Turning on Hyperthreading for meshing and setting only physical cores for simulation

AidealZohary · October 23, 2019, 06:38

Hi guys,

Im having 4 physical cores and 8 virtual cores. I read that HT is not recommended for ansys simulations. Unfortunately i cant turn it of because i dont have the option for this in my BIOS and this i believe is the same case for many out there who accidentally turned on HT.

However, if i set 4 cores for parallel simulation in fluent, will this be equivalent to HT turned off?

LuckyTran · October 23, 2019, 08:53

Run 8 to be safe and not think about it. 4 works too, but if you run on 4, then the nuances of being hyperthreaded will come up from time to time. You'll need to develop a habit of checking your system performance.

Before giving up, check if there are any bios updates for your system. Lenovo was notorious for disabling the option to turn off hyperthreading by locking the user out of the option in the Bios. The company's trademark response was "no one would ever need it." Then Meltdown & Spectre were revealed...

flotus1 · October 23, 2019, 08:53

As long as you don't oversubscribe the cores (i.e. running more threads than physical cores), you should be ok.
One of the main problems with SMT was when operating systems did not properly distinguish between cores and hyperthreads, e.g. assigning two threads to the same physical core. This should be a thing of the past, unless you use bleeding edge hardware or ancient operating systems.
There can still be a small difference in performance between SMT on/off, but it is usually nothing to write home about.

AidealZohary · October 23, 2019, 09:11

Hi LuckyTran,

Thanks for the reply. Say if i have the opportunity to disable HT in the future. Should i go for it or it does not actually make much difference? or maybe turn on HT for meshing and then turn it off again for Fluent

AidealZohary · October 23, 2019, 09:12

Thanks Flotus1!

That was a great relief

LuckyTran · October 23, 2019, 09:33

It makes almost no sense to turn hyperthreading on for meshing and then off for simulating. You have to restart your system each time you do this. Either you have HT on or off.

As long as you keep your cores properly loaded (running always on 4 or 8 and not 5/6/7/9+) the difference is somewhere between 0%-20% (more likely <10%). It's hard to say whether you should run on 4 or 8 because it depends on a lot of things. You need a stopwatch.

AidealZohary · October 23, 2019, 09:49

I see, okay thanks a lot. At first why i thought of turning on HT for meshing was because maybe i can get faster meshing when decreasing the mesh size and then since fluent simulation prefers HT off that would justify the latter. Anyways the question is answered. Thanks again

October 23, 2019, 06:38	Turning on Hyperthreading for meshing and setting only physical cores for simulation	#1
AidealZohary New Member Aideal Zohary Join Date: Feb 2019 Location: Malaysia Posts: 28 Rep Power: 7	Hi guys, Im having 4 physical cores and 8 virtual cores. I read that HT is not recommended for ansys simulations. Unfortunately i cant turn it of because i dont have the option for this in my BIOS and this i believe is the same case for many out there who accidentally turned on HT. However, if i set 4 cores for parallel simulation in fluent, will this be equivalent to HT turned off?

October 23, 2019, 08:53		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,762 Rep Power: 66	Run 8 to be safe and not think about it. 4 works too, but if you run on 4, then the nuances of being hyperthreaded will come up from time to time. You'll need to develop a habit of checking your system performance. Before giving up, check if there are any bios updates for your system. Lenovo was notorious for disabling the option to turn off hyperthreading by locking the user out of the option in the Bios. The company's trademark response was "no one would ever need it." Then Meltdown & Spectre were revealed... AidealZohary likes this.

October 23, 2019, 08:53		#3
flotus1 Super Moderator Alex Join Date: Jun 2012 Location: Germany Posts: 3,428 Rep Power: 49	As long as you don't oversubscribe the cores (i.e. running more threads than physical cores), you should be ok. One of the main problems with SMT was when operating systems did not properly distinguish between cores and hyperthreads, e.g. assigning two threads to the same physical core. This should be a thing of the past, unless you use bleeding edge hardware or ancient operating systems. There can still be a small difference in performance between SMT on/off, but it is usually nothing to write home about. AidealZohary likes this.

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October 23, 2019, 09:11		#4
AidealZohary New Member Aideal Zohary Join Date: Feb 2019 Location: Malaysia Posts: 28 Rep Power: 7	Hi LuckyTran, Thanks for the reply. Say if i have the opportunity to disable HT in the future. Should i go for it or it does not actually make much difference? or maybe turn on HT for meshing and then turn it off again for Fluent

October 23, 2019, 09:12		#5
AidealZohary New Member Aideal Zohary Join Date: Feb 2019 Location: Malaysia Posts: 28 Rep Power: 7	Thanks Flotus1! That was a great relief

October 23, 2019, 09:33		#6
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,762 Rep Power: 66	It makes almost no sense to turn hyperthreading on for meshing and then off for simulating. You have to restart your system each time you do this. Either you have HT on or off. As long as you keep your cores properly loaded (running always on 4 or 8 and not 5/6/7/9+) the difference is somewhere between 0%-20% (more likely <10%). It's hard to say whether you should run on 4 or 8 because it depends on a lot of things. You need a stopwatch.

October 23, 2019, 09:49		#7
AidealZohary New Member Aideal Zohary Join Date: Feb 2019 Location: Malaysia Posts: 28 Rep Power: 7	I see, okay thanks a lot. At first why i thought of turning on HT for meshing was because maybe i can get faster meshing when decreasing the mesh size and then since fluent simulation prefers HT off that would justify the latter. Anyways the question is answered. Thanks again