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fix collectBoundarySeeds: snapshot phaseFab to host before scanning #272

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jameslehoux merged 1 commit into from
May 6, 2026
Merged

fix collectBoundarySeeds: snapshot phaseFab to host before scanning #272

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23 changes: 19 additions & 4 deletions src/props/FloodFill.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -35,16 +35,31 @@ void collectBoundarySeeds(const amrex::iMultiFab& phaseFab, int phaseID, int dir
amrex::Vector<amrex::IntVect> local_inlet;
amrex::Vector<amrex::IntVect> local_outlet;

// Seed collection must run on CPU (building host-side IntVect vectors).
// Synchronize first to ensure phase data is available on host.
// Seed collection runs on CPU (it builds host-side IntVect vectors).
// On a CUDA build, phaseFab.const_array(mfi) is a view into device-resident
// memory — reading it from a host LoopOnCpu segfaults on T4 / A100. We
// therefore snapshot phaseFab into a pinned-host iMultiFab once, and
// iterate that copy on the CPU. On CPU builds the snapshot is a no-op
// assignment (host data already accessible) so we just alias the input.
amrex::Gpu::streamSynchronize();

#ifdef AMREX_USE_GPU
amrex::iMultiFab phaseFab_host_storage(phaseFab.boxArray(), phaseFab.DistributionMap(),
phaseFab.nComp(), phaseFab.nGrow(),
amrex::MFInfo().SetArena(amrex::The_Pinned_Arena()));
amrex::Copy(phaseFab_host_storage, phaseFab, 0, 0, phaseFab.nComp(), phaseFab.nGrow());
amrex::Gpu::streamSynchronize();
const amrex::iMultiFab& phaseFab_host = phaseFab_host_storage;
#else
const amrex::iMultiFab& phaseFab_host = phaseFab;
#endif

#ifdef AMREX_USE_OMP
#pragma omp parallel if (amrex::Gpu::notInLaunchRegion())
#endif
for (amrex::MFIter mfi(phaseFab); mfi.isValid(); ++mfi) {
for (amrex::MFIter mfi(phaseFab_host); mfi.isValid(); ++mfi) {
const amrex::Box& validBox = mfi.validbox();
const auto phase_arr = phaseFab.const_array(mfi);
const auto phase_arr = phaseFab_host.const_array(mfi);

amrex::Box inlet_intersect = validBox & domain_lo_face;
if (!inlet_intersect.isEmpty()) {
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