Question about RAM speed, don't make sense to me

[Edgar R. Zakarian]

So I have this motherboard

X570S AERO G (rev. 1.x) Specification | Motherboard - GIGABYTE Global

 

Spoiler

 

Using G'skill TridentZ 2x32GB DDR4 4000MHz memory

The motherboard has 4 memory slots.

 

If i place the memory slots like so:

[ ][x] [ ] [x]

I can get the XMP profile to activate and work at 4000MHz.

Spoiler

If i place the memory like so:

[x][ ] [x] [ ]

After activating XMP or trying to increase clock to 4000MHz, the PC will just reboot a few times and reset to 2666MHz and stay there, I could only deactivate and increase it to max 3200MHz until i started resetting to 2666 again..

 

Any ideas as to why that may be? I did try reseating the memory.

Could it just be some small speck of dust that I haven't removed?

Other ideas?

 

Just really wondering.

[Nena Trinity]

 

[Tetras]

It is likely due to the memory topology, and/or BIOS optimisations and is not unusual.

 

 

[Nena Trinity]

Simple answer is A2 & B2 is the fastest since your motherboard has a daisy-chain layout.

[adm0n]

If you want to actually study this look up something like RAM topology.

 

But if you want a super simple analogy, you can always imagine the traces of your motherboard like highways with cars that all go at the same speed (the speed being a stand-in for the frequency). Each car can transport one data package.

 

Since every car goes at the same speed, for everything to work out in the dual channel setup, you have to make sure, that the highways from your CPU to the first memory model is exactly the same length as the distance from your CPU to the second module.

 

In a daisy chain topology (which most motherboards use), the first and second RAM slot and the third and fourth slot are linked after another. Which causes some problems. Because as soon as you insert another RAM stick into one of the daisy chained slots, the cars don't have to drive as fast anymore. To compensate for that everything has to go slower, so the cars that go to the second module can catch up.

 

Since there isn't much the board makers can do to alleviate that problem, they just accept that in case of a 4 DIMM scenario they all go slower. So they only really optimize the 2 DIMM version. And for that they chose the two slots at the end of the signal trace. So if you run the scenario that doesn't work for you, you are likely running into some signal integrity issues and your motherboard falls back into one of the slower JEDEC standards.

 

One option for all 4 slots to have the same distance to the CPU is using a T topology, but since most people run with two DIMMs anyway, daisy chain is more popular and it is also better to optimize just two trace runs instead of four.

[Kilrah]

That's expected, and almost every mobo manual will tell you that if you only use 2 sticks they have to be in the "2" slots.

 

If you put them in the "1" slots then the bus continues to the 2nd slot where it isn't terminated, causing signal reflections that degade integrity.

[FI Fheonix]

1 hour ago, Nena Trinity said:

Simple answer is A2 & B2 is the fastest since your motherboard has a daisy-chain layout.

Is that true for most standard motherboards? (Just wondering)

[Nena Trinity]

45 minutes ago, FI Fheonix said:

Is that true for most standard motherboards? (Just wondering)

Yes 2 & 4 or A2 & B2 as they are called is always faster on daisy chain, on T-Topology all 4 dimms are same.

[Edgar R. Zakarian]

12 hours ago, Nena Trinity said:

Simple answer is A2 & B2 is the fastest since your motherboard has a daisy-chain layout.

But does that mean, if I wanted to have 128gb and added identical 2x32gb g'skill tridentz 4000MHz ram to it, it wouldn't work?
Cause that seems strange, since the motherboards website says it should be able to handle up to 5600MHz overclock. 

[Edgar R. Zakarian]

12 hours ago, Tetras said:

It is likely due to the memory topology, and/or BIOS optimisations and is not unusual.

 

 

Basically two of the 4 slots will SUCK. It clocks significantly worse than the other set. 

WHAT THE HECK.. So I CAN'T Run 4 DIMM's at 4000MHz on this motherboard? Isn't that misleading advertising???

[Edgar R. Zakarian]

12 hours ago, adm0n said:

If you want to actually study this look up something like RAM topology.

 

But if you want a super simple analogy, you can always imagine the traces of your motherboard like highways with cars that all go at the same speed (the speed being a stand-in for the frequency). Each car can transport one data package.

 

Since every car goes at the same speed, for everything to work out in the dual channel setup, you have to make sure, that the highways from your CPU to the first memory model is exactly the same length as the distance from your CPU to the second module.

 

In a daisy chain topology (which most motherboards use), the first and second RAM slot and the third and fourth slot are linked after another. Which causes some problems. Because as soon as you insert another RAM stick into one of the daisy chained slots, the cars don't have to drive as fast anymore. To compensate for that everything has to go slower, so the cars that go to the second module can catch up.

 

Since there isn't much the board makers can do to alleviate that problem, they just accept that in case of a 4 DIMM scenario they all go slower. So they only really optimize the 2 DIMM version. And for that they chose the two slots at the end of the signal trace. So if you run the scenario that doesn't work for you, you are likely running into some signal integrity issues and your motherboard falls back into one of the slower JEDEC standards.

 

One option for all 4 slots to have the same distance to the CPU is using a T topology, but since most people run with two DIMMs anyway, daisy chain is more popular and it is also better to optimize just two trace runs instead of four.

So basically 4 dimms at 4000MHz it not achievable on this motherboard and MANY MANY other motherboards, only reasonably on 2 DIMMS placed in the right slots. 
If I wanna run 4 identical DIMMs the speed is gonna suffer to like 2133 or 2666 something something. 
That the gist of it right?

[Kilrah]

No. The problem from your first post is an empty slot at the end of the bus. If everything's populated there isn't one...

Achievable speeds can go down a little for other reasons with 4 sticks but it won't be all the way down and that'll depend on everything from CPU to board to sticks.

[adm0n]

37 minutes ago, Edgar R. Zakarian said:

So basically 4 dimms at 4000MHz it not achievable on this motherboard and MANY MANY other motherboards, only reasonably on 2 DIMMS placed in the right slots. 
If I wanna run 4 identical DIMMs the speed is gonna suffer to like 2133 or 2666 something something. 
That the gist of it right?

What @Kilrah wrote is correct.

 

 

[Nena Trinity]

2 hours ago, Edgar R. Zakarian said:

But does that mean, if I wanted to have 128gb and added identical 2x32gb g'skill tridentz 4000MHz ram to it, it wouldn't work?
Cause that seems strange, since the motherboards website says it should be able to handle up to 5600MHz overclock. 

This is DDR4, realistically the infinity fabric of Ryzen cannot handle much more than 3800 to 4000, if on Intel you would had the same issue potentially with this type of motherboard.

 

The motherboard is just half of the issue tough the other half is the CPUs memory controller.

[Tetras]

4 hours ago, Edgar R. Zakarian said:

Basically two of the 4 slots will SUCK. It clocks significantly worse than the other set. 

WHAT THE HECK.. So I CAN'T Run 4 DIMM's at 4000MHz on this motherboard? Isn't that misleading advertising???

I have no idea if Gigabyte are right about what memory OC the board can support. because I doubt there are many CPUs capable of running that speed with 4 dual rank (32GB) sticks.

 

AMD's official rating for Zen 3 is:

2x single rank: 3200

2x dual rank: 3200

4x single rank: 2933

4x dual rank: 2666

 

Zen 4 isn't much better with DDR5, it goes from 5200 with 2x single rank sticks, to just 3600 with 4x dual rank sticks.

[Edgar R. Zakarian]

21 hours ago, Tetras said:

I have no idea if Gigabyte are right about what memory OC the board can support. because I doubt there are many CPUs capable of running that speed with 4 dual rank (32GB) sticks.

 

AMD's official rating for Zen 3 is:

2x single rank: 3200

2x dual rank: 3200

4x single rank: 2933

4x dual rank: 2666

 

Zen 4 isn't much better with DDR5, it goes from 5200 with 2x single rank sticks, to just 3600 with 4x dual rank sticks.

Ahhh Thanks very much for the breakdown.