[SiemenDiemon]

Hey everyone,

My colleagues and I have noticed that we're only able to run STAR-CCM+ in parallel with a maximum of half the number of cores there are on whatever machine we're using. So, for example, a machine with eight cores could only run STAR-CCM+ in parallel with four cores. Is there some way to change this?

Cheers

[Ana Herrero]

I have the same question! any help??

[cwl]

Originally Posted by SiemenDiemon

Hey everyone,

My colleagues and I have noticed that we're only able to run STAR-CCM+ in parallel with a maximum of half the number of cores there are on whatever machine we're using
Cheers

Are you referring to launching Star-CCM+ itself - or to the performance?

[bluebase]

Originally Posted by SiemenDiemon

Hey everyone,

My colleagues and I have noticed that we're only able to run STAR-CCM+ in parallel with a maximum of half the number of cores there are on whatever machine we're using. So, for example, a machine with eight cores could only run STAR-CCM+ in parallel with four cores. Is there some way to change this?

Cheers

Are you sure that you have 8 physical cores? Or do you have 8 virtual cores and only 4 physical cores. You might need to check the specs for your cpu.

[SiemenDiemon]

Originally Posted by cwl

Are you referring to launching Star-CCM+ itself - or to the performance?

STAR-CCM+ launches and runs but tells me that I've assigned to many cores and defaults to running with some set number of cores (not sure what this is, maybe one). For example if I'm running a virtual machine with 16 cores and I try to run STAR-CCM+ with 10 cores, it will tell me that I've assigned 2 too many cores.

[SiemenDiemon]

Originally Posted by bluebase

Are you sure that you have 8 physical cores? Or do you have 8 virtual cores and only 4 physical cores. You might need to check the specs for your cpu.

I'm running on AWS so can vary the number of virtual cores. I have 4 physical cores on my computer, though I don't see how this should matter. No matter how many cores I assign to my virtual instance, STAR-CCM+ still won't run with any more than half that number of cores. Weird.

[bluebase]

Originally Posted by SiemenDiemon

STAR-CCM+ launches and runs but tells me that I've assigned to many cores and defaults to running with some set number of cores (not sure what this is, maybe one). For example if I'm running a virtual machine with 16 cores and I try to run STAR-CCM+ with 10 cores, it will tell me that I've assigned 2 too many cores.

Could you include the error messages?

[SiemenDiemon]

Originally Posted by bluebase

Could you include the error messages?

The important part of the error is this:

Starting STAR-CCM+ parallel server
************************************************** **********************************
Warning: The host machine [...] has been over-subscribed.
The host has 8 too many processes allocated to it.

Over-subscribing a host can severely affect performance.
************************************************** **********************************

This is from trying to run STAR-CCM+ with 16 cores on a virtual machine with 16 cores.

[Sebastian3232]

Star CCM+ comercial licence mode is per cores, maybe licence you are using is only able to use 4 cores and you need to pay for more capacity, check licence setting.

[bluebase]

Originally Posted by SiemenDiemon

Starting STAR-CCM+ parallel server
************************************************** **********************************
Warning: The host machine [...] has been over-subscribed.
The host has 8 too many processes allocated to it.

Over-subscribing a host can severely affect performance.
************************************************** **********************************

Well, i can confirm this behaviour in StarCCM 2020.1 (and up), It does not want to oversubsribe a host. 2019.1 does not complain obviously.

However, this is just a warning in my case. StarCCM will open anyway with the given number of workers.
I believe this is just a reminder for users, that you should only use the actual number of pyhsical cores per host. Using additional Hyperthreading (virtual) cores and alikes will not increase the performance. In most cases it will actually increase the run time.

So if you are sure, that you have more physical cores than StarCCM recoqnizes, then just go on.

In any case, i suggest to run a brief benchmark on your virtualized system to make sure. Run a simulation with 4,8,16 cores (or something like that) and see for yourself which number of workers result in the fastest simulation for that respective system. I believe, there is also a command-line option for benchmarking the number or cores - have a look in the manual.

Best,
Sebastian

[SiemenDiemon]

Originally Posted by bluebase

Well, i can confirm this behaviour in StarCCM 2020.1 (and up), It does not want to oversubsribe a host. 2019.1 does not complain obviously.

However, this is just a warning in my case. StarCCM will open anyway with the given number of workers.
I believe this is just a reminder for users, that you should only use the actual number of pyhsical cores per host. Using additional Hyperthreading (virtual) cores and alikes will not increase the performance. In most cases it will actually increase the run time.

So if you are sure, that you have more physical cores than StarCCM recoqnizes, then just go on.

In any case, i suggest to run a brief benchmark on your virtualized system to make sure. Run a simulation with 4,8,16 cores (or something like that) and see for yourself which number of workers result in the fastest simulation for that respective system. I believe, there is also a command-line option for benchmarking the number or cores - have a look in the manual.

Best,
Sebastian

Thanks Sebastian! It's good to know that this is in fact just a warning and not a limitation, that's where most of my confusion was.

On your point about using additional cores, are you saying that while it's possible that a simulation may run faster on a system with 16 physical cores than on a system with 8 physical cores, running the simulation with 8 physical cores and 8 virtual cores might actually be slower than just running with 8 physical cores?

Cheers

[bluebase]

Originally Posted by SiemenDiemon

running the simulation with 8 physical cores and 8 virtual cores might actually be slower than just running with 8 physical cores?

Yes, i am pretty sure that is true for the majority of use cases in cfd.


But again, to be certain an easy benchmark can be used.

[trampoCFD]

Hi
I thought I'd clarify a few points:
1/ You cannot access physical and virtual cores at the same time.
Virtual cores is basicaly giving access to only a % of a physical core because of a VM or hyperthreading.
Through a VM, you will only ever have access to a virtual core, that's the whole idea of a VM.
2/Most cloud provider runs a VM with hyperthreading ON. That means that you get < 45% of the performance of a physical core per virtual core.
3/You might find that you get better performance running on 16 virtual cores, depending on your model.
4/STAR-CCM+ detects physical cores in its warning message.
5/trampoCFD.com runs on physical cores, we don't run through a VM. If you'd like to run your simulations fast, we can give you access to more than 150,000 cores that scale down to 10,000 cells per core.
That's sufficient for most users :-)
Please email me gui@trampocfd.com if you'd like to learn more.

Best regards,
gui@trampoCFD