modules.

Apparently so!

Thanks again for your help.

Mal

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Answer 4

DJstylus Author

Level 1

15 points

May 14, 2020 8:18 AM in response to Grant Bennet-Alder

Thanks Grant,

I've installed the 4 x 8Gb sticks (Hynix DDR3 1333Mhz ECC Reg) in slots 1,2,5,6.

I did try install as above with 4 x 4Gb (Kingston KTA-MP1333/4G) in slots 3,4,7,8

but it wouldnt boot - just got a flashing power button so I assume they didnt like each other.

I also torn with keeping the 8Gb sticks in the 12 core as I plan on selling the 4 core & it will only

go with 16Gb, which will make it not worth as much as a 32Gb machine...

Performance wise though, there should be no noticeable difference between 4 x 8Gb, & 8 x 4Gb?

Thanks again

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5,1 Mac Pro: What is better - 4 x 8Gb or 8 x 4Gb memory modules?

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