[user421]

What is better for CFD one CPU with 4GHZ or two CPU with 3.5GHz?

What is more important for CFD number of CPUs or clock speed?

[moving-boundaries]

I think what matters more is the memory bandwidth, that is, number of memory channels and supported RAM speed ([memory bandwidth] = 2 x [number of channels] x [RAM speed] x [bus width]). So, if we assume that CPUs in your single and dual setups have similar memory characteristics, then dual CPU would be better because it means twice as many memory channels and, thus, twice as higher memory bandwidth. But if your single CPU is, say, EPYC 9554 (bandwidth ~= 460.8 GB/s) and your dual CPUs are, for example, Xeon Platinum 8284 (bandwidth ~= 2 x 140 GB/s = 280 GB/s), then the single CPU would be better.

[user421]

Originally Posted by moving-boundaries

I think what matters more is the memory bandwidth, that is, number of memory channels and supported RAM speed ([memory bandwidth] = 2 x [number of channels] x [RAM speed] x [bus width]). So, if we assume that CPUs in your single and dual setups have similar memory characteristics, then dual CPU would be better because it means twice as many memory channels and, thus, twice as higher memory bandwidth. But if your single CPU is, say, EPYC 9554 (bandwidth ~= 460.8 GB/s) and your dual CPUs are, for example, Xeon Platinum 8284 (bandwidth ~= 2 x 140 GB/s = 280 GB/s), then the single CPU would be better.

@flotus1

What is better option ?



1x Intel Xeon W-2145 8-core 3.7 / 4.5 GHz,
1x Intel Xeon W-2125 4.0 GHz Quad Core,
1x Intel Xeon Gold 12-core 6136 3.0 / 3.7 GH
or

2x Xeon E5-2678W v4 3.0Ghz 12 core

[moving-boundaries]

1x Intel Xeon W-2145 8-core 3.7 / 4.5 GHz: 4 channels at 2666MHz -> max bandwidth = 85 GB/s
1x Intel Xeon W-2125 4.0 GHz Quad Core: 4 channels at 2666MHz -> max bandwidth = 85 GB/s
1x Intel Xeon Gold 12-core 6136 3.0 / 3.7 GH: 6 channels at 2666MHz -> max bandwidth = 128 GB/s
2x Xeon E5-2678W v4 3.0Ghz 12 core: 2 x 4 channels at 2400MHz -> max bandwidth = 154 GB/s

I would personally go for the 2x Xeon E5-2678W v4 option. But perhaps more experienced users on this forum can also chime in

Note that to use full memory bandwidth you will have to have the same number of RAM sticks as there are channels. So, for 2x Xeon E5-2678W v4 options you would need 8 identical memory sticks plugged in.

[user421]

Originally Posted by moving-boundaries

1x Intel Xeon W-2145 8-core 3.7 / 4.5 GHz: 4 channels at 2666MHz -> max bandwidth = 85 GB/s
1x Intel Xeon W-2125 4.0 GHz Quad Core: 4 channels at 2666MHz -> max bandwidth = 85 GB/s
1x Intel Xeon Gold 12-core 6136 3.0 / 3.7 GH: 6 channels at 2666MHz -> max bandwidth = 128 GB/s
2x Xeon E5-2678W v4 3.0Ghz 12 core: 2 x 4 channels at 2400MHz -> max bandwidth = 154 GB/s

I would personally go for the 2x Xeon E5-2678W v4 option. But perhaps more experienced users on this forum can also chime in

Note that to use full memory bandwidth you will have to have the same number of RAM sticks as there are channels. So, for 2x Xeon E5-2678W v4 options you would need 8 identical memory sticks plugged in.

But if workstation have 16 ram slots and I plug only 8 ram sticks, there will 8 slots empty, is this problem?
What happened if I plug more ram stick then CPU has channels,for example 10 or all 16 ?

Do I need plug in these 8 ram sticks in every slots I wont or must be some order?

[flotus1]

Please refrain from opening new threads, if you feel like you are not getting an answer fast enough. It won't help.

We have two goals when populating memory
1) Fill all available memory channels with at least one DIMM (channels, not slots)
2) Make it a balanced configuration - i.e. identical DIMMs on each memory channnel.

On a motherboard with 2 CPUs, 4 channels per CPU, and a total of 16 DIMM slots:
Each memory channel has two DIMM slots associated to it. We can either use 8 or 16 identical DIMMs to achieve both goals.
With 16 DIMMs it's easy. Just fill all slots.
With 8 DIMMs, one slot for each memory channel needs to be populated. They are often color-coded. But you should definitely look up in the manual which slots to fill. You can't just pick either of them.

Using 10 DIMMs on such a board would result in an unbalanced memory configuration. It usually works in the sense that the system POSTs, and all memory is recognized by the operating system. But if you care about memory performance (which we do for CFD) this has to be avoided.

[user421]

Originally Posted by flotus1

Please refrain from opening new threads, if you feel like you are not getting an answer fast enough. It won't help.

We have two goals when populating memory
1) Fill all available memory channels with at least one DIMM (channels, not slots)
2) Make it a balanced configuration - i.e. identical DIMMs on each memory channnel.

On a motherboard with 2 CPUs, 4 channels per CPU, and a total of 16 DIMM slots:
Each memory channel has two DIMM slots associated to it. We can either use 8 or 16 identical DIMMs to achieve both goals.
With 16 DIMMs it's easy. Just fill all slots.
With 8 DIMMs, one slot for each memory channel needs to be populated. They are often color-coded. But you should definitely look up in the manual which slots to fill. You can't just pick either of them.

Using 10 DIMMs on such a board would result in an unbalanced memory configuration. It usually works in the sense that the system POSTs, and all memory is recognized by the operating system. But if you care about memory performance (which we do for CFD) this has to be avoided.

DIMM is RAM stick? Where can I find diagram of Z840 memory slots?
If someone put 8 sticks in "wrong" slots, will Windows inform me that something is not OK?

Maybe is better to buy first 8 sticks so one day If I want double RAMs than I can buy more 8 sticks..

Quote:

"It is critical that all memory channels in a processor are being populated with equal amounts of memory, otherwise both memory bandwidth and performance will be significantly compromised."

I was thought that number of RAM sticks must be same as CPU memory channels...

What is your answer on my first post, do you agree that 2x Xeon E5-2678W v4 is the fastest option?

So 8x16GB I must plug sticks at 1,5,3,7,8,4,6,2 position?They are all at side of CPU2
https://www.youtube.com/watch?v=Z_wf9sDUrTo
https://www.youtube.com/watch?v=Z_wf9sDUrTo

[flotus1]

DIMM, memory module, RAM "stick"... all the same thing

If someone put 8 sticks in "wrong" slots, will PC inform me that something is not OK?

No. It is your responsibility to look that up beforehand. Either in the manual of the motherboard. Or if it's an OEM workstation, in the manual for the machine.

Maybe is better to buy first 8 sticks so one day If I want double RAMs than I can buy more 8 sticks..

You could do that. But considering how cheap DDR4 memory is these days even when buying new, you might as well buy twice as much RAM now.

What is your answer on my first post, do you agree that 2x Xeon E5-2678W v4 is the fastest option?

Among the options listed, 2x Xeon E5-2687W v4 is the fastest for parallel CFD.

[user421]

Originally Posted by flotus1

No. It is your responsibility to look that up beforehand. Either in the manual of the motherboard. Or if it's an OEM workstation, in the manual for the machine.

for dual CPU Z840, 8x16GB sticks I must plug in slots: 4,8,6,2,3,7,5,1 ?

at 1:25sec
https://www.youtube.com/watch?v=Z_wf9sDUrTo

[flotus1]

Yeah. Same information in the manual, page 68
https://h10032.www1.hp.com/ctg/Manual/c04823811.pdf