Mixing RAM sizes?

CH12671

macrumors 6502
Original poster
Dec 29, 2013
350
0
Southern US
I'm considering just ordering the minimum (12g) of RAM in my nMP, and promptly ordering a 16g upgrade kit from crucial (2x8 gig). My understanding is that the 8 gigs are still non-registered, so they could technically mix with the two 4 gig factory chips, couldn't they? I would only do it for a few months until I can order another 2x8 chips to complete the 32 gig set....I just didn't want to waste the two factory chips in the meantime, or the two open slots.
Would it work?

Thanks
 

Umbongo

macrumors 601
Sep 14, 2006
4,934
54
England
I'm considering just ordering the minimum (12g) of RAM in my nMP, and promptly ordering a 16g upgrade kit from crucial (2x8 gig). My understanding is that the 8 gigs are still non-registered, so they could technically mix with the two 4 gig factory chips, couldn't they? I would only do it for a few months until I can order another 2x8 chips to complete the 32 gig set....I just didn't want to waste the two factory chips in the meantime, or the two open slots.
Would it work?

Thanks
Yes it should work. See my comment below on performance.
 
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Anim

macrumors 6502a
Dec 16, 2011
614
22
Macclesfield, UK
I'm considering just ordering the minimum (12g) of RAM in my nMP, and promptly ordering a 16g upgrade kit from crucial (2x8 gig). My understanding is that the 8 gigs are still non-registered, so they could technically mix with the two 4 gig factory chips, couldn't they? I would only do it for a few months until I can order another 2x8 chips to complete the 32 gig set....I just didn't want to waste the two factory chips in the meantime, or the two open slots.
Would it work?

Thanks
I'm pretty sure that the Xeon CPU memory controller does not support mixing of ram. I read it somewhere on the intel website when looking at the chip features. What this does in the real world I have no idea unless somebody else has tested it using the same CPU. I would treat it as risky until you get some solid info.
 

Umbongo

macrumors 601
Sep 14, 2006
4,934
54
England
I found the link.

I think this is the 8 core cpu used, look at the spec for Flex memory in the features:

http://ark.intel.com/products/77912/
Flex is something for dual-channel boards, don't think it is on any triple or quad channel boards, probably because of how it works. It lets you mix capacities and part of the larger capacity DIMM, up to the size of the smaller one, work in dual channel and the other part in single channel. So 8GB and 4GB would be 2x4GB dual and 4GB single. With 8, 8, 4, 4 in a Mac Pro it looks like it will all go in single channel, but the extra capacity may well offer more overall performance than 16GB in dual-channel mode would.

Probably worth someone benchmarking.
 
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deconstruct60

macrumors G3
Mar 10, 2009
8,951
1,778
I'm considering just ordering the minimum (12g) of RAM in my nMP, and promptly ordering a 16g upgrade kit from crucial (2x8 gig). My understanding is that the 8 gigs are still non-registered, so they could technically mix with the two 4 gig factory chips, couldn't they?
Crucial Vendor-> Product -> specific version selector doesn't show them at first but if press the "show all compatible offerings" button it shows. As long as both subsets are Unbuffered ECC and meets Apple's guidelines it should work.

Holding onto the 1-2 DIMMs you pulled for a couple of weeks wouldn't hurt so that have something to fall back to and/or use for elimination of glitch (root cause) testing.
 

Anim

macrumors 6502a
Dec 16, 2011
614
22
Macclesfield, UK
Flex is something for dual-channel boards, don't think it is on any triple or quad channel boards, probably because of how it works. It lets you mix capacities and part of the larger capacity DIMM, up to the size of the smaller one, work in dual channel and the other part in single channel. So 8GB and 4GB would be 2x4GB dual and 4GB single. With 8, 8, 4, 4 in a Mac Pro it looks like it will all go in single channel, but the extra capacity may well offer more overall performance than 16GB in dual-channel mode would.

Probably worth someone benchmarking.
Good post, yes, Flex comes in when dual/single channel mode is being used and allows mixed ram. From what I understand, all ram slots on the nMP will be used for 4 channel mode but 12GB (3x4) will be triple channel mode? So, if you buy 2x8 and use the 2x4 (removing one of the 4's) that would give you 24GB of total memory with dual channel on the 2 x 8 and 1 channel on the 2 x 4?

There is a good breakdown here:
http://www.intel.com/support/motherboards/desktop/sb/cs-011965.htm

PS. Sorry for the edits
 
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Umbongo

macrumors 601
Sep 14, 2006
4,934
54
England
So, if you buy 2x8 and use the 2x4 (removing one of the 4's) that would give you 24GB of total memory with dual channel on the 2 x 8 and 1 channel on the 2 x 4?
Honestly don't know. The documentation suggests the following would occur with 8, 8, 4, 4:

Single-channel (asymmetric) mode
This mode provides single-channel bandwidth operations and is used when only one DIMM is installed or when the memory capacities of more than one DIMM are unequal. When using different speed DIMMs between channels, the slowest memory timing is also used.
However we know from the 2009 Mac Pro that there are other things possible. than Intel document (2, 1, 1 DIMMs per channel giving an interleave with dual-channel performance). It could be that 8, 8, 4, 4 would act like 8+4 on one channel and 8+4 on a second channel rather than only allowing access to one DIMM at a time.

12GB/s read/6GB/s write or whatever the real world performance of a single DIMM is probably won't be a bother for a temporary solution although I wouldn't do it long term.

I only have one capacity of DIMM for my workstations or I'd test it, hopefully someone will as I'm curious and it is a logical thing many will consider - getting 2x8GB DIMMs and then another 2 later on. Annoyingly if it had 8 slots it wouldn't even be an issue, you'd easily get dual-channel performance :/
 

CH12671

macrumors 6502
Original poster
Dec 29, 2013
350
0
Southern US
So, Umbongo, you are saying that I may have to install the chips as 8,4 and 8,4 rather than 8,8 and 4,4? I guess the machine will let me know quickly if it doesn't like the way they are matched.

And I definitely plan to hang onto the one 4 gig chip I pull out. I am going to just go with the base model, and upgrade everything but the ram. It makes more sense to me to spend 229 dollars and have 24 gigs of ram than to spend 100 dollars and have 16 gigs of ram......plus it gives me cheaper upgrade options later. I don't see myself needing more than 32 gigs ultimately.
 

Umbongo

macrumors 601
Sep 14, 2006
4,934
54
England
So, Umbongo, you are saying that I may have to install the chips as 8,4 and 8,4 rather than 8,8 and 4,4? I guess the machine will let me know quickly if it doesn't like the way they are matched.

And I definitely plan to hang onto the one 4 gig chip I pull out. I am going to just go with the base model, and upgrade everything but the ram. It makes more sense to me to spend 229 dollars and have 24 gigs of ram than to spend 100 dollars and have 16 gigs of ram......plus it gives me cheaper upgrade options later. I don't see myself needing more than 32 gigs ultimately.
Yeah, OS X may tell you how to place them when you boot with mixed capacities. Someone may have tested this before you buy anyway so keep an eye out.
 
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