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Korican100

macrumors 65816
Original poster
Oct 9, 2012
1,131
478
So I am about to do the 4,1 to 5,1 upgrade w/ a w3690 3.46 hex.
I know on my mac pro3,1, I was advised to fill all the slots for increased speeds.

I heard that to use my new ram @ 1333hz, I would only be able to use slots 1-3. How much of a performance loss can i expect using only 3 slots? vs 4 slots @ 1066hz?



I feel I maybe misinformed also. If the speed differences are completely negligible, please inform me.
 

Studio K

macrumors 6502
Feb 17, 2013
361
7
United States
I know on my mac pro3,1, I was advised to fill all the slots for increased speeds.
I heard that to use my new ram @ 1333hz, I would only be able to use slots 1-3. How much of a performance loss can i expect using only 3 slots? vs 4 slots @ 1066hz?
I feel I maybe misinformed also. If the speed differences are completely negligible, please inform me.

What's most important is that you have enough ram.
If you can achieve that by using the 3 slots (for tri-channel @1333Mhz), then that would be ideal, I suppose.
If you need to use the fourth slot to have enough ram, then I believe that the additional memory would outweigh the speed penalty.

The consensus seems to be that the speed difference b/n 1066 and 1333 is nominal.
 
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MacVidCards

Suspended
Nov 17, 2008
6,096
1,054
Hollywood, CA
Don't know where this 1066 speed comes from.

You can use 4th slot at 1333 unless system profiler is lying. (It does sometimes)

With a "W3xxx" processor you are limited to 3@16 GB and 1 @ 8 GB for Max of 56 GB.

To get to 64, use a X5xxx CPU.

Nobody has found a 32 GB module that works yet but I am still hopeful.
 
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Studio K

macrumors 6502
Feb 17, 2013
361
7
United States
Don't know where this 1066 speed comes from.

You can use 4th slot at 1333 unless system profiler is lying. (It does sometimes)
QUOTE]

Someone here recently posted that using that 4th slot will drop your speed to 1066 (when using four 1333Mhz sticks).
They said System Profiler will falsely report your speed at 1333.

I don't know how to confirm this, unless you have Windows installed. Perhaps the Windows version of System profiler will tell the "truth".
 
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Korican100

macrumors 65816
Original poster
Oct 9, 2012
1,131
478
Don't know where this 1066 speed comes from.

You can use 4th slot at 1333 unless system profiler is lying. (It does sometimes)

Someone here recently posted that using that 4th slot will drop your speed to 1066 (when using four 1333Mhz sticks).
They said System Profiler will falsely report your speed at 1333.

I don't know how to confirm this, unless you have Windows installed. Perhaps the Windows version of System profiler will tell the "truth".

this is also what I read. That's why I wanted to know if 1333 w/ 3 slots is better than 1066 @ 4. Or does the "must fill up all slots for optimal speeds" rule apply to '09-'11 machines?
 
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reco2011

macrumors 6502a
May 25, 2014
531
0
So I am about to do the 4,1 to 5,1 upgrade w/ a w3690 3.46 hex.
I know on my mac pro3,1, I was advised to fill all the slots for increased speeds.

I heard that to use my new ram @ 1333hz, I would only be able to use slots 1-3. How much of a performance loss can i expect using only 3 slots? vs 4 slots @ 1066hz?



I feel I maybe misinformed also. If the speed differences are completely negligible, please inform me.

Aside from benchmarks and very high memory demand applications you won't be able to tell the difference. As Studio K said...if you memory demands can be met with three slots that's ideal. If not then load up all four. Either way you're unlikely to see a difference.
 
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bxs

macrumors 65816
Oct 20, 2007
1,141
516
Seattle, WA
Slots 1, 2 and 3 are best for each of the processor's 3 memory channels. If all slots are used then simultaneous access to some of the memory by the processors will have to share a memory channel. This in effect can slow down memory access at times.

The budget may dictate 64GB so in this case 4x 8GB sticks for each processor would be done.

If the budget can stand it, then use 96GB and place 3x 16GB in slots 1/2/3 for each processor.
 
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flowrider

macrumors 604
Nov 23, 2012
6,541
2,526
For optimal performance use slots 1,2, & 3. The 35XX, 36XX, 55XX, @ 56XX Xeons have three memory channels. On a 4,1 and 5,1 Mac Pro, Slots 3 & 4 share the third channel. Therefore if you fill channel 1, 2, & 4 you won't see the RAM in the 4th slot. But, using the fourth slot along with the third probably won't be a big performance hit, but there will be some slowdown that you may not notice. However, my preference for my Dual CPU 5,1, I've left slots 4 and 8 MT.

Lou
 
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brand

macrumors 601
Oct 3, 2006
4,373
432
127.0.0.1
Or does the "must fill up all slots for optimal speeds" rule apply to '09-'11 machines?

There is no 2011 Mac Pro. The 2009 model is identified as 4,1 while the 2010 and 2012 models are both identified as 5,1. The 4,1 is technically different than the 5,1.
 
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fredr500

macrumors regular
Apr 12, 2007
223
19
Just to be clear

I have a quad 2.8 5,1. It came with 3GB (WTF?) so I ordered 16GB (2x8GB) from OWC, it arrives today.

I was planning on the 2 new 8's in front, 2 of the old 1GBs in back.

I assume it is better to have the extra gig at a reduced speed than just use one of the old sticks. It's better to have slightly slower 2GB than a faster 1GB when I need the extra space, correct?
 
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flowrider

macrumors 604
Nov 23, 2012
6,541
2,526
^^^^Since you only ordered Two DIMMs you won't get the benefit of 3 channel memory any way, so yes your plan is OK.

Lou
 

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fredr500

macrumors regular
Apr 12, 2007
223
19
I didn't realize that when I ordered the 2 DIMMs. Bummer.

How much real-world difference would I see with 3 matching DIMMs? Besides the increase from 16 to 24GB.
 
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