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About RAM slots used in 4-,8-, and 12 core machines..

MrAverigeUser

MrAverigeUser

macrumors 6502a
Original poster
May 20, 2015
895
397
europe

There is no question about the fact that 4- and 8-core classic MacPros should have a configuration with just 3 of4 RAM slots used ... So "3of4-slots-configuration is prferable, BUT:



I remember the argument why a 6-core or a 12-core MP would profit from 4of4-configuration:

The Xeons ar able to create a virtual 3rd core. To max that out, you need a sum of real hardre cores that you can divide by 3 so that any pair of the real existing cores can create and use a third one. if this is not the case, the 4th occupied slot will only brake instead of helping the system.

-----> could someone with a 12-core MBP help,us tomverify this?
Would be easy for him:

Just run a performance test with both configurations and report the result....
Imthinkmthis would be very interesting for a lot of people.

As for "as much RAM as possible is the best and not enough RAM in 3 slots is worse than enough RAM in 4 slots" :

i agree, having 32 GB of RaM since I purchased my MP with it (yes, I was really that naive and burned money because at that time this was horribly expensive...)

Last question:
Does it matter which of the 3 slots are used for RAM or not?

Thanks in advance​
 
Last edited:
Use the 4th slot with my W3690 (6 cores) actually will decrease the memory performance (proved in Geekbench).

So, the magic of multiple of 3 won't work. I am pretty sure it's the same on the 12 cores, but I can't test it by myself.

And yes, which 3 slots does matter. The 3rd and 4th slots share the same channel. Therefore, for optimum performance, you can to put the RAM in only slot 1,2, and 3. Also, AFAIK, the 4th slot only work when there is a RAM stick in slot 3.

Since the 3rd and 4th slots are sharing the same channel, no matter how advance the CPU, how many cores it has. I believe use both slots can only result in decrease in memory performance.
 
r6mile said:
Sorry if this sounds naive but what's the point of Apple putting in a fourth RAM slot then?
Click to expand...

There was none just marketing BS to claim higher ram capacity it should have been six slots like every other manufacturer correctly did ie. two triple channel channels or twelve on the dual core machines.
 
h9826790 said:
Use the 4th slot with my W3690 (6 cores) actually will decrease the memory performance (proved in Geekbench).

So, the magic of multiple of 3 won't work. I am pretty sure it's the same on the 12 cores, but I can't test it by myself.

And yes, which 3 slots does matter. The 3rd and 4th slots share the same channel. Therefore, for optimum performance, you can to put the RAM in only slot 1,2, and 3. Also, AFAIK, the 4th slot only work when there is a RAM stick in slot 3.

Since the 3rd and 4th slots are sharing the same channel, no matter how advance the CPU, how many cores it has. I believe use both slots can only result in decrease in memory performance.
Click to expand...



Thanks for sharing ! :)

I did only mention the 12-core and forgot to mention the 6-core (added the 6-core MB).
So if the hypothesis I presented in my first posting is right, the 6-core should perform more either with 4 slots…

jerwin said:
Ahem.

http://macperformanceguide.com/Mac-Upgrade-CaseStudy-MacPro-Memory.html

Sometimes the performance increase associated with having enough memory for the task at hand outweighs the slight performance decrease associated with using 4 dimms.
Click to expand...


Thank you as well.

I add one more page about RAM-upgrade for MacPro in this article:

http://macperformanceguide.com/Mac-Upgrade-MacPro-Memory.html

What I didn´t know yet:
Everybody talked bout performance-loss with 4/4-slot configuration compared to 3/4-slot configuration…. in fact, the difference in performance (in use of PS5 which was not capable at that time - in 2011 - to use all the cores) was only ridiculous 2,7% !!!

Nowadays apps may use more cores than old apps - so the results in performance could (!) perhaps be the opposite, as well as for more than 24 GB RAM….

here the citation of the article (INFO: In the article 24GB RAM = 3x8 GB, 32 GB RAM = 4x8GB) :

"ENOUGH memory means that the particular task involved has its needs fully satisfied with memory to spare for other system activities.

Using 32GB instead of 24GB reduces performance by 2.7%. That’s because 24GB is already ENOUGH, and 3 X 8GB runs in triple channel mode, yielding faster memory access times the dual-channel mode used with 4 X 8GB.

HOWEVER, if the problem size were to increase to about 24GB or larger, then the 32GB configuration would be much faster, because the ENOUGH criterion would not have been satisfied with 24GB."

 
Last edited:
With triple channel enabled the performance gain is so small that in the end it makes no difference (at least for me). Should be somewhere between 5 ~ 7%.
I'm currently with 24GB (6x4) but soon i want to upgrade to 64GB (8x8), in my case having more memory is more important.
 
dimobr said:
With triple channel enabled the performance gain is so small that in the end it makes no difference (at least for me). Should be somewhere between 5 ~ 7%.
I'm currently with 24GB (6x4) but soon i want to upgrade to 64GB (8x8), in my case having more memory is more important.
Click to expand...


I think this is right.
 
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