RemappedFFTSolver Class ReferenceΒΆ

adc_cpp: pops::RemappedFFTSolver Class Reference
adc_cpp 0.3.0
Model-free C++23 core for coupled hyperbolic-elliptic systems on adaptive (AMR) meshes, with MPI and GPU (Kokkos) backends
Public Member Functions | List of all members
pops::RemappedFFTSolver Class Reference

DIRECT periodic Poisson solver (spectral FFT) under MPI, presenting the SYSTEM LAYOUT, models EllipticSolver (ADC-287). More...

#include <poisson_fft_solver.hpp>

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Public Member Functions

 RemappedFFTSolver (const Geometry &geom, const BoxArray &ba, const BCRec &=BCRec{}, std::function< bool(Real, Real)>={}, bool spectral=false)
 spectral: Laplacian symbol forwarded to PoissonFFT (false = discrete 5-point stencil, bit-identical default; true = continuous symbol -(kx^2+ky^2)).
 
MultiFab & rhs ()
 
MultiFab & phi ()
 
const Geometry & geom () const
 
void solve ()
 
Real residual ()
 

Detailed Description

DIRECT periodic Poisson solver (spectral FFT) under MPI, presenting the SYSTEM LAYOUT, models EllipticSolver (ADC-287).

Role: let System.set_poisson(..., "fft"|"fft_spectral") run on n_ranks() > 1 WITHOUT touching the system field solver. Unlike DistributedFFTSolver (slabs, DistributionMapping(np, np)), this solver allocates rhs()/phi() on the System's own BoxArray/DistributionMapping (the single round-robin box, DistributionMapping(1, np) -> box on one owner rank). The whole box<->slab transpose is HIDDEN inside solve(): the System path (RHS assembly via add_poisson_rhs, the phi->aux derivation loop aligned on fab(li), fill_boundary, the device_fence ordering) keeps reading rhs()/phi() on the SAME layout as the aux, so system_field_solver.hpp needs no change beyond selecting this type.

Remap mechanics (the box lives ENTIRELY on the owner rank, so this is a scatter/gather, NOT a symmetric alltoall): solve() (1) the owner packs its full Nx*Ny rhs fab into per-slab contiguous buffers (slab r = global rows [r*nyl, (r+1)*nyl), full x); (2) MPI_Scatter -> each rank gets nyl*Nx row-major rho_local; (3) every rank calls the proven PoissonFFT::solve (its internal MPI_Alltoall does the y<->x transpose); (4) MPI_Gather phi_local -> owner; (5) owner unpacks into phi's valid cells; (6) owner fill_boundary fills the periodic ghosts (a pure intra-box wrap since only the owner has a fab -> no inter-rank messages, deadlock-free) so the centered grad phi reads correct halos.

Preconditions / constraints:

  • the System box: ba.size() == 1 (the round-robin single box); rejected otherwise (collective throw);
  • geom.domain.ny() % n_ranks() == 0 (equal slabs for the internal transpose); collective throw, no deadlock since ensure_elliptic is called collectively and every rank constructs the object;
  • periodic BCs, constant coefficient (the BCRec and wall predicate are ignored; the System guards wall / variable-eps / anisotropic-eps / kappa BEFORE selecting this solver);
  • Nx, Ny powers of two for the fast radix-2 path; otherwise PoissonFFT's correct O(n^2) DFT fallback.

Conservation / gauge: the scatter/gather is a pure permutation of doubles (no FP arithmetic, exact), so the charge integral is preserved bit-exactly through the remap; PoissonFFT zeroes the k=0 mode, so phi has zero mean. The only FP difference vs the single-rank PoissonFFTSolver is PoissonFFT's internal alltoall reassociation, already characterised at machine zero by test_mpi_fft_distributed (np=1/2/4).

NOTE: STRUCTURAL change pending the ADC-273 design vote (new EllipticSolver type in the System ell_ variant, new MPI collective pattern). Behind the existing "fft"/"fft_spectral" tokens; geometric_mg stays the MPI default and the only option for walls / variable-eps / kappa.

Constructor & Destructor Documentation

◆ RemappedFFTSolver()

pops::RemappedFFTSolver::RemappedFFTSolver ( const Geometry &  geom,
const BoxArray &  ba,
const BCRec &  = BCRec{},
std::function< bool(Real, Real)>  = {},
bool  spectral = false 
)
inline

spectral: Laplacian symbol forwarded to PoissonFFT (false = discrete 5-point stencil, bit-identical default; true = continuous symbol -(kx^2+ky^2)).

Same constructor shape as PoissonFFTSolver (BCRec and wall predicate accepted but ignored: periodic constant coefficient).

Member Function Documentation

◆ geom()

const Geometry & pops::RemappedFFTSolver::geom ( ) const
inline

◆ phi()

MultiFab & pops::RemappedFFTSolver::phi ( )
inline

◆ residual()

Real pops::RemappedFFTSolver::residual ( )
inline
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◆ rhs()

MultiFab & pops::RemappedFFTSolver::rhs ( )
inline

◆ solve()

void pops::RemappedFFTSolver::solve ( )
inline
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