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I've got a Mac Pro 4,1.

I had two KVR1066D3Q8R7S/8G planks in it and it ran on 1066 Mhz.

Now I've bought two planks of KVR1066D3Q8R7S/8GI (the same with "I" at the end - stands for intel certification). I've got 32 Gbs, but the system says they run at 800Mhz.

Why? Is there a way to fix it? Is it because I use four slots instead of a triple setup? Will 3x8Gbs perfom better than 4x8 or is it better to have more RAM?

UPD1: If I use either three of the sticks, they all run on 1066, when i insert the fourth one, everything drops to 800 Mhz.

UPD2: That's what I have found here:

Single-processor computers have four memory slots. Depending on the model, you can install 1 GB, 2 GB DIMMs, or 4 GB DIMMs. The processor’s memory controller has three memory channels. DIMM slots 1 and 2 have their own channels; slots 3 and 4 share a channel. For optimal memory bandwidth, all three memory channels should be used, and memory should be balanced across the three channels.

Note: Populating slot 4 slightly drops maximum memory bandwidth, but depending on the applications used, overall system performance may benefit from the larger amount of memory.

But it is still confusing. I thought the bandwidth that is being described is overall bandwidth not like all chips are dropping to 800 Mhz...

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My guess is one (or both) or the new memorys are not rated at 1066 or higher. (As far as the Mac Pro can see.) –  GEdgar May 13 '13 at 17:15
    
What do you mean? There are two KVR1066D3Q8R7S/8G at 1066 , and two KVR1066D3Q8R7S/8GI at 1066Mhz. Wait, do you mean they could be fake? –  DataGreed May 13 '13 at 17:20
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2 Answers 2

up vote 6 down vote accepted

I believe this issue is caused by the fact that the Nehalem based Xeon processor you have is one of the "Triple Channel" variants, meaning that when you install the fourth stick of RAM you are forcing the memory into the non-channel diversified mode, similar to using 3 sticks of memory in a dual channel system.

This is supported by the fact that you're experiencing the issue only on the 4th stick as well as a fairly large amount of of the supporting documentation surrounding the MacPro and the processor itself.

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seems very likely. I'll stay with three sticks for a while –  DataGreed May 14 '13 at 18:14
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@Gedgar is saying that RAM always runs at the slowest speed available amongst all your chips. I'd suggest this.

One stick at a time (8GB should be plenty to boot)

  • put one stick in
  • boot the computer
  • check speed
  • remove the stick

repeat with the second stick and so on.

I'm expecting you to find a stick that's labeled wrong or not functioning properly and running at a 800mhz...

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Thanks for the answer, but I got no luck. Just pulled out one of the new chips, booted - 1066Mhz. Switched the one i pulled out with the other new one, booted - 1066Mhz. Put all four sticks together - 800 Mhz... It's confusing –  DataGreed May 13 '13 at 18:33
    
i've updated the question –  DataGreed May 13 '13 at 18:40
    
everymac.com/systems/apple/mac_pro/specs/… This your Mac? States you can do up to 48GB of RAM. (If you couldn't power 32 you'd have some issues, possibly explaining your problem) I'd suggest exchanging the RAM. If you bought from a place like crucial.com they'll ask that you do the test I just suggested prior to returning an item. –  Ryan Hollingsworth May 13 '13 at 18:41
    
Ryan, slots 3 and 4 share the same channel. Can this be an explanation? Yeah, that's my mac –  DataGreed May 13 '13 at 19:21
    
I did the test - they all run at 1066 separatedely –  DataGreed May 13 '13 at 19:32
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