From fe7c359da5e3a56f729e3d8eeca09f7883501117 Mon Sep 17 00:00:00 2001
From: "Alex R. Long" <along@lanl.gov>
Date: Mon, 4 Nov 2024 08:55:25 -0700
Subject: [PATCH] Try to fix MPI error

+ Call resize portion even if no particles in buffer (like old
 behavior)
---
 src/interface/swarm_comms.cpp | 35 +++++++++++++++++++----------------
 1 file changed, 19 insertions(+), 16 deletions(-)

diff --git a/src/interface/swarm_comms.cpp b/src/interface/swarm_comms.cpp
index 7b236b6e4e1f2..d216fd2f16e29 100644
--- a/src/interface/swarm_comms.cpp
+++ b/src/interface/swarm_comms.cpp
@@ -178,6 +178,13 @@ void Swarm::LoadBuffers_() {
   auto &y = Get<Real>(swarm_position::y::name()).Get();
   auto &z = Get<Real>(swarm_position::z::name()).Get();
 
+  // Zero out number of particles to send before accumulating
+  pmb->par_for(
+      PARTHENON_AUTO_LABEL, 0, NMAX_NEIGHBORS - 1, KOKKOS_LAMBDA(const int n) {
+        num_particles_to_send[n] = 0;
+        buffer_start[n] = 0;
+      });
+
   if (max_active_index_ >= 0) {
     auto &buffer_sorted = buffer_sorted_;
     auto &buffer_start = buffer_start_;
@@ -201,12 +208,6 @@ void Swarm::LoadBuffers_() {
     auto &num_particles_to_send = num_particles_to_send_;
     auto max_active_index = max_active_index_;
 
-    // Zero out number of particles to send before accumulating
-    pmb->par_for(
-        PARTHENON_AUTO_LABEL, 0, NMAX_NEIGHBORS - 1, KOKKOS_LAMBDA(const int n) {
-          num_particles_to_send[n] = 0;
-          buffer_start[n] = 0;
-        });
 
     pmb->par_for(
         PARTHENON_AUTO_LABEL, 0, max_active_index_, KOKKOS_LAMBDA(const int n) {
@@ -228,19 +229,21 @@ void Swarm::LoadBuffers_() {
     // copy values back to host for buffer sizing
     auto num_particles_to_send_h = num_particles_to_send_.GetHostMirrorAndCopy();
     auto buffer_start_h = buffer_start.GetHostMirrorAndCopy();
+  }
 
-    // Resize send buffers if too small
-    for (int n = 0; n < pmb->neighbors.size(); n++) {
-      num_particles_to_send_h(n) -= buffer_start_h(n);
-      const int bufid = pmb->neighbors[n].bufid;
-      auto sendbuf = vbswarm->bd_var_.send[bufid];
-      if (sendbuf.extent(0) < num_particles_to_send_h(n) * particle_size) {
-        sendbuf = BufArray1D<Real>("Buffer", num_particles_to_send_h(n) * particle_size);
-        vbswarm->bd_var_.send[bufid] = sendbuf;
-      }
-      vbswarm->send_size[bufid] = num_particles_to_send_h(n) * particle_size;
+  // Resize send buffers if too small
+  for (int n = 0; n < pmb->neighbors.size(); n++) {
+    num_particles_to_send_h(n) -= buffer_start_h(n);
+    const int bufid = pmb->neighbors[n].bufid;
+    auto sendbuf = vbswarm->bd_var_.send[bufid];
+    if (sendbuf.extent(0) < num_particles_to_send_h(n) * particle_size) {
+      sendbuf = BufArray1D<Real>("Buffer", num_particles_to_send_h(n) * particle_size);
+      vbswarm->bd_var_.send[bufid] = sendbuf;
     }
+    vbswarm->send_size[bufid] = num_particles_to_send_h(n) * particle_size;
+  }
 
+  if (max_active_index_ >= 0) {
     auto &bdvar = vbswarm->bd_var_;
     auto neighbor_buffer_index = neighbor_buffer_index_;
     // Loop over active particles buffer_sorted, use index n and buffer_start to