X299 Tomahawk AC: Cannot fully utilize 2x16 GB memory

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May 8, 2022
Messages
3
Hello everyone,

Issue (short):
Recently, I have bought a box set of Corsair VENGEANCE LPX DDR4 3600MHz 32GB (2x16GB) RAM (P/N:CMK32GX4M2D3600C18) which cannot be fully utilized

Original Hardware
-MotherBoard:
X299 Tomahawk AC (with latest bios 1G1)
BIOS default configuration, no overclocking

-CPU:
Intel Core i7-7800X

-RAM:
ADATA DDR4 2400MHz 8GB (1 Stick) CL 17 (P/N: AD4U240038G17-B)

Recently bought Hardware

-RAM:
Corsair VENGEANCE LPX DDR4 3600MHz 32GB (2x16GB kit) (P/N:CMK32GX4M2D3600C18) C18

Issue (Detail):
Install new bought Corsair DDR4 3600 2x16GB ram kit to DIMM slot B1 and A1 of X299 Tomahawk AC MB respectively and my original ADATA DDR4 2400 8GB is installed to C1 slot. However, it runs at 2133MHz only and only 24576MB (24GB) can be used (instead of 40GB).

-see screen attachment MSI_SnapShot1.jpg

Both ADATA and Corsair DDR4 RAM sticks are well detected

-see screen attachment MSI_SnapShot2.jpg

I have tried to remove ADATA RAM stick from C1 before and Corsair RAM kit are still keeping on B1 and A1, it is same, BIOS also shows memory runs at 2133MHz and 16384MB (16GB) can be used only. If Corsair RAM Kit are installed on C1 and A1 according to manual guide line respectively, (no ADATA), it cannot be booted up after power on (no BIOS logo and none of EZ debug LED is on). All memory can be well detected in different cases.

Questions:
Do i miss some configuration? Or my MB have an issue? Is there a way to utilize both ADATA DDR4 2400 8GB and Corsair DDR4 3600 32GB (2x16GB) RAM kit to max. up memory available to 40GB? Can someone help me please? Thank you.
 

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citay

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You're running a quad-channel-memory platform in a dual-channel setup with three modules. This is bad on several levels.

1) The dual-channel operation, even though the mainboard and your CPU are quad-channel-capable, leaves significant memory performance on the table. Especially thruput, which is basically half of what it could achieve. You should equip it with a quad-channel memory kit, like 4x8GB.

2) Mixing RAM of different capacity and models is not good at all. The memory system has to find a compromise between an old DDR4-2400 module and brand new DDR4-3600 modules, which is bound to fail. As a result, speeds are limited to what the old module can handle at maximum, but actually you get a lot more problems, since the memory controllers gets massively confused with such a configuration. Mixing different RAM is like putting different sized wheels on your car. Sometimes it might work kind of alright, but other times everything will fall apart when you try to go fast.

3) Using three modules: This is not good, neither on a dual- nor on a quad-channel platform. The third module will be addressed in single-channel mode, while simultaneously ruining the memory system's electrical properties and making everything work at whatever the slowest module's specification is.

4) As i said, for 32 GB total, you want 4x 8GB, to make use of the quad-channel architecture. You also want to check if the RAM kit in question is on the QVL at MSI. For quad-channel platforms, this is more important than usual. Ideally, you want to get slightly older RAM that perhaps was already available when the board was still sold, this way, it's optimally supported by the BIOS.

5) See my thread about RAM for some more explanations, even if some don't apply to your board (as i only talk about dual-channel boards there). But some of the more general explanations about single- vs. dual-rank modules might interest you. It depends on your individual CPU's IMC (integrated memory controller) how well it deals with dual-rank modules at this relatively high frequency of DDR4-3600. So this is another reason why 4x 8GB might be better: Not only will this utilize all memory channels, but 8GB modules are also pretty much guaranteed single-rank, which makes it easier on the IMC. It might be worth restricting yourself to 4x8GB DDR4-3200.

Observe the population rule with four modules:

Screenshot 2022-05-08 at 17-37-33 E7B05v1.2.pdf.png


In conclusion: I'm afraid you made the wrong purchase with your kit of 2x 16GB. Your mainboard is quad-channel, your CPU is quad-channel, so you have to get a kit of four, otherwise you might as well not have gotten this platform to begin with. One of the main advantages is quad-channel memory capability.

You cannot re-use your old 1x 8GB. It has no place together with whatever new RAM you buy. I would return the 2x16 kit and get 4x8GB, and sell your old 1x8GB or give it away. This old 1x8GB will do nothing but cause problems and slow everything down. As for the 2x16GB kit, it is not the right fit for this system, it would only be good for a dual-channel system.

P.S. Why are you writing with such a small font?
 
Last edited:
Joined
May 8, 2022
Messages
3
You're running a quad-channel-memory platform in a dual-channel setup with three modules. This is bad on several levels.

1) The dual-channel operation, even though the mainboard and your CPU are quad-channel-capable, leaves significant memory performance on the table. Especially thruput, which is basically half of what it could achieve. You should equip it with a quad-channel memory kit, like 4x8GB.

2) Mixing RAM of different capacity and models is not good at all. The memory system has to find a compromise between an old DDR4-2400 module and brand new DDR4-3600 modules, which is bound to fail. As a result, speeds are limited to what the old module can handle at maximum, but actually you get a lot more problems, since the memory controllers gets massively confused with such a configuration. Mixing different RAM is like putting different sized wheels on your car. Sometimes it might work kind of alright, but other times everything will fall apart when you try to go fast.

3) Using three modules: This is not good, neither on a dual- nor on a quad-channel platform. The third module will be addressed in single-channel mode, while simultaneously ruining the memory system's electrical properties and making everything work at whatever the slowest module's specification is.

4) As i said, for 32 GB total, you want 4x 8GB, to make use of the quad-channel architecture. You also want to check if the RAM kit in question is on the QVL at MSI. For quad-channel platforms, this is more important than usual. Ideally, you want to get slightly older RAM that perhaps was already available when the board was still sold, this way, it's optimally supported by the BIOS.

5) See my thread about RAM for some more explanations, even if some don't apply to your board (as i only talk about dual-channel boards there). But some of the more general explanations about single- vs. dual-rank modules might interest you. It depends on your individual CPU's IMC (integrated memory controller) how well it deals with dual-rank modules at this relatively high frequency of DDR4-3600. So this is another reason why 4x 8GB might be better: Not only will this utilize all memory channels, but 8GB modules are also pretty much guaranteed single-rank, which makes it easier on the IMC. It might be worth restricting yourself to 4x8GB DDR4-3200.

Observe the population rule with four modules:

View attachment 159595

In conclusion: I'm afraid you made the wrong purchase with your kit of 2x 16GB. Your mainboard is quad-channel, your CPU is quad-channel, so you have to get a kit of four, otherwise you might as well not have gotten this platform to begin with. One of the main advantages is quad-channel memory capability.

You cannot re-use your old 1x 8GB. It has no place together with whatever new RAM you buy. I would return the 2x16 kit and get 4x8GB, and sell your old 1x8GB or give it away. This old 1x8GB will do nothing but cause problems and slow everything down. As for the 2x16GB kit, it is not the right fit for this system, it would only be good for a dual-channel system.

P.S. Why are you writing with such a small font?
Thank you Citay, your explanation is very detail. And I have read your thread about RAM. It makes me have a clear picture of my situation. From the thread, that means my case should be replaced with 4 x 8GB DDR4 2400 (i7-7800X only support up to DDR4-2400)to reach max. attainable frequency rather than 2 modules or single module due to my MB and CPU are Quad channel architecture and would not cause bios to enable divider for memory controller (If i 'm right)

P.S) Sorry for difficult reading of my last post, hope it would be better this time. :-)
 

citay

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Joined
Oct 12, 2016
Messages
7,802
Yes, for the most part that's right. Two slight corrections:

Official RAM speed support of the CPU is always conservative. They are doing this for liability/support reasons. In reality, the CPUs will do more than what Intel lists, often way more. Even XMP, Extreme Memory Profile, was developed by Intel, so you see that they support this, but unofficially. I would say, you can get a DDR4-3200 kit (4x 8GB), preferably one from the QVL list for the RAM, and that should usually work even at DDR4-3200. It will boot with DDR4-2133 or -2400, then when you enable XMP, it goes to -3200. Even if it doesn't work at -3200, you can manually set -3000, -2800, -2666...

Secondly, the divider for the memory controller is only a factor in the newest CPU generations, Intel 11th gen and 12th gen. Your Skylake-X CPU has a conventional memory controller that doesn't need a divider. So the restriction to DDR4-3200 that i suggested was only for better compatibilty and chance to work, not to avoid any divider.
 
Joined
May 8, 2022
Messages
3
Yes, for the most part that's right. Two slight corrections:

Official RAM speed support of the CPU is always conservative. They are doing this for liability/support reasons. In reality, the CPUs will do more than what Intel lists, often way more. Even XMP, Extreme Memory Profile, was developed by Intel, so you see that they support this, but unofficially. I would say, you can get a DDR4-3200 kit (4x 8GB), preferably one from the QVL list for the RAM, and that should usually work even at DDR4-3200. It will boot with DDR4-2133 or -2400, then when you enable XMP, it goes to -3200. Even if it doesn't work at -3200, you can manually set -3000, -2800, -2666...

Secondly, the divider for the memory controller is only a factor in the newest CPU generations, Intel 11th gen and 12th gen. Your Skylake-X CPU has a conventional memory controller that doesn't need a divider. So the restriction to DDR4-3200 that i suggested was only for better compatibilty and chance to work, not to avoid any divider.
Got it, thanks your correction(y)
 
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