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MP 1,1-5,1 Best way to arrange 48GB of 3,1 Mac Pro RAM?

r6mile

macrumors 6502a
Original poster
Feb 3, 2010
979
466
London, UK
Hi all,

I currently have 16GB of DDR2-800 RAM in my 3,1 Mac Pro, in four sticks of 4GB. It is Kingston RAM, with the big Apple heatsinks.
I have just bought on eBay 64GB of server DDR2-800 RAM (8x8GB sticks), with the thin heatsinks. DDR2-800 RAM at a reasonable price seems exceedingly rare, so I am quite pleased with my find, even if it's 'thin' PC server RAM.

I am inclined to keep four of these sticks (selling the rest - if you are in Europe and interested let me know!), add it to my Apple RAM with the big heatsinks (as I assume it is better thermally), and have 48GB of RAM - 4x4GB 'fat' RAM and 4x8GB 'thin' RAM.

1. Is this the best idea? I know that I could get 56GB or even slightly more before I suffer a performance hit, but I assume the more 'fat' RAM the better for cooling? I will never need as much RAM as this, I only just bought it it because it was fairly cheap.

2. What is the best way to arrange these modules, both to maximise performance (dual channel etc?) and reduce thermal output?

Thanks a lot!
 

mikas

macrumors 6502
Sep 14, 2017
344
138
Finland
Heatsinks are under debate which ones are better for cooling (maybe you want to try search within Mac Pro forum). I chose to keep the flat ones I bought. I do have bigger ones too, but I don't think it's worth the hassle changing them.

First I had some trouble to get dimms recognized, but with trial and error I got all of them working, 48 GB. Please see pic. I chose 667 MHz because 800 MHz was many times the price of these slower ones.

I bought 8 pcs and the other half is in my 1,1 (flashed to 2,1). It sees max. 40 GB though, and I have no idea why.

Mac_Pro_2008_48GB_667.png


Risers A and B are both populated the same way:
slot 1: 8 GB
slot 2: 8 GB
slot 3: 4 GB
slot 4: 4 GB

These tend to get a little hot, but that's normal behavior for FB-DIMMs I have understood.
 
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r6mile

macrumors 6502a
Original poster
Feb 3, 2010
979
466
London, UK
Thank you. Yes I would have opted for FB-677s, but found 8*8GB FB-800s (thin) for £60 so I jumped on it.

I will follow your advice on how to populate the riders. RAM should be arriving today!
 
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r6mile

macrumors 6502a
Original poster
Feb 3, 2010
979
466
London, UK
Thanks all. Now all my RAM has arrived, but I have a strange issue.
No matter what combination I try it (the server RAM on its own, or together with some of the 'fat' Kingston RAM), the RAM only runs at 667 Mhz. It is definitely PC2-6400F RAM though. This does not occur with the 'fat' Kingston RAM only. What could cause the Mac Pro to down clock 800Mhz RAM to 667Mhz?

I have tried inserting each of the risers separately with only 2 sticks of RAM, and it is still only seen as 667Mhz. I am resetting the PRAM every time I change the RAM.

Just to clarify, I am not mixing any frequencies - all the RAM should be PC2-6400F 800Mhz RAM.

All the sticks are like this:
 

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r6mile

macrumors 6502a
Original poster
Feb 3, 2010
979
466
London, UK
4R4 is the wrong type
a cMP wants 2R4

****. Ok thank you. I had no idea - I guess I took a gamble on this server ram but it didn't pay out. Is the 4R4 what causes the RAM to downclock then? Is there any way to make the RAM run at 800Mhz, or should I just sell it and get different modules?
 
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Ludacrisvp

macrumors 6502a
May 14, 2008
741
319
due to it being 4r4 it will only do 667 in the cMP, that being said most would say having more ram is better than faster ram and you'll probably never see the difference outside of a synthetic benchmark.

So why do you want to run 48Gb vs just going all out with 64GB?
I know there is a potential for a performance hit with the drive controller at 64GB but just booting with a maxmem kernel boot flag can overcome that problem for you

Or do a 56Gb config as mentioned in this post:


Also see this thread for other possible tips:
 
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r6mile

macrumors 6502a
Original poster
Feb 3, 2010
979
466
London, UK
due to it being 4r4 it will only do 667 in the cMP, that being said most would say having more ram is better than faster ram and you'll probably never see the difference outside of a synthetic benchmark.

So why do you want to run 48Gb vs just going all out with 64GB?
I know there is a potential for a performance hit with the drive controller at 64GB but just booting with a maxmem kernel boot flag can overcome that problem for you

Or do a 56Gb config as mentioned in this post:


Also see this thread for other possible tips:

Thank you - I wanted to run 48GB so I could keep the 16GB (4*4) with the fat heatsinks, and only run 4 sticks of the RAM with thinner heatsinks. I thought that would be better for thermal management etc, and prevent the risers from burning etc. In any case, I tried putting all 64GB in and it just wouldn't boot.

Beyond the 4R4 downclocking to 667mhz, is there any other performance disadvantage? Would it be better to just run 2R4 FB-5300s?
Many thanks again!
 
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Ludacrisvp

macrumors 6502a
May 14, 2008
741
319
All of the RAM will run at the slowest speed.

You can actually have lower latency running faster ram at slower speeds, just depends on exactly how the RAM was made.

meaning you may have something like this (made up numbers coming)
CL7 @ 2000MHz
CL5 @ 1400Mhz

so while the clock is slower the latency is less so you may not feel the clock difference, depending on what you're doing.

Might have failed to boot due to 4r4 or maybe one or more of those 'new' DIMMs are bad.
 
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Nguyen Duc Hieu

macrumors 6502
Jul 5, 2020
438
103
Ho Chi Minh City, Vietnam
There are the same restriction in the server/workstation world: When you populate all of the RAM banks with RDIMMs, the system will automatically reduce the RAM frequency to lower one to ensure system stability.
 
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r6mile

macrumors 6502a
Original poster
Feb 3, 2010
979
466
London, UK
All of the RAM will run at the slowest speed.

You can actually have lower latency running faster ram at slower speeds, just depends on exactly how the RAM was made.

meaning you may have something like this (made up numbers coming)
CL7 @ 2000MHz
CL5 @ 1400Mhz

so while the clock is slower the latency is less so you may not feel the clock difference, depending on what you're doing.

Might have failed to boot due to 4r4 or maybe one or more of those 'new' DIMMs are bad.

I don't think the RAM was bad - I tried all the sticks separately and they worked. It was just when all were installed that it didn't boot. I am now leaning towards the option of installing 56GB of PC2-5300F RAM, maybe something like the picture below. It has Cas Latency 5.

There are the same restriction in the server/workstation world: When you populate all of the RAM banks with RDIMMs, the system will automatically reduce the RAM frequency to lower one to ensure system stability.

Thank you, I am fully aware of this.
The reason why I was suggestion to keep by FB-800 sticks is not because of the frequency - I know they will downclock to FB-667 - but because they have the big fat Apple-recommended heatsinks. So I thought it is less likely to burn!

But then again, FB-667 memory runs less hot than FB-800, so perhaps what I should do is just sell my RAM (which still commands a premium) and get cheaper server FB-667. This Kingston RAM below looks like it has marginally better cooling than most of the server modules, which look to have cheap slabs of metal and that's it. Anyone have experience of this?

s-l1600-2.jpg
 
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Nguyen Duc Hieu

macrumors 6502
Jul 5, 2020
438
103
Ho Chi Minh City, Vietnam
72C is normal, I think... I have 2 sticks which are always at 85ish degrees.
Where I live, old 2GB FB-DIMM 667 modules are pretty cheap. If you can really sell them for a premium, better do it as soon as possible.
One guy offered me 8x4GB FB-DIMM 667 for 35$, and I'm still thinking about that.
They are mostly considered as PC wastes, as DDR3 RDIMM are flooding the second-hand market here.
 
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mikas

macrumors 6502
Sep 14, 2017
344
138
Finland
Starting cold from 42 °C idling 15 minutes it climbs slowly to 58 °C (those 8GB samsung ones).
Running cinebench and geekbench in a row they heat up to 73+ °C. But those programs are not memory stress tests.

I found the contrary claim about heatsinks: Fat apple ones against industry/enterprise standard heatspreaders:
Apr 10, 2020
 
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