FFT OpenBC Solver: Communication optimization

Src/Base/AMReX_NonLocalBC.H

@@ -528,7 +528,7 @@ unpack_recv_buffer_gpu (FabArray<FAB>& mf, int scomp, int ncomp,
 struct PackComponents {
     int dest_component{0};
     int src_component{0};
-    int n_components{0};
+    int n_components{1};
 };
 
 //! \brief Dispatch local copies to the default behaviour that knows no DTOS nor projection.

Src/FFT/AMReX_FFT_OpenBCSolver.H

@@ -163,7 +163,9 @@ void OpenBCSolver<T>::solve (MF& phi, MF const& rho)
     inmf.setVal(T(0));
     inmf.ParallelCopy(rho, 0, 0, 1);
 
+    m_r2c.m_openbc_half = true;
     m_r2c.forward(inmf);
+    m_r2c.m_openbc_half = false;
 
     auto scaling_factor = m_r2c.scalingFactor();
 
@@ -199,7 +201,9 @@ void OpenBCSolver<T>::solve (MF& phi, MF const& rho)
         }
     }
 
+    m_r2c.m_openbc_half = true;
     m_r2c.backward_doit(phi, phi.nGrowVect());
+    m_r2c.m_openbc_half = false;
 }
 
 }

Src/FFT/AMReX_FFT_R2C.H

@@ -186,6 +186,8 @@ private:
     std::unique_ptr<MultiBlockCommMetaData> m_cmd_z2y; // (z,x,y) -> (y,x,z)
     std::unique_ptr<MultiBlockCommMetaData> m_cmd_x2z; // (x,y,z) -> (z,x,y)
     std::unique_ptr<MultiBlockCommMetaData> m_cmd_z2x; // (z,x,y) -> (x,y,z)
+    std::unique_ptr<MultiBlockCommMetaData> m_cmd_x2z_half; // for openbc
+    std::unique_ptr<MultiBlockCommMetaData> m_cmd_z2x_half; // for openbc
     Swap01 m_dtos_x2y{};
     Swap01 m_dtos_y2x{};
     Swap02 m_dtos_y2z{};
@@ -209,6 +211,7 @@ private:
     Info m_info;
 
     bool m_slab_decomp = false;
+    bool m_openbc_half = false;
 };
 
 template <typename T, Direction D, DomainStrategy S>
@@ -415,7 +418,24 @@ void R2C<T,D,S>::forward (MF const& inmf)
     }
 #if (AMREX_SPACEDIM == 3)
     else if (                     m_cmd_x2z) {
-        ParallelCopy(m_cz, m_cx, *m_cmd_x2z, 0, 0, 1, m_dtos_x2z);
+        if (m_openbc_half) {
+            Box upper_half = m_spectral_domain_z;
+            // Note that z-direction's index is 0 because we z is the unit-stride direction here.
+            upper_half.growLo (0,-m_spectral_domain_z.length(0)/2);
+            if (! m_cmd_x2z_half) {
+                Box bottom_half = m_spectral_domain_z;
+                bottom_half.growHi(0,-m_spectral_domain_z.length(0)/2);
+                m_cmd_x2z_half = std::make_unique<MultiBlockCommMetaData>
+                    (m_cz, bottom_half, m_cx, IntVect(0), m_dtos_x2z);
+            }
+            NonLocalBC::ApplyDtosAndProjectionOnReciever packing
+                {NonLocalBC::PackComponents{}, m_dtos_x2z};
+            auto handler = ParallelCopy_nowait(m_cz, m_cx, *m_cmd_x2z_half, packing);
+            m_cz.setVal(0, upper_half, 0, 1);
+            ParallelCopy_finish(m_cz, std::move(handler), *m_cmd_x2z_half, packing);
+        } else {
+            ParallelCopy(m_cz, m_cx, *m_cmd_x2z, 0, 0, 1, m_dtos_x2z);
+        }
     }
 #endif
     m_fft_fwd_z.template compute_c2c<Direction::forward>();
@@ -439,7 +459,22 @@ void R2C<T,D,S>::backward_doit (MF& outmf, IntVect const& ngout)
     }
 #if (AMREX_SPACEDIM == 3)
     else if (                     m_cmd_z2x) {
-        ParallelCopy(m_cx, m_cz, *m_cmd_z2x, 0, 0, 1, m_dtos_z2x);
+        if (m_openbc_half) {
+            Box upper_half = m_spectral_domain_x;
+            upper_half.growLo (2,-m_spectral_domain_x.length(2)/2);
+            if (! m_cmd_z2x_half) {
+                Box bottom_half = m_spectral_domain_x;
+                bottom_half.growHi(2,-m_spectral_domain_x.length(2)/2);
+                m_cmd_z2x_half = std::make_unique<MultiBlockCommMetaData>
+                    (m_cx, bottom_half, m_cz, IntVect(0), m_dtos_z2x);
+            }
+            NonLocalBC::ApplyDtosAndProjectionOnReciever packing
+                {NonLocalBC::PackComponents{}, m_dtos_z2x};
+            auto handler = ParallelCopy_nowait(m_cx, m_cz, *m_cmd_z2x_half, packing);
+            ParallelCopy_finish(m_cx, std::move(handler), *m_cmd_z2x_half, packing);
+        } else {
+            ParallelCopy(m_cx, m_cz, *m_cmd_z2x, 0, 0, 1, m_dtos_z2x);
+        }
     }
 #endif