NCORE

NCORE = [integer]
Default: NCORE = 1 

Description: NCORE controls how many MPI ranks collaborate on a single band, parallelizing the FFTs for that band.

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Mind: Available as of VASP 5.2.13

VASP distributes the available MPI ranks into band groups that each work on one band, parallelizing the FFTs for that band. For the common case that IMAGES = 1 and no other algorithm-dependent parallelization (e.g., NOMEGAPAR) is active:

[math]\displaystyle{ \text{available ranks} = \frac{\text{total MPI ranks}}{\text{KPAR}} }[/math]

NCORE sets the size of each band group. The number of bands treated in parallel is then:

NPAR = [math]\displaystyle{ \text{available ranks} }[/math] / NCORE

This makes NCORE and NPAR strict inverses of one another for a given number of available ranks. Do not set NCORE and NPAR at the same time. If both are present in the INCAR, NPAR takes precedence for legacy reasons. Nevertheless, we strongly encourage using NCORE instead of NPAR for all modern VASP versions.

Common settings

• NCORE = [math]\displaystyle{ \sim \sqrt{\text{available ranks}} }[/math]: each band group of NCORE ranks parallelizes its FFTs internally. This reduces both memory requirements (projector functions are shared within the group) and the cost of orthogonalization. This is the recommended regime for modern multi-core machines.

• NCORE = 1 (default): each band is handled by a single rank. The maximum number of bands is treated in parallel, but the non-local projector functions must be stored in full on every rank, leading to high memory usage. In addition, orthogonalizing the bands requires heavy all-to-all communication between all ranks. This setting is appropriate for small unit cells or machines with limited communication bandwidth.

• NCORE = [math]\displaystyle{ \text{available ranks} }[/math]: all ranks collaborate on a single band, distributing only the plane-wave coefficients. No band parallelization occurs. This is almost always very slow and should be avoided.

Warning: When running with OpenMP threading or any of the GPU-offloaded code paths, NCORE is automatically reset to NCORE = 1, regardless of the value set in the INCAR file. FFT parallelization is then handled by OpenMP threads or the GPU. Use the number of OpenMP threads per rank for fine-grained control over FFT parallelization in those cases.

Recommendations

Tip: Consult the optimizing the parallelization page for a step-by-step guide on how to optimize your parallelization. The examples below are only rough guidelines. For any non-trivial production run, perform a short benchmarking scan over a few values of NCORE before committing to one setting.

General guidelines:

• Small systems or slow networks (up to ~8 cores, Gbit Ethernet interconnect): use NCORE = 1. The limited number of ranks and slow interconnect make FFT parallelization within band groups inefficient.

• Modern multi-core clusters (Infiniband or equivalent fast interconnect): set NCORE to a value between 2 and the number of cores per node. NCORE should be a factor of the number of cores per node to ensure that all intra-group FFT communication stays within a single node. As a rule of thumb, NCORE = 4 works well for ~100 atoms; NCORE = 12–NCORE = 16 is often better for unit cells with more than 400 atoms.

• NUMA-aware setting: setting NCORE equal to the number of cores per NUMA domain is often a particularly good choice, because all intra-group FFT communication then stays within the fastest memory domain of the hardware. Use lstopo or numactl --hardware to determine the NUMA layout of your machine.

Related tags and articles

NPAR, KPAR, LPLANE, LSCALU, NSIM, LSCALAPACK, LSCAAWARE,

GPU ports of VASP, Combining MPI and OpenMP,

Optimizing the parallelization, Parallelization, Energy cutoff and FFT meshes

Workflows that use this tag