Q: 2010 8-core MacPro asymmetrical RAM config?
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Jan 18, 2014 12:41 PMby The hatter,Helpful8GB RDIMMs are also supported. The computer will not start if both types of DIMMS are installed in the same computer. For more information, see Mac Pro (Mid 2010): Registered DIMM support.
If the system detects it is installed wrong it will tell you.
The ideal right way is to use triple channel configurations for 3 DIMM slots and slot 3&4 are shared.
Yes yours is very unbalanced and thought you could start with using the tech article.
6 or 8 x 4GB though many - depends on what you run and use it for and how heavy t he memory stress but 24-32GB for graphics might be all though some find they need more and don't want to be locked in so they go with 8GB or 16GB DIMMs.
Depends on what type of 8GB DIMM too.
Difference between RDIMM and UDIMM
R = Registered, U = Unregistered, FB = Fully Buffered, 5520 always needs ECC registered fully buffered.
please strictly stick to the memory compatibility list for your motherboard - there are even more differences than buffered vs. unbuffered (eg. single/dual/quad rank) - when purchasing memory not on the compatibility list get a written confirmation from the dealer the modules will work for a certain motherboard.
When you populate slots 3 & 4 you do take a hit as they share one channel.
If 2 memory DIMMs are used per channel, this "...results in a reduction of maximummemory bandwidth for 2DPC (DIMMs per channel) configurations with UDIMM by some 5% in comparison to RDIMM." [1] (p. 14). This is because "...when you go to 2 DIMMs per memory channel, due to the high electrical loading on the address and control lines, the memory controller uses something called a “2T” or “2N” timing for UDIMMs. Consequently every command that normally takes a single clock cycle is stretched to two clock cycles to allow for settling time. Therefore, for two or more DIMMs per channel, RDIMMs will have lower latency and better bandwidth than UDIMMs." [2] http://en.wikipedia.org/wiki/Unbuffered_memory
Mac Performance Guide News -
Jan 18, 2014 6:06 PMby Grant Bennet-Alder,Unequal amounts on the two sides are not an issue, which is why it is not mentioned.
When you get new DIMMs, it is important to use a two-step process to introduce them.
The first step is to determine whether these DIMMs work on their own on one side or the other.
Assuming they work, the second step is to determine whether, and under what conditions, these new DIMMs "play nice with others".
The hatter has pointed out that slots 3 and 4 share a single channel, and that may become an issue. Also, RDIMMs and UDIMMs do not always work together.
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Jan 21, 2014 5:08 PMby A H1,Grant, Hatter
thanks for the info. I don't know whether the 8Gb module is an RDIMM or an UDIMM (or may be 8G modules are automatically RDIMMs)... All I know is that it's an original Apple part, but just looking at the part number it's hard for me to tell. but it seems like it's better not to use the 8Gb module, at least by itself.
I have heard about the last two slots being shared. currently the setup is like this. we were on a tight budget, so instead of buying 6x2G modules, we opted to just get two 4G module from OWC and kept the stock 6x1G module...:
CPU #1
Slot 1. 4G
Slot 2. 1G
Slot 3. 1G
Slot 4. 1G
CPU #2
Slot 1. 4G
Slot 2. 1G
Slot 3. 1G
Slot 4. 1G
At this point, the machine recognizes all 14Gb RAM, and the memory configuration utility seem to verify it's the best configuration. On my 2008 MP, we were happy with just 10Gb RAM, so whether we stay wtih 14Gb, or reduce it to 12G is probably not going to make a whole lot of difference to us (not a heavy RAM use. casual photoshop, occasional Handbrake video conversion, iDVD, that sort of thing).
I guess the next question is, for a casual user, should I just leave the 1G modules in both slot 4? Assumption is that memory is accessed from Slot1 first and then go up the ranks (serial access)? or are all slots 1-3 accessed simultaneously and thus having 1G with the 4G makes things very bad?
but we really don't need all that much RAM, so...
I guess i should get some of those RAM efficiency benchmark software to see whether having 1G in slots 4 makes the system noticeably slower...
i'm rambling...
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Jan 21, 2014 5:39 PMby Grant Bennet-Alder,HelpfulWhen we first started talking about this, this subject came up, and the statements made at the time were that the slowdown penalty (provided all would work together) for the extra DIMM in slot 4 was small. It would be more than overcome by the speedup provided by having the extra needed memory.
We have since seen that in certain circumstances, the DIMMs will actually clock down if the Bus loading is too large, and this will be reflected in the stated speeds of the modules. If you are not seeing this down-clocking, and all modules are showing, I expect you are seeing less than about ten percent slowdown from having the last modules present.
The calculations of size of these slowdowns came from memory tests intended to pound the pudding out of memories, and the real-world impact of this may be much smaller.
