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fix non-deterministic axis regularisation algorithm #413

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fix non-deterministic axis regularisation algorithm #413

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aa0644a
fix non-deterministic axis regularisation algorithm
glesur Sep 21, 2026
26594a9
using Kahan summation strategy to limit roundoff errors
glesur Sep 21, 2026
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87 changes: 59 additions & 28 deletions src/fluid/boundary/axis.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -27,15 +27,25 @@ void Axis::SymmetrizeEx1Side(int jref, IdefixArray3D<real> Ex1) {

auto Ex1Avg = this->Ex1Avg.deviceView();

idefix_for("Ex1_ini",0,data->np_tot[IDIR],
KOKKOS_LAMBDA(int i) {
Ex1Avg(i) = ZERO_F;
});

idefix_for("Ex1_Symmetrize",data->beg[KDIR],data->end[KDIR],0,data->np_tot[IDIR],
KOKKOS_LAMBDA(int k,int i) {
Kokkos::atomic_add(&Ex1Avg(i), Ex1(k,jref,i));
// Deterministic reduction: one thread per i, fixed k loop order

const int kbeg = data->beg[KDIR];
const int kend = data->end[KDIR];

idefix_for("Ex1_SymmetrizeDet", 0, data->np_tot[IDIR],
KOKKOS_LAMBDA(int i) {
real sum = ZERO_F;
real c = ZERO_F; // Kahan compensation
for(int k = kbeg; k < kend; k++) {
real y = Ex1(k, jref, i) - c;
real t = sum + y;
c = (t - sum) - y;
sum = t;
}
Ex1Avg(i) = sum;
});


if(needMPIExchange) {
#ifdef WITH_MPI
Kokkos::fence();
Expand Down Expand Up @@ -94,14 +104,20 @@ void Axis::RegularizeCurrentSide(int side) {
IdefixArray1D<real> dx3 = data->dx[KDIR];
IdefixArray1D<real> dx2 = data->dx[JDIR];

idefix_for("B_ini",0,data->np_tot[IDIR],
KOKKOS_LAMBDA(int i) {
BAvg(i) = ZERO_F;
});
idefix_for("Compute_Bcirculation",data->beg[KDIR],data->end[KDIR],0,data->np_tot[IDIR],
KOKKOS_LAMBDA(int k,int i) {
Kokkos::atomic_add(&BAvg(i), Vs(BX3s,k,jc,i)*dx3(k) ); // Compute the circulation of
// Bphi around the pole
const int kbeg = data->beg[KDIR];
const int kend = data->end[KDIR];
idefix_for("Compute_BcirculationDet", 0, data->np_tot[IDIR],
KOKKOS_LAMBDA(int i) {
real sum = ZERO_F;
real c = ZERO_F; // Kahan compensation
for(int k = kbeg; k < kend; k++) {
real term = Vs(BX3s, k, jc, i) * dx3(k);
real y = term - c;
real t = sum + y;
c = (t - sum) - y;
sum = t;
}
BAvg(i) = sum;
});

if(needMPIExchange) {
Expand Down Expand Up @@ -191,19 +207,34 @@ void Axis::FixBx2sAxis(int side) {
sign = -1;
}

idefix_for("B_ini",0,data->np_tot[IDIR],0,2,
KOKKOS_LAMBDA(int i, int n) {
BAvg(i,n) = ZERO_F;
});
idefix_for("BHorizontal_compute",data->beg[KDIR],data->end[KDIR],0,data->np_tot[IDIR],
KOKKOS_LAMBDA(int k,int i) {
real Bthmid = sign*HALF_F*(Vs(BX2s,k,jaxe-1,i) + Vs(BX2s,k,jaxe+1,i));
real Bphimid = HALF_F*(Vs(BX3s,k,jin,i) + Vs(BX3s,k,jout,i));
//Bthmid = 0.0;
//Bphimid = 0.0;
const int kbeg = data->beg[KDIR];
const int kend = data->end[KDIR];
idefix_for("BHorizontal_computeDet", 0, data->np_tot[IDIR],
KOKKOS_LAMBDA(int i) {
real sumX = ZERO_F;
real cX = ZERO_F; // Kahan compensation for X
real sumY = ZERO_F;
real cY = ZERO_F; // Kahan compensation for Y

for(int k = kbeg; k < kend ; k++) {
real Bthmid = sign*HALF_F*(Vs(BX2s,k,jaxe-1,i) + Vs(BX2s,k,jaxe+1,i));
real Bphimid = HALF_F*(Vs(BX3s,k,jin,i) + Vs(BX3s,k,jout,i));

real termX = Bthmid * cos(phi(k)) - Bphimid * sin(phi(k));
real yX = termX - cX;
real tX = sumX + yX;
cX = (tX - sumX) - yX;
sumX = tX;

real termY = Bthmid * sin(phi(k)) + Bphimid * cos(phi(k));
real yY = termY - cY;
real tY = sumY + yY;
cY = (tY - sumY) - yY;
sumY = tY;
}

Kokkos::atomic_add(&BAvg(i,IDIR), Bthmid * cos(phi(k)) - Bphimid * sin(phi(k)));
Kokkos::atomic_add(&BAvg(i,JDIR), Bthmid * sin(phi(k)) + Bphimid * cos(phi(k)));
BAvg(i,IDIR) = sumX;
BAvg(i,JDIR) = sumY;
});
if(needMPIExchange) {
Kokkos::fence();
Expand Down
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