1066Mhz DDR3 Ram down clocked by Mac Pro to 800Mhz!!!

All Taken

macrumors 6502a
Original poster
Dec 28, 2009
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I purchased 3 sticks of 1066Mhz DDR3 ram for my 3.2Ghz Nahalem Mac Pro. The sticks were perfect and have the temperature sensors on board, it's hynix ram and is ECC Registered.

I decided to buy a fourth stick making 16GB 4x4GB - When I installed the fourth stick the Mac down clocked the ram to 800mhz!. Any idea why? or better yet, how to get it running at 1066mhz again?

I have tried using the new stick on its own and it's fine running along at 1066Mhz, if you add a fourth stick with any module being that fourth stick it auto downclocks the ram......

Many Thanks
 

linuxcooldude

macrumors 68020
Mar 1, 2010
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The Mac pro uses a triple channel memory controller that is optimized to use three DIMM's.

Adding the fourth DIMM forces the CPU to split the memory bandwidth between the third and fourth DIMMs, which drops the effective throughput to those DIMMs.

The vast majority of applications don't saturate the memory bandwidth. If more memory is a priority I would not worry about lower memory speed.

To get it back up to 1066Mhz speed would have to remove fourth DIMM and use 8 GIG DIMMS. Not sure if mixing of different size DIMMS in the three channels will effect anything.
 
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Umbongo

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Sep 14, 2006
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First I'd find out if your memory is single rank, dual rank or quad rank. Also check what voltage each DIMM is: 1.35 or 1.5. Check each DIMM is exactly the same in those regards. There are a few ways where memory will run at 800MHz. Unfortunately as registered memory isn't supported on X58 boards (single processor) there is no documentation for it, just for dual socket ones.
 
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cjoy

macrumors member
Oct 24, 2008
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First I'd find out if your memory is single rank, dual rank or quad rank. Also check what voltage each DIMM is: 1.35 or 1.5. Check each DIMM is exactly the same in those regards. There are a few ways where memory will run at 800MHz. Unfortunately as registered memory isn't supported on X58 boards (single processor) there is no documentation for it, just for dual socket ones.
just stumbled over the above statement. :eek:
can someone clarify wether the 2x8GB kit would work in a 2010 quad (2,8)?

Kingston ValueRAM
KVR1066D3Q8R7SK2/16G

DIMM
SDRAM-DDR3 - 1066 (PC3 - 8500)
ECC-Support
Registered
CL 9
240-Pin
1024Mx72
1,5 Volt (1,48 Volt - 1,52 Volt)
Quad Rank x8
Thermal Sensor


also the whole quad-rank thing is not entirely clear to me... will this impact a future upgrade to 24GB (3 modules) of this type?
thanks.
 
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Umbongo

macrumors 601
Sep 14, 2006
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just stumbled over the above statement. :eek:
can someone clarify wether the 2x8GB kit would work in a 2010 quad (2,8)?

Kingston ValueRAM
KVR1066D3Q8R7SK2/16G

DIMM
SDRAM-DDR3 - 1066 (PC3 - 8500)
ECC-Support
Registered
CL 9
240-Pin
1024Mx72
1,5 Volt (1,48 Volt - 1,52 Volt)
Quad Rank x8
Thermal Sensor


also the whole quad-rank thing is not entirely clear to me... will this impact a future upgrade to 24GB (3 modules) of this type?
thanks.
Three quad rank modules will be fine. Adding a fourth would downclock it all to 800MHz.
 
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cjoy

macrumors member
Oct 24, 2008
83
3
Three quad rank modules will be fine. Adding a fourth would downclock it all to 800MHz. Quad rank modules offer no benefit on a Mac Pro and you should have no problem finding dual rank 8GB DIMMs for the same price.
actually the quad ranks are offered 50% cheaper than the identical kingston modules with dual rank... since I have no plans to go beyond 24GB (3x8) on this machine, this is good news.

thank you very much.
 
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Umbongo

macrumors 601
Sep 14, 2006
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actually the quad ranks are offered 50% cheaper than the identical kingston modules with dual rank... since I have no plans to go beyond 24GB (3x8) on this machine, this is good news.

thank you very much.
Yeah edited my post after I checked prices.
 
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handheldgames

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Apr 4, 2009
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That should not be happening.... I am running 4x4GB sticks for 16gb@1066 on a 2009 MacPro Quad.

IF your original set is a triple channel part, you must add an identical part to make it properly function. I had a similar issue adding in a part of OWC that was not matched, except my MacPro would not even boot up. Once I had the right chip, I was running just fine.

Based on the benchmarks, the benefits in triple channel vs dual channel are so insignificant, it was more of marketing hype than anything that equates to a measurable boost in the user experience.
 

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cjoy

macrumors member
Oct 24, 2008
83
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That should not be happening.... I am running 4x4GB sticks for 16gb@1066 on a 2009 MacPro Quad.

IF your original set is a triple channel part, you must add an identical part to make it properly function. I had a similar issue adding in a part of OWC that was not matched, except my MacPro would not even boot up. Once I had the right chip, I was running just fine.

Based on the benchmarks, the benefits in triple channel vs dual channel are so insignificant, it was more of marketing hype than anything that equates to a measurable boost in the user experience.

keep in mind this is not about tripple/dual channel, but the type of memory itself. there have been conflicting statements concerning certain module types (quad-rank, registred) - this has nothing to do with dual-channel or triple-channel support.
 
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handheldgames

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keep in mind this is not about tripple/dual channel, but the type of memory itself. there have been conflicting statements concerning certain module types (quad-rank, registred) - this has nothing to do with dual-channel or triple-channel support.
Exactly... But if you have ram rated for triple channel, you must get a 4th chip of the same type for it to properly work in the 2009 MacPro. This is probably why the RAM is slowing down - I'd call it lucky that it even works.
 
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cjoy

macrumors member
Oct 24, 2008
83
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ram itself is not "triple-channel".. thats just marketing dust on the kits they sell. as long as the rank-type and size are equal, you can mix any ram from any brand you like and maintain triple channel support.. thats a chipset feature and not bound to certain ram modules.

if, however, you add a fourth module the triple channel access to ram is no longer supported, as #3 and #4 have to share a lane... but again, this happens also when adding a module from a ram kit labled "triple channel bla kit"
 
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Umbongo

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Sep 14, 2006
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ram itself is not "triple-channel".. thats just marketing dust on the kits they sell. as long as the rank-type and size are equal, you can mix any ram from any brand you like and maintain triple channel support.. thats a chipset feature and not bound to certain ram modules.

if, however, you add a fourth module the triple channel access to ram is no longer supported, as #3 and #4 have to share a lane... but again, this happens also when adding a module from a ram kit labled "triple channel bla kit"
Using quad-ranked DIMMs while also using more than one slot on any channel will see speed drop to 800MHz. This is a technical limitation of the platform. Doesn't matter if they are matched, or if you have 4, 5 or 6 DIMMs on the memory branch.
 
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cjoy

macrumors member
Oct 24, 2008
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Using quad-ranked DIMMs while also using more than one slot on any channel will see speed drop to 800MHz. This is a technical limitation of the platform. Doesn't matter if they are matched, or if you have 4, 5 or 6 DIMMs on the memory branch.
does that mean using 2 quad-rank modules in a nehalem quad would result in the mentioned speed drop?

thanks,
cj
 
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Umbongo

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Sep 14, 2006
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does that mean using 2 quad-rank modules in a nehalem quad would result in the mentioned speed drop?

thanks,
cj
It is only when you have more than 1 on any channel. So with only 2 you'd have one on channel 1 and one on channel 2. No problem here. Really it's only when using 4 or 8 on Mac Pros that there should be any issue.
 
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Umbongo

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So what ram modules should I use if I want to go 24gb (6x8) in a 2009? Can someone specify some models?

-Thanks
KVR1066D3Q8R7SK3/24GI - That is a 3x8GB kit of quad ranked 1066MHz registered DIMMs with the thermal sensor. Goes for about €400. I can't say that anyone has tested it, but there is no technical reason why it won't work, Mac Pros aren't any more picky than other Intel systems and you can just return it using the EU distance selling laws (within 7 days) if there was any problem. Crucial are like twice that price.
 
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handheldgames

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So what ram modules should I use if I want to go 24gb (6x8) in a 2009? Can someone specify some models?

-Thanks
Just head over to macsales.com (other world computing) They have a great interface for selecting ram for the Mac Pro, it's that simple....

Btw... Im running all 4 slots at 1066, not 800.
 

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Umbongo

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Sep 14, 2006
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Just head over to macsales.com (other world computing) They have a great interface for selecting ram for the Mac Pro, it's that simple....
OWC are great, but if you only plan on using 3 or 6 memory slots you can save 33% on the cost of 8GB DIMMs by going quad-ranked. 48GB of Kingston memory would be €800 vs around €1250 imported to Spain from OWC.

Btw... Im running all 4 slots at 1066, not 800.
:) All of the memory branded as suitable for the Mac Pro will be dual or single rank so shouldn't have issues. This only applies to those who want to save money by getting non-certified memory.
 
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MacMolly

macrumors newbie
Feb 10, 2008
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Safe to MIX?

My first Mac Pro Quad has always run with 4 x 4 @1066 Mhz. About a week ago, I picked up a large amount of Hynix 4 GB chips and yesterday a 2nd Mac Pro arrived. I popped in 4 of the new sticks and sure enough, everything dropped to 800 mhz and back to 1066 mhz when I removed the 4th stick. I spent half a day (literally) trying to figure out what was wrong before I stumbled upon this post.

I figured out on my own that the sharing of the 3rd & 4th sockets with "some" RAM brought everything down to 800mhz. But I also found out that "apparently" I could use 2 sticks of the "new" RAM in slots 1 & 2 and 2 sticks of the "old" RAM in slots 3 & 4 and end up with 1066mhz registering.

Two questions:

1) Is the RAM that down regulates to 800 Mhz called "quad core", and, if so, what type of RAM doesn't do that?

2) Is it ok (wise) to mix and match to get to 1066 Mhz. Seems like a cheaper alternative to use a 50/50 split.
 
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Umbongo

macrumors 601
Sep 14, 2006
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Two questions:

1) Is the RAM that down regulates to 800 Mhz called "quad core", and, if so, what type of RAM doesn't do that?
It is "quad-ranked". Other types are dual-rank and single-rank. You may see it as SR, DR or QR withing a model name or list of specs.


2) Is it ok (wise) to mix and match to get to 1066 Mhz. Seems like a cheaper alternative to use a 50/50 split.
If it works, it works.

My guess would be that it works on the Mac Pro because there is nothing put it place in the EFI to handle the situation so if it isn't going over the number of memory ranks the memory controller can access at 1066MHz then it won't downclock. On other boards the limit appears to be 12 ranks at 1066MHz. Either 3xQR or 6xDR. They likely set it in the BIOS that using QR and DR together drops it to 800MHz. 2xQR and 2xDR of course is 12 ranks total on a Mac Pro.
 
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Daud

macrumors regular
Oct 23, 2008
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Low cost options for 2009 2.66 Nehalem Quad

Before getting this MP, I am trying to estimate the immediate additional cost and one of course is memory, as it "boasts" meager 3GB (3x1GB).

Based of the info above, if I get the original memory (e.g. from somebody upgrading), then having 4x1GB, the clock speed will be maintained but the memory will be accessed on two channels only, correct ?

Or, I can remove 1 stick and get two 3GB ones, for an arrangement
2x1 and 2x3 for a total of 8GB. Not certain whether this would impair
something..
(8GB should be enough for me)
 
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Umbongo

macrumors 601
Sep 14, 2006
4,934
54
England
Before getting this MP, I am trying to estimate the immediate additional cost and one of course is memory, as it "boasts" meager 3GB (3x1GB).

Based of the info above, if I get the original memory (e.g. from somebody upgrading), then having 4x1GB, the clock speed will be maintained but the memory will be accessed on two channels only, correct ?

Or, I can remove 1 stick and get two 3GB ones, for an arrangement
2x1 and 2x3 for a total of 8GB. Not certain whether this would impair
something..
(8GB should be enough for me)
You can't get 3GB DIMMs.

If you are only mixing 1GB and 2GB DIMMs that are branded for use with the Mac Pro then you will have no issues really, mix and match as much as you want.
 
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MacMolly

macrumors newbie
Feb 10, 2008
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SF BAY area
Great Explanation

It is "quad-ranked". Other types are dual-rank and single-rank. You may see it as SR, DR or QR withing a model name or list of specs.




If it works, it works.

My guess would be that it works on the Mac Pro because there is nothing put it place in the EFI to handle the situation so if it isn't going over the number of memory ranks the memory controller can access at 1066MHz then it won't downclock. On other boards the limit appears to be 12 ranks at 1066MHz. Either 3xQR or 6xDR. They likely set it in the BIOS that using QR and DR together drops it to 800MHz. 2xQR and 2xDR of course is 12 ranks total on a Mac Pro.
I want to say thank you for posting such a clear and concise explanation for WHY this is happening and how to avoid it in the future. I've posted to other forums and NOBODY has come even close to explaining or even suggesting the "12" Rank maximum to maintain full bandwith. What this means to me next time I buy memory is that I'll need to know the "RANK" of the memory in order to be sure I can get optimal bandwith.

Kudos, sir! I really appreciate it!
 
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