View source for Parallelization for non-linear response calculations

By default yambo_nl is parallelized on frequencies, that is the most efficient way to distribute calculations among the different processors,
other two parallelizations are available in the code:

'''K-points parallelization'''<br>
if your system is large and requires more memory or you have few frequencies you can change the parallelization strategy. By using the flag "-V par" you will get the parallelization options in your input, you can decide to turn on the parallelization on k-points in such a way that the product of cores in k-space and in frequency-space is equal to the total number of cores. For example if have 16 cores you can set:
 .....
 NL_CPU= "4 4"                   # [PARALLEL] CPUs for each role
 NL_ROLEs= "w k"                 # [PARALLEL] CPUs roles (w,k)
 DIP_CPU= "4 2 2"                      # [PARALLEL] CPUs for each role
 DIP_ROLEs= "k c v"                    # [PARALLEL] CPUs roles (k,c,v)
 .....                           
in this way the code will distribute the wave-function on 4 cores and reduce the amount of memory.
If this is not enough you can use the Open-MP parallelization, see below.<br>

'''Open-MP'''<br>
Another possibility is active openMP in the configure with the flags <span style="color:blue">--enable-open-mp --enable-openmp-int-linalg</span>,
recompile the code and then you can use the [https://www.openmp.org/ OpenMP] parallelization. <br>
For example set the number of threads to 2 with the command:

 export OMP_NUM_THREADS=2                     
and yambo_nl automatically will use the threads available. In the log file will find:
 
 .....
 <---> P1: MPI Cores-Threads   : 16(CPU)-2(threads)
 <---> P1: MPI Cores-Threads   : NL(environment)-4 4(CPUs)-w k(ROLEs)
 .....
Notice the setting of threads number can depend from the queue system and configuration of your machine.<br>

Using all these parallelization you can use a large number of cores.
Image you want to calculate the response for 40 different frequencies you could set

"4 cores Open-MP" x "10 cores k-points" x "40 cores frequencies" = 1600 cores.

We advice not to use more than 4 open-MP threads,  unless you have memory problems  in your calculations, in order to have an efficient parallelization.<br>
Finally remember that the restart for interrupted calculations works only on frequencies.