[Wien] 24k points on 36processors ??. (a Fractional k-point per core)

[Pavel Ondračka]

Hi,

do you have hyperthreading or not (in other words does this number of
12 already mean there are 6 physical CPUs and 12 virtuals, or 12
physical)? This would influence the advice maybe a bit...

Otherwise you need to experiment, the optimal setting is heavily
dependent on your specific CPU, memory speed and what you are
calculating (system size). 

When talking about the 24 kpoints and 36 processors, than running
4kpoints on each node  (12 kpoints in parallel) with 3 openmp threads
each might be a reasonable setting.

It is also possible that just leaving some cores idle might be the best
thing to do (as running a lot of k-points in parallel you can get
limited by the memory speed so leaving some cores idle means more
memory bandwidth for the others):
This would correspond to running 8 kpoints on each node or 4 kpoints on
each node with 2 openmp threads each.

The linux kernel and modern processors are also usually good at
handling some small overload and load balancing so you can also try to
overload the system a bit, i.e., 8kpoints per node with 2 openmp
threads each. 

Just try the different settings (single lapw1 run for each should be
enough to get some idea) and compare the timings.

Best regards
Pavel

BTW for lapw0 I would go with something like 3 MPI processes per node
with 4 OpenMP threads for each in this case.

On Thu, 2019-12-12 at 12:28 +0530, Ashwani Kumar wrote:
> Hi, 
>    This is related to no. of k-points which we provide during the
> initilization. No. of k-gen points given ; 120 with shifted mesh.
> Irr. k-points : 24k points. Running job on 3 nodes (12 x3 processors,
> 48 gb x 3 Ram). Job running on 24 processors only (with granularity:
> 1, extrafine:1 in *.machine file) which means 1kpoint/1-core. How can
> 24 k-points be made to run on 36  cores ?. Or how can 24 kpoints can
> be distributed equally between 36 cores (or let's say 12 kpoints on
> 24 processors to make calculation converge faster). 
> 
> thanks,
> A. Kumar
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