[Wien] OMP_NUM_THREADS

[Dr Qiwen YAO]

Dear Wien2k users,

We were told in the WIEN workshop that for  mkl+multi-core cases, it might be better having a setting of $OMP_NUM_THREADS =2.

I have two questions in my mind:

Q1.  Does this apply to a 2 core system with 4GB RAM that is not running parallel calculation (not K-point parallel nor mpi-parallel )? 


Q2. Or this only apply to eg a quad core machine that runs on k-point parallel or mpi-parallel calculation?

I have a 4-Core Dell T7500 PC with 12GB RAM, each core is of two threads, so in Susie/Linux or even in Windows, it all displays as a 8 CPU machine (so it is in actuality a four-core CPU but each core is with 2 threads, so all the OS sees it as a 8-core CPU). The actual info for this CPU is here if you like to see the details of it: http://ark.intel.com/products/37111

I am setting up this machine running k-parallel calculation (not mpi-parallel as I have got only one of this machine for the moment), I am pondering:

Which of the following 2 scenarios is a better choice for a 90 atom supercell calculation?
 
Scenario  1. 
.machines files is this:
-------
granularity:1
1:localhost
1:localhost
1:localhost
1:localhost
1:localhost
1:localhost
1:localhost
1:localhost
extrafine:1
----------
and the OMP_NUM_THREADS=1 as default in my .bashrc file.
so no multi-threading but all k-parallelism. (With this setting, I do notice after running the job for while - more than an hour say, the 8 CPUs shown in the System Monitor says only two CPUs were really utilized at a time (and it keep switching CPUs for the full-loading status, but mostly only two fully loaded at a time) and the rest of the 6 CPUs weren't really doing much - some of a few percentage of the load and others even on 0% - so I was wondering maybe this setting isn't optimized?

Scenario  2. 
.machines files would be like this:
-------
granularity:1
1:localhost
1:localhost
1:localhost
1:localhost
extrafine:1
----------
and set the OMP_NUM_THREADS=2 in my .bashrc file - I have not tried this setting as I am not sure if this would be a workable setting? 

Or, both settings would work and won't make much difference in calculation time length for a supercell calculation of 90 atoms? I am new to WIEN so I could not fully understand the THREAD'ings in WIEN's aspect.

On addition, for the above two .machines file setting, would it make any difference if I put the real hostname in the place of "localhost"?

Any comment would be greatly appreciated.

Thank you!

Kind regards,
Qiwen

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Dr QiWen YAO

JSPS Fellow
Multifunctional Materials Group
Optical and Electronic Materials Unit
Environment and Energy Materials Research Division

National Institute for Materials Science

1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
Phone: +81-29-851-3354, ext. no. 6482, Fax: +81-29-859-2501

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