CFD Workstation - RAM size & CORES number

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  • Last Post 09 July 2019
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Rosi posted this 26 June 2019

Hi!

      I need to select a good workstation for CFD modeling in ANSYS for my Lab to buy. 

I understand that there's 25-40% decrease in simulation performances if all memory slots are not filled with the same amount of memory. - The specs of a DELL workstation option are listed below. I have 2 quick questions related to RAM choice:

1. Are 64GB (8x8GB) or 96GB (4x16GB + 4x8GB additional sticks) valid options? 

Or is it worth to consider reducing the cores number (for instance, 8C instead of 10C) and invest in larger RAM (8x16GB=128GB, for instance)?

2. Do I really need 192GB of RAM to match the minimum memory requirement?

 

Budget is ~ $6-7k. The workstation is needed for time-dependent, rigid-wall simulations (2M element mesh). As reference, I used to run these simulations on *2 CPU of 8 cores each (DELL Intel Xeon CPU 2650 0 @ 2.0 GHz), 64 GB of RAM, and 2 HHDs of 2TB each*. - The simulation would run for 2-3cycles in 4-6days.

Thanks a lot in advance for your help!

____________________________________

Full specs (DELL Precision 5820 Tower):

Intel Xeon W-2115 (3.3GHz, 4.5 GHz Turbo, 10 Cores, 13.75MB Cache)

64-96 GB RAM

INVIDIA video card P4000 (8GB, 4 Displayport 1.4)

Operating System: Windows 10 Pro

Precision 5820 Tower 950 Chassis

Hard Drive Controllers: Integrated Intel AHCI SATA chipset controller (8x 6.0Gb/s), SW RAID 0,1,5,10

Hard Drive: 2.5” 512GB SATA SSD

2nd Hard Drive: 2.5” 1T SATA SSD

3rd Hard Drive: 2.5” 1T SATA SSD

 

 

*edited for clarity - apologize for confusion*

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rwoolhou posted this 26 June 2019

I've moved to installation as it'll get more attention. On physics support we're more likely be responsible for breaking computers...... 

I'd aim for around 4-6GB per core for most CFD applications (should be sufficient for around 2 million cells) : if you need more RAM the load on each core will be high so more parallel makes sense for speed. If you expect to build really big models for use on a cluster a high RAM machine is very useful. It depends on how big a model you're expecting to build, what physics you want to run and how much you can/want to spend. 

 

Rosi posted this 26 June 2019

Okay, thanks. The budget is ~ $6-7k. And the workstation is needed for time-dependent simulations (fluid hypotheses: newtonian, laminar flow regime), under rigid-wall conditions. The number of mesh elements might go up to 4-5 million, but I don't think we'll run models much more complex than this. 

 

From your answer, I understand that 64GB (8x8GB) on that DELL Precision 5820 Tower might be sufficient for our scope (with 8-10 cores), and if we want to get larger RAM we might prefer parallel processors. Is this correct?

The goal is to have simulation time down to 1-2days.. Would that be the case with 64GB?

Thank you in advance for your help!

 

rwoolhou posted this 27 June 2019

For 4-5M cells 64GB will be plenty. 

Can't comment on the speed, but steady should run well with that configuration. You'll see a benefit with more cores too: typically you get good speed up down to 50-100k cells per core. 

Rosi posted this 03 July 2019

Thanks.. Just wanted to highlight that I'd run transient CFD simulations.

My previous workstation actually had  2 CPU of 8 cores each, 64GB of RAM, and 2 HHDs of 2TB each, and on this workstation similar simulations would run for 4-6 days.

My goal is to buy a new workstation to run faster simulations (ideally simulation time below 2 days).

 

Any suggestions on whether I should prefer 2 CPU, increase the number of cores and/or RAM size for the new workstation to achieve that goal?

 Thanks for your help!

Rosi posted this 09 July 2019

For anyone interested in a solution for this post, after a little more research, I opted for a DELL T7820 2-CPU (6 memory slots/CPU):

• DUAL INTEL XEON SILVER (2.1GHz, 3.0GHz Turbo, 8 Cores, 11MB Cache) 

• NVIDIA video card P4000 (8GB, 4DP)

• 2 2TB-SSDs

• RAM size: either 96GB or 192GB  (~ $7k vs. $10k)

 

Main concepts I took in account for final choice are:

1. Parallel cores improve performances, so I opted for 2 CPU (8 Cores each).

2. Having all memory slots filled with the same memory maximizes simulation performances (otherwise 25-40% decrease in performances has been observed) - The DELL T7820 is a great option because it has only 6 memory slots/CPU (instead of 8 or 12), so they are easy to fill with the amount of memory needed

3. RAM: 96GB should be more than sufficient for our scope, 192GB has been suggested by the ANSYS performance team. - I wonder if the simulation time would actually decrease by a factor of 2 with 192GB for this 8-Core DELL T7820 2-CPU.

4. A minimum of P4000 Graphic card (8GB GDDR-VRAM) has been recommended.

 

A big thank to both the ANSYS team and the DELL customers service, who have been of great help!

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