[JohHaas]

Hello,

I am currently planning a new workstation for CFD calculations in ANSYS Fluent 15.0.
I recently changed my employer and have to take care of things like licenses and hardware myself, whereas in the past I had a colleague managing all these things.
Concerning my budget, I will be able to get an ANSYS Fluent license (Solver and Pre/Post split) and one additional HPC-Pack-License which supports up to 8 parallel processes for a single case (I hope that's right so far).
From what I always thought "8 parallel processes" means that you can run ONE CASE on up to EIGHT CORES at the same time. Instead my colleague told me that it means that you can run ONE CASE on up to EIGHT THREADS at the same time (which would mean that I would not necessarily need an 8-Core processor to utilize my 8 HPC-Pack-License to the full).

So, which of the two statements is right? (or are they both right/wrong?!)

It would be nice if someone could shed some light on this topic for me, since I need to get my hardware together in the next few weeks and need to decide which CPU to get.

Thanks in advance,
Johannes

[gfoam]

Hi, for what I know 8 CORES or 8 THREADS is the same for one HPC pack, but if you want more speed some people recomends that 8 CORES is better than 8 THRADS (I don't know because there is a white paper who states otherwise). But if you want to run a case and in the same time do Post-Processing is better to have a machine with more than 8 cores. I hope this helps you. regards.
Gonzalo

[sbaffini]

Dear Johannes,

Confusion is the first step when approaching the Licensing policy for parallel Fluent, then comes anger and denial

Coming back to your matter, according to my knowledge, Fluent has separate licensing approaches for HPC or non HPC cases, where HPC stands for parallel computations.

Let however assume that you simply want N licenses (non HPC case). In this case, Fluent does not care what machine you are using. You can launch a parallel computation with N processes on a single core machine (i usually test the license installation by launching, not running, a parallel Fluent with up to hundreds processes on a single core machine), or a machine with up to N cores, or a machine with up to N/2 cores in hyperthreading. This is just because nowadays the threads are managed by the O.S. Fluent only checks that you have up to the licensed N processes, not more. You can basically launch N processes on any kind of machine you can think of.


With a basic non-HPC license with N processes, you can also launch N different serial instances of fluent working on different cases.

HPC licenses are exactly the same, they do not check the hardware and you can launch your parallel case on any given machine. However, but i'm not sure, HPC licenses might have limitations in the number of separate fluent instances you can launch. That is, you can launch a single parallel run with N processes but not N serial runs.

[JohHaas]

Hello,

Thanks for your responses!

You are definitely right, Paolo. HPC-Licenses can only be used as a pack. So you can not run 8 serial cases but only 1 parallel case. That's why the 8 "standard parallel" licenses are more expensive than an HPC-Pack (which is by default 8 parallel processes). HPC-Packs stack differently, though. If you buy 2 HPC-Packs you can either run 2 parallel cases in 8x parallel or run 1 case in 32x parallel. 3 packs stacked together equal 124x parallel and so on...
But since my resources are limited I can only manage to get on pack ;-)

So, from your replies I can guess that my question concerning the right hardware cant be answered that easily. I guess I will have to give the forum's hardware-section a try!

Thanks a lot for the answers, maybe I will see you in the hardware-section ;-)

Best regards,
Johannes

[sbaffini]

What i meant to say is that you don't have to worry about the hardware, in the sense that, of course, the more processors you get the best it is. There are no consequences for your license.

Let's say you get an 8x HPC pack. You can run an 8-process parallel run on a single core machine (don't do that!), you can run on an 8-core/2-processor machine or, if you can afford it, you can run on a node with 8 physical processors. The license only gives you the maximum number of Fluent processes. They can be allocated on nearly all the hardware.

[Kokemoor]

sbaffini has it right here, but to tie it a little more closely to your original phrasing, the HPC Pack gives you 8 threads. If you try put 8 threads on 4 cores, at best it will work the same as if you did 4 threads, but it will probably be slower and possibly not work at all. There's no benefit to having more threads than cores, so to get the most out of the Pack, you'll want 8 cores. This could be a single 8 core processor, two 4 core processors, two 4 core networked machines, etc.

[diamondx]

SBAFFINI is absolutely right !

HPC stands for parallel computations

I think it's high performance computations

[JohHaas]

Hello everyone,

Thanks for your help!

So I guess I was initially right here. Just one more question:
I asked my hardware provider to configure a workstation with 2x Xeon E5-2637V2 as CPUs, which makes it 2 x 4 x 3.5 GHz. My provider keeps telling me that it would not really make sense to go for 2 quadcores and suggests me to go for 2 hexacore cpus with lower clock speed (Xeon E5-2630V2 with only 2.6 GHz) instead.
His explanation is that 12 cores in partial load would outperform 8 cores in full load even if they have a higher clockspeed (and only 8 cores can be adressed license-wise, if I got that right from what you have said here). Is that true? I am pretty wary about what he is saying.

Thanks in advance!
Johannes

[sbaffini]

In theory you get more cores, so you can still use your machine while using Fluent, which is not bad. Also, this machines usually are dimensioned for using all the cores but not for CFD. This means that you are less likely to incur in cache misses for a hexacore than for a quadcore.

[JohHaas]

Well, the thing about more cores giving you more performance outside of fluent while it's solving makes sense to me. But if I was to run calculations over the weekend and I would not need to work on the computer while fluent is solving, would it be smarter to go for 8 x 3.4 GHz or 12 x 2.6 GHz (just based on fluent's calculation time). I guess that I will need to squeeze every tiny bit of solving performance out of the hardware since I am limited to only 8 licenses.

Best regards,
Johannes

[sbaffini]

It really depends on the specific CPU and when cache misses start to become an issue. Usually, if you have more cores you also get more cache, which means that the same 8 processes will work faster when occupying 4/6 cores per CPU than two full 4/4 CPUs.

Is the cache miss more important than the CPU speed? It is if you get into cache misses. In practice, it also depends from how large are your expected cases.

However, the actual clock speed is not usually a limiting factor, to the extent that hyperthreading is possible. Hence, i would certainly go hexacore.