3x8GB or 6x4GB RAM

[rdru]

Is there any performance differences between 3x8GB or 6x4GB RAM in the
MacPro 12-core?

[Ravich]

My guess is 3x8GB performs better because of triple channel.

[Umbongo]

My guess is 3x8GB performs better because of triple channel.


6x4GB is also triple channel. Each processor would also have it's own memory and the memory would be unbuffered meaning 6x4GB will be faster but offers lack of expansion.

[rdru]

I asked one of the Mac memory vendors and they say there are no differences?
(I can not post their answers publicly ...)

My doubt is whether the 2 processors can access memory in parallel.

If they can the 6x4GB is better.

I'll run multithreaded algorithms that do access RAM constantly through 8 or more threads.

This can not be an issue for most applications, since they do not access large chunks of RAM in parallel.

[MFDoom]

Mac Performance Guide has a write up where they basically say that there is now real world difference between triple and dual channel. Based on this I'd go with the lower cost option.

[rdru]

MFDoom,

The use of 3x8GB or 6x4GB runs in triple channel.

The matter is whether the 2 RAM banks can be accessed in parallel.

If 3x8GB only one bank is used whereas with 6x4GB the 2 banks are used.
I heard people saying that there is an association between each processor
and a RAM bank. This is why of my question.

[alust2013]

MFDoom,

The use of 3x8GB or 6x4GB runs in triple channel.

The matter is whether the 2 RAM banks can be accessed in parallel.

If 3x8GB only one bank is used whereas with 6x4GB the 2 banks are used.
I heard people saying that there is an association between each processor
and a RAM bank. This is why of my question.

6x4 would give you better performance, as it would give each cpu its own memory, and the CPUs can access the memory in their bank faster than the other

[rdru]

6x4 would give you better performance, as it would give each cpu its own memory, and the CPUs can access the memory in their bank faster than the other

Can you point me to an explanation how this is actually done?
I tend to believe it is as you described but I would like to know precisely
how it is done.

[alust2013]

Can you point me to an explanation how this is actually done?
I tend to believe it is as you described but I would like to know precisely
how it is done.

That I am not exactly sure of, I just read a thread about it the other day