choosin higher number of processes than actual number of cores

alinik · February 26, 2019, 17:45

Hi, all,

What happens if I choose higher number of processes than actual number of cores on my machine to run fluent in parallel?

Thanks,

LuckyTran · February 26, 2019, 19:35

It will launch that many processes. When you click calculate and it is iterating, it'll end up running slower since all the threads have to complete their task. The threads will wait on one another to finish before moving on.

alinik · February 27, 2019, 10:00

Thank you LuckyTran
So the most optimum case would be to have as many threads as the number of cores that I have. Correct?
Because somewhere I heard that the number of cores is actually twice the number of processors. So if I have 8 cores, it means I can launch 16 processes on fluent. By your account, that is not correct. Am I right?

LuckyTran · February 27, 2019, 11:01

Note that what ppl call cpu/core/processor means a lot of things.

If you have a processor die/socket with 8 cores (an oct-core for example) and you have hyperthreading on you will have 8 physical processes + 8 virtual processes = 16 logical processes. So you will have 16 possible processor threads to which tasks can be assigned. You need to know your hardware to know whether hyperthreading is on or not. If you are using windows, go to task manager and performance tab and you can find out how many logical processes you have total. In win7 it shows how many logical processes there are (e.g. 16 if it is 8+8). In win10, it will tell you number of sockets, cores, logical processes.

If you plan to do serious CFD calculations, hyperthreading should be off. But there are various reasons why hyperthreading might be on (for example, you never turned it off, or you cannot turn it off because you're not the admin). If hyperthreading is on and needs to stay on, then you should run on 16 processes to saturate the cpu.

Roh · February 27, 2019, 12:10

I think you can refer to the datasheet or Wiki-page of your processor to know the cores and threads that it handles. e.g.

https://en.wikipedia.org/wiki/Intel_Core

alinik · February 27, 2019, 14:59

LuckTran,
That was the most comprehensive answer to my question. Thank you

Roh,
Thanks for sharing the info. That was very helpful

February 26, 2019, 17:45	choosin higher number of processes than actual number of cores	#1
alinik Senior Member ali Join Date: Oct 2009 Posts: 318 Rep Power: 18	Hi, all, What happens if I choose higher number of processes than actual number of cores on my machine to run fluent in parallel? Thanks,

February 26, 2019, 19:35		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,750 Rep Power: 66	It will launch that many processes. When you click calculate and it is iterating, it'll end up running slower since all the threads have to complete their task. The threads will wait on one another to finish before moving on. racing_student likes this.

February 27, 2019, 11:01		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,750 Rep Power: 66	Note that what ppl call cpu/core/processor means a lot of things. If you have a processor die/socket with 8 cores (an oct-core for example) and you have hyperthreading on you will have 8 physical processes + 8 virtual processes = 16 logical processes. So you will have 16 possible processor threads to which tasks can be assigned. You need to know your hardware to know whether hyperthreading is on or not. If you are using windows, go to task manager and performance tab and you can find out how many logical processes you have total. In win7 it shows how many logical processes there are (e.g. 16 if it is 8+8). In win10, it will tell you number of sockets, cores, logical processes. If you plan to do serious CFD calculations, hyperthreading should be off. But there are various reasons why hyperthreading might be on (for example, you never turned it off, or you cannot turn it off because you're not the admin). If hyperthreading is on and needs to stay on, then you should run on 16 processes to saturate the cpu. alinik and racing_student like this. Last edited by LuckyTran; February 27, 2019 at 14:45.

February 27, 2019, 12:10		#5
Roh Senior Member Join Date: Sep 2017 Posts: 130 Rep Power: 9	I think you can refer to the datasheet or Wiki-page of your processor to know the cores and threads that it handles. e.g. https://en.wikipedia.org/wiki/Intel_Core alinik likes this.

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February 27, 2019, 10:00		#3
alinik Senior Member ali Join Date: Oct 2009 Posts: 318 Rep Power: 18	Thank you LuckyTran So the most optimum case would be to have as many threads as the number of cores that I have. Correct? Because somewhere I heard that the number of cores is actually twice the number of processors. So if I have 8 cores, it means I can launch 16 processes on fluent. By your account, that is not correct. Am I right?

February 27, 2019, 14:59		#6
alinik Senior Member ali Join Date: Oct 2009 Posts: 318 Rep Power: 18	LuckTran, That was the most comprehensive answer to my question. Thank you Roh, Thanks for sharing the info. That was very helpful