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| For many complex problems, a single core is not enough to finish the calculation in a reasonable time.
| | #REDIRECT [[:Category:Parallelization]] |
| VASP makes use of parallel machines splitting the calculation into many tasks.
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| By default, VASP distributes the number of bands ({{TAG|NBANDS}}) over the available cores.
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| But it is often beneficial to add parallelization over the FFTs ({{TAG|NCORE}}), the '''k''' points ({{TAG|KPAR}}), and separate calculations ({{TAG|IMAGES}}).
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| All these tags default to 1 and divide the number of cores among the parallelization options.
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| There are also additional parallelization options for some algorithms in VASP.
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| ::<math> | |
| \text{total cores} = \text{cores parallelizing bands} \times \text{NCORE} \times \text{KPAR} \times \text{IMAGES} \times \text{other algorithm-dependent tags}
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| </math>
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| VASP makes use of [[Hybrid_MPI/OpenMP_parallelization|OpenMP]] and [[OpenACC_GPU_port_of_VASP|OpenACC]] when possible.
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| Note that these options conflict with the {{TAG|NCORE}} parallelization.
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| ==Optimizing the parallelization==
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| {{NB|tip|We offer only general advice here. The performance for specific systems may be significantly different. However, in many cases, one is interested in similar calculations. Then run a few of these cases varying the parallel setup and use the optimal choice of parameters for the rest.}}
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| When choosing the optimal performance try to get as close as possible to the actual system.
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| This includes both the physical system (atoms, cell size, cutoff, ...) as well as the computational hardware (CPUs, interconnect, number of nodes, ...).
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| If too many parameters are different, the parallel configuration may not be transferable to the production calculation.
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| Nevertheless, a few steps of repetitive tasks give a good idea of an optimal choice for the full calculation.
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| For example, running only a few electronic or ionic self-consistency steps instead of finishing the convergence.
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| Often, combining multiple parallelization options yields the fastest results because the parallel efficiency of each level drops near its limit.
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| For the default option (band parallelization), the limit is {{TAG|NBANDS}} divided by a small integer.
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| Note that VASP will increase {{TAG|NBANDS}} to match the number of cores.
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| Choose {{TAG|NCORE}} as a factor of the cores per node to avoid communicating between nodes for the FFTs.
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| The '''k'''-point parallelization is efficient but requires additional memory.
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| Given sufficient memory, increase {{TAG|KPAR}} up to the number of irreducible '''k''' points.
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| Keep in mind that {{TAG|KPAR}} should factorize the number of '''k''' points.
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| Finally, {{TAG|IMAGES}} is required to split several VASP runs into separate calculations.
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| The limit is dictated by the number of desired calculations.
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| ==Caveat about the MPI setup==
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| ==Additional parallelization options==
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| ==OpenMP/OpenACC==
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