Add complicated distributed shared array test

Author: hmenke

c++/mpi/mpi.hpp

@@ -79,6 +79,8 @@ namespace mpi {
 
     [[nodiscard]] MPI_Comm get() const noexcept { return _com; }
 
+    [[nodiscard]] bool is_null() const noexcept { return _com == MPI_COMM_NULL; }
+
     [[nodiscard]] int rank() const {
       if (has_env) {
         int num = 0;

test/c++/mpi_window.cpp

@@ -14,7 +14,8 @@
 //
 // Authors: Philipp Dumitrescu, Olivier Parcollet, Nils Wentzell
 
-#include "mpi/mpi.hpp"
+#include <mpi/mpi.hpp>
+#include <mpi/vector.hpp>
 #include <gtest/gtest.h>
 #include <numeric>
 
@@ -95,10 +96,10 @@ TEST(MPI_Window, SharedArray) {
   auto shm = world.split_shared();
   int const rank_shm = shm.rank();
 
-  constexpr int const size = 20;
+  constexpr int const array_size = 20;
   constexpr int const magic = 21;
 
-  mpi::shared_window<int> win{shm, rank_shm == 0 ? size : 0};
+  mpi::shared_window<int> win{shm, rank_shm == 0 ? array_size : 0};
   std::span array_view{win.base(0), static_cast<std::size_t>(win.size(0))};
 
   win.fence();
@@ -110,7 +111,77 @@ TEST(MPI_Window, SharedArray) {
   win.fence();
 
   int sum = std::accumulate(array_view.begin(), array_view.end(), int{0});
-  EXPECT_EQ(sum, size * magic);
+  EXPECT_EQ(sum, array_size * magic);
+}
+
+TEST(MPI_Window, DistributedSharedArray) {
+  mpi::communicator world;
+  auto shm = world.split_shared();
+
+  // Number of total array elements (prime number to make it a bit more exciting)
+  constexpr int const array_size_total = 197;
+
+  // Create a communicator between rank0 of all shared memory islands ("head node")
+  auto head = world.split(shm.rank() == 0 ? 0 : MPI_UNDEFINED);
+
+  // Determine number of shared memory islands and broadcast to everyone
+  int head_size = (world.rank() == 0 ? head.size(): -1);
+  mpi::broadcast(head_size, world);
+
+  // Determine rank in head node communicator and broadcast to all other ranks
+  // on the same shared memory island
+  int head_rank = (head.get() != MPI_COMM_NULL ? head.rank() : -1);
+  mpi::broadcast(head_rank, shm);
+
+  // Determine number of ranks on each shared memory island and broadcast to everyone
+  std::vector<int> shm_sizes(head_size, 0);
+  if (!head.is_null()) {
+      shm_sizes.at(head_rank) = shm.size();
+      shm_sizes = mpi::all_reduce(shm_sizes, head);
+  }
+  mpi::broadcast(shm_sizes, world);
+
+  // Chunk the total array such that each rank has approximately the same number
+  // of array elements
+  std::vector<int> array_sizes(head_size, 0);
+  for (auto &&[shm_size, array_size]: itertools::zip(shm_sizes, array_sizes)) {
+      array_size = array_size_total / world.size() * shm_size;
+  }
+  // Last shared memory island will get the excess
+  array_sizes.back() += array_size_total % world.size();
+
+  // Determine the global index offset on the current shared memory island
+  auto begin = array_sizes.begin();
+  std::advance(begin, head_rank);
+  std::ptrdiff_t offset = std::accumulate(array_sizes.begin(), begin, std::ptrdiff_t{0});
+
+  // Allocate memory
+  mpi::shared_window<int> win{shm, shm.rank() == 0 ? array_sizes.at(head_rank) : 0};
+  std::span array_view{win.base(0), static_cast<std::size_t>(win.size(0))};
+
+  // Fill array with global index (= local index + global offset)
+  // We do this in parallel on each shared memory island by chunking the total range
+  win.fence();
+  auto slice = itertools::chunk_range(0, array_view.size(), shm.size(), shm.rank());
+  for (auto i = slice.first; i < slice.second; ++i) {
+      array_view[i] = i + offset;
+  }
+  win.fence();
+
+  // Calculate partial sum on head node of each shared memory island and
+  // all_reduce the partial sums into a total sum over the head node
+  // communicator and broadcast result to everyone
+  std::vector<int> partial_sum(head_size, 0);
+  int sum = 0;
+  if (!head.is_null()) {
+      partial_sum[head_rank] = std::accumulate(array_view.begin(), array_view.end(), int{0});
+      partial_sum = mpi::all_reduce(partial_sum, head);
+      sum = std::accumulate(partial_sum.begin(), partial_sum.end(), int{0});
+  }
+  mpi::broadcast(sum, world);
+
+  // Total sum is just sum of numbers in interval [0, array_size_total)
+  EXPECT_EQ(sum, (array_size_total * (array_size_total - 1)) / 2);
 }
 
 MPI_TEST_MAIN;