diff --git a/CMakeLists.txt b/CMakeLists.txt
index 05cbfd2..0f65e4a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -55,4 +55,12 @@ if ("${GTENSOR_DEVICE}" STREQUAL "sycl")
   add_executable(mpi_stencil2d_sycl)
   target_sources(mpi_stencil2d_sycl PRIVATE mpi_stencil2d_sycl.cc)
   target_link_libraries(mpi_stencil2d_sycl MPI::MPI_CXX)
+  target_compile_options(mpi_stencil2d_sycl PRIVATE -fsycl -x c++)
+  target_link_options(mpi_stencil2d_sycl PRIVATE -fsycl)
+
+  add_executable(mpi_stencil2d_sycl_oo)
+  target_sources(mpi_stencil2d_sycl_oo PRIVATE mpi_stencil2d_sycl_oo.cc)
+  target_link_libraries(mpi_stencil2d_sycl_oo MPI::MPI_CXX)
+  target_compile_options(mpi_stencil2d_sycl_oo PRIVATE -fsycl -x c++)
+  target_link_options(mpi_stencil2d_sycl_oo PRIVATE -fsycl)
 endif()
diff --git a/mpi_stencil2d_sycl_oo.cc b/mpi_stencil2d_sycl_oo.cc
new file mode 100644
index 0000000..b366c70
--- /dev/null
+++ b/mpi_stencil2d_sycl_oo.cc
@@ -0,0 +1,649 @@
+/*
+ * Test GPU aware MPI on different platforms using a distributed
+ * 1d stencil on a 2d array. The exchange in second (non-contiguous)
+ * direction forces use of staging buffers, which replicates what
+ * is needed for all but the innermost dimension exchanges in the
+ * GENE fusion code.
+ *
+ * Takes optional command line arg for size of each dimension of the domain
+ * n_global, in 1024 increments. Default is 8 * 1024 (so 256K plus ghost points
+ * in size for doulbles per array), which should fit on any system but may not
+ * be enough to tax larger HPC GPUs and MPI impelmentations.
+ *
+ * There will be four exchange buffers of size 2 * n_global, i.e. 128K each
+ * by default.
+ *
+ * Modified version that uses minimal owning (array2d) and non-owning (span2d)
+ * classes to make indexing handling less error prone, without using all of
+ * gtensor. Note that the owning class is not trivially copyable and not device
+ * copyable, because it must have a non-trivial destructor.
+ *
+ * TODO: Since no temparories are used, perhaps a helper that allocates and
+ * returns a span is a simpler option to create this minimal example?
+ */
+
+#include <cassert>
+#include <cmath>
+#include <memory>
+#include <mpi.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+#include <type_traits>
+
+#include "sycl/sycl.hpp"
+
+constexpr std::size_t idx2(int n, int row, int col)
+{
+  return row + col * n;
+}
+
+template <typename T, sycl::usm::alloc Alloc>
+class span2d
+{
+public:
+  using value_type = T;
+  using pointer = value_type*;
+  using const_pointer = const value_type*;
+  using reference = value_type&;
+  using const_reference = const value_type&;
+  using size_type = std::size_t;
+
+  span2d(T* data, const int nrows, const int ncols)
+    : data_(data), nrows_(nrows), ncols_(ncols)
+  {}
+
+  // use default copy and move ctor. Ideall move ctor would better invalidate
+  // the moved from object, but this is supposed to be a small example...
+  span2d(const span2d& other) = default;
+  span2d& operator=(const span2d& other) = default;
+  span2d(span2d&& other) = default;
+  span2d& operator=(span2d&& other) = default;
+
+  // Note: shallow const
+  reference operator()(int row, int col) const
+  {
+    return data_[idx2(nrows_, row, col)];
+  }
+
+  // Note: shallow const
+  reference operator[](size_type i) const { return data_[i]; }
+
+  int nrows() const { return nrows_; }
+  int ncols() const { return ncols_; }
+  size_type size() const { return nrows_ * ncols_; }
+
+  span2d to_span() { return *this; }
+
+  // Note: shallow const
+  pointer data() const { return data_; }
+
+private:
+  const sycl::usm::alloc alloc_ = Alloc;
+  T* data_;
+  const int nrows_;
+  const int ncols_;
+};
+
+template <typename T, sycl::usm::alloc Alloc>
+class array2d : public span2d<T, Alloc>
+{
+public:
+  using base_type = span2d<T, Alloc>;
+  using value_type = T;
+  using pointer = value_type*;
+  using const_pointer = const value_type*;
+  using reference = value_type&;
+  using const_reference = const value_type&;
+  using size_type = std::size_t;
+
+  array2d(sycl::queue& q, const int nrows, const int ncols)
+    : base_type(sycl::malloc<value_type>(nrows * ncols, q, Alloc), nrows,
+                ncols),
+      q_(q)
+  {}
+
+  ~array2d() { sycl::free(this->data(), q_); }
+
+  // skip these to keep the example simple, pass by reference everywhere
+  array2d(const array2d& other) = delete;
+  array2d& operator=(const array2d& other) = delete;
+  array2d(array2d&& other) = delete;
+  array2d& operator=(array2d&& other) = delete;
+
+  base_type to_span()
+  {
+    return base_type(this->data(), this->nrows(), this->ncols());
+  }
+
+private:
+  sycl::queue& q_;
+};
+
+template <typename SrcArray, typename DestArray>
+auto copy(sycl::queue& q, SrcArray& src, DestArray& dest)
+{
+  static_assert(std::is_same_v<typename SrcArray::value_type,
+                               typename DestArray::value_type>,
+                "value types must match");
+  assert(src.size() == dest.size());
+  return q.copy(src.data(), dest.data(), dest.size());
+}
+
+template <typename Array>
+auto copy_dest_slice(sycl::queue& q, Array& src, Array& dest, int dim,
+                     int start, int end)
+{
+  auto s_src = src.to_span();
+  auto s_dest = dest.to_span();
+  assert(dim == 0 || dim == 1);
+  if (dim == 0) {
+    assert(src.ncols() == dest.ncols());
+    if (start < 0) {
+      start += dest.nrows();
+    }
+    if (end == 0 && start > end) {
+      end = dest.nrows();
+    }
+    assert(start < end);
+    auto range = sycl::range<2>(dest.ncols(), end - start);
+    auto e = q.submit([&](sycl::handler& cgh) {
+      cgh.parallel_for(range, [=](sycl::item<2> item) {
+        int row = item.get_id(1);
+        int col = item.get_id(0);
+        s_dest(start + row, col) = s_src(row, col);
+      });
+    });
+    return e;
+  } else {
+    assert(src.nrows() == dest.nrows());
+    if (start < 0) {
+      start += dest.ncols();
+    }
+    if (end == 0 && start > end) {
+      end = dest.ncols();
+    }
+    auto range = sycl::range<2>(end - start, dest.nrows());
+    auto e = q.submit([&](sycl::handler& cgh) {
+      cgh.parallel_for(range, [=](sycl::item<2> item) {
+        int row = item.get_id(1);
+        int col = item.get_id(0);
+        s_dest(row, start + col) = s_src(row, col);
+      });
+    });
+    return e;
+  }
+}
+
+template <typename T, sycl::usm::alloc Alloc>
+auto copy_src_slice(sycl::queue& q, span2d<T, Alloc>& src,
+                    span2d<T, Alloc>& dest, int dim, int start, int end)
+{
+  assert(dim == 0 || dim == 1);
+  auto s_src = src.to_span();
+  auto s_dest = dest.to_span();
+  if (dim == 0) {
+    assert(src.ncols() == dest.ncols());
+    if (start < 0) {
+      start += src.nrows();
+    }
+    if (end == 0 && start > end) {
+      end = src.nrows();
+    }
+    auto range = sycl::range<2>(dest.ncols(), end - start);
+    auto e = q.submit([&](sycl::handler& cgh) {
+      cgh.parallel_for(range, [=](sycl::item<2> item) {
+        int row = item.get_id(1);
+        int col = item.get_id(0);
+        s_dest(row, col) = s_src(start + row, col);
+      });
+    });
+    return e;
+  } else {
+    assert(src.nrows() == dest.nrows());
+    if (start < 0) {
+      start += src.ncols();
+    }
+    if (end == 0 && start > end) {
+      end = src.ncols();
+    }
+    auto range = sycl::range<2>(end - start, dest.nrows());
+    auto e = q.submit([&](sycl::handler& cgh) {
+      cgh.parallel_for(range, [=](sycl::item<2> item) {
+        int row = item.get_id(1);
+        int col = item.get_id(0);
+        s_dest(row, col) = s_src(row, start + col);
+      });
+    });
+    return e;
+  }
+}
+
+inline void check(const char* fname, int line, int mpi_rval)
+{
+  if (mpi_rval != MPI_SUCCESS) {
+    printf("%s:%d error %d\n", fname, line, mpi_rval);
+    exit(2);
+  }
+}
+
+#define CHECK(x) check(__FILE__, __LINE__, (x))
+
+static constexpr double stencil5[] = {1.0 / 12.0, -2.0 / 3.0, 0.0, 2.0 / 3.0,
+                                      -1.0 / 12.0};
+
+/*
+ * Calculate 1d stencil of second dimension of 2d array on GPU. Out array must
+ * be contiguous column major nrows x ncols array, while in array must be
+ * (nrows)x(ncols+4) to accomodate 2 ghost points in each direction for the
+ * second dimension.
+ *
+ * Returns sycl event, async with respect to host.
+ */
+template <typename Array>
+auto stencil2d_1d_5(sycl::queue& q, Array& out2d, Array& in2d, double scale)
+{
+  // Note: swap index order; SYCL is row-major oriented, and this example
+  // is col-major
+  int in_nrows = in2d.nrows();
+  auto range = sycl::range<2>(out2d.ncols(), out2d.nrows());
+  auto s_in2d = in2d.to_span();
+  auto s_out2d = out2d.to_span();
+  auto e = q.submit([&](sycl::handler& cgh) {
+    cgh.parallel_for(range, [=](sycl::item<2> item) {
+      int row = item.get_id(1);
+      int col = item.get_id(0);
+      int out_idx = idx2(s_out2d.nrows(), row, col);
+      int in_base_idx = idx2(s_in2d.nrows(), row, col);
+      s_out2d[out_idx] = (stencil5[0] * s_in2d[in_base_idx + 0] +
+                          stencil5[1] * s_in2d[in_base_idx + 1] +
+                          stencil5[2] * s_in2d[in_base_idx + 2] +
+                          stencil5[3] * s_in2d[in_base_idx + 3] +
+                          stencil5[4] * s_in2d[in_base_idx + 4]) *
+                         scale;
+    });
+  });
+  return e;
+}
+
+/*
+ * Calculate the norm of the difference of two arrays, as sqrt of sum of
+ * squared distances.
+ */
+double diff_norm(sycl::queue& q, std::size_t size, double* d_a, double* d_b)
+{
+  double result = 0.0;
+  sycl::buffer<double> result_buf{&result, 1};
+  {
+    sycl::range<1> range(size);
+    auto e = q.submit([&](sycl::handler& cgh) {
+      auto reducer = sycl::reduction(result_buf, cgh, 0.0, std::plus<>{});
+      cgh.parallel_for(range, reducer, [=](sycl::id<1> idx, auto& r) {
+        double diff = d_a[idx] - d_b[idx];
+        r.combine(diff * diff);
+      });
+    });
+    e.wait();
+  }
+  return std::sqrt(result_buf.get_host_access()[0]);
+}
+
+sycl::queue get_rank_queue(int n_ranks, int rank)
+{
+  int n_devices, device_idx, ranks_per_device;
+
+  sycl::context ctx{};
+  auto devices = ctx.get_devices();
+  n_devices = devices.size();
+
+  if (n_ranks > n_devices) {
+    if (n_ranks % n_devices != 0) {
+      printf(
+        "ERROR: Number of ranks (%d) not a multiple of number of GPUs (%d)\n",
+        n_ranks, n_devices);
+      exit(EXIT_FAILURE);
+    }
+    ranks_per_device = n_ranks / n_devices;
+    device_idx = rank / ranks_per_device;
+  } else {
+    ranks_per_device = 1;
+    device_idx = rank;
+  }
+
+  // printf("n_devices = %d\n", n_devices);
+  // printf("device_idx = %d\n", device_idx);
+
+  return sycl::queue{devices[device_idx], sycl::property::queue::in_order()};
+}
+
+// exchange in first dimension, staging into contiguous buffers on device
+template <typename T, sycl::usm::alloc Alloc>
+void boundary_exchange_x(MPI_Comm comm, int world_size, int rank,
+                         sycl::queue& q, int n_bnd, array2d<T, Alloc>& d_z,
+                         bool stage_host = false)
+{
+  static array2d<double, sycl::usm::alloc::device> sbuf_l{q, n_bnd,
+                                                          d_z.ncols()};
+  static array2d<double, sycl::usm::alloc::device> sbuf_r{q, n_bnd,
+                                                          d_z.ncols()};
+  static array2d<double, sycl::usm::alloc::device> rbuf_l{q, n_bnd,
+                                                          d_z.ncols()};
+  static array2d<double, sycl::usm::alloc::device> rbuf_r{q, n_bnd,
+                                                          d_z.ncols()};
+
+  static array2d<double, sycl::usm::alloc::host> h_sbuf_l{q, n_bnd,
+                                                          d_z.ncols()};
+  static array2d<double, sycl::usm::alloc::host> h_sbuf_r{q, n_bnd,
+                                                          d_z.ncols()};
+  static array2d<double, sycl::usm::alloc::host> h_rbuf_l{q, n_bnd,
+                                                          d_z.ncols()};
+  static array2d<double, sycl::usm::alloc::host> h_rbuf_r{q, n_bnd,
+                                                          d_z.ncols()};
+
+  int buf_size = sbuf_l.size();
+
+  MPI_Request req_l[2];
+  MPI_Request req_r[2];
+
+  int rank_l = rank - 1;
+  int rank_r = rank + 1;
+
+  // start async copy of ghost points into send buffers
+  if (rank_l >= 0) {
+    // printf("rank_l = %d\n", rank_l); fflush(nullptr);
+    // sbuf_l = d_z.view(_all, _s(n_bnd, 2 * n_bnd));
+    copy_src_slice(q, d_z, sbuf_l, 0, n_bnd, 2 * n_bnd);
+    if (stage_host) {
+      copy(q, sbuf_l, h_sbuf_l);
+      /*
+      for (int i = 0; i < n_bnd; i++) {
+        for (int j = 0; j < n_global; j++) {
+          int idx = idx2(n_global, j, i);
+          printf("sbuf_l[%d, %d] = %f\n", j, i, h_sbuf_l[idx]);
+          fflush(nullptr);
+        }
+      }
+      */
+    }
+  }
+  if (rank_r < world_size) {
+    // printf("rank_r = %d\n", rank_r); fflush(nullptr);
+    // sbuf_r = d_z.view(_all, _s(-2 * n_bnd, -n_bnd));
+    copy_src_slice(q, d_z, sbuf_l, 0, -2 * n_bnd, -n_bnd);
+    if (stage_host) {
+      copy(q, sbuf_r, h_sbuf_r);
+      /*
+      for (int i = 0; i < n_bnd; i++) {
+        for (int j = 0; j < n_global; j++) {
+          int idx = idx2(n_global, j, i);
+          printf("sbuf_r[%d, %d] = %f\n", j, i, h_sbuf_r[idx]);
+          fflush(nullptr);
+        }
+      }
+      */
+    }
+  }
+
+  // initiate async recv
+  if (rank_l >= 0) {
+    double* rbuf_l_data = nullptr;
+    if (stage_host) {
+      rbuf_l_data = h_rbuf_l.data();
+    } else {
+      rbuf_l_data = rbuf_l.data();
+    }
+    MPI_Irecv(rbuf_l_data, buf_size, MPI_DOUBLE, rank_l, 123, comm, &req_l[0]);
+  }
+
+  if (rank_r < world_size) {
+    double* rbuf_r_data = nullptr;
+    if (stage_host) {
+      rbuf_r_data = h_rbuf_r.data();
+    } else {
+      rbuf_r_data = rbuf_r.data();
+    }
+    MPI_Irecv(rbuf_r_data, buf_size, MPI_DOUBLE, rank_r, 456, comm, &req_r[0]);
+  }
+
+  // wait for send buffer fill
+  q.wait();
+
+  // initiate async sends
+  if (rank_l >= 0) {
+    double* sbuf_l_data = nullptr;
+    if (stage_host) {
+      sbuf_l_data = h_sbuf_l.data();
+    } else {
+      sbuf_l_data = sbuf_l.data();
+    }
+    MPI_Isend(sbuf_l_data, buf_size, MPI_DOUBLE, rank_l, 456, comm, &req_l[1]);
+  }
+
+  if (rank_r < world_size) {
+    double* sbuf_r_data = nullptr;
+    if (stage_host) {
+      sbuf_r_data = h_sbuf_r.data();
+    } else {
+      sbuf_r_data = sbuf_r.data();
+    }
+    MPI_Isend(sbuf_r_data, buf_size, MPI_DOUBLE, rank_r, 123, comm, &req_r[1]);
+  }
+
+  // wait for send/recv to complete, then copy data back into main data array
+  int mpi_rval;
+  if (rank_l >= 0) {
+    mpi_rval = MPI_Waitall(2, req_l, MPI_STATUSES_IGNORE);
+    if (mpi_rval != MPI_SUCCESS) {
+      printf("send_l error: %d\n", mpi_rval);
+    }
+    if (stage_host) {
+      /*
+      for (int i = 0; i < n_bnd; i++) {
+        for (int j = 0; j < n_global; j++) {
+          int idx = idx2(n_global, j, i);
+          printf("rbuf_l[%d, %d] = %f\n", j, i, h_rbuf_l[idx]);
+          fflush(nullptr);
+        }
+      }
+      */
+      copy(q, h_rbuf_l, rbuf_l);
+    }
+    // d_z.view(_all, _s(0, n_bnd)) = rbuf_l;
+    copy_dest_slice(q, rbuf_l, d_z, 0, 0, n_bnd);
+  }
+  if (rank_r < world_size) {
+    mpi_rval = MPI_Waitall(2, req_r, MPI_STATUSES_IGNORE);
+    if (mpi_rval != MPI_SUCCESS) {
+      printf("send_r error: %d\n", mpi_rval);
+    }
+    if (stage_host) {
+      /*
+      for (int i = 0; i < n_bnd; i++) {
+        for (int j = 0; j < n_global; j++) {
+          int idx = idx2(n_global, j, i);
+          printf("rbuf_r[%d, %d] = %f\n", j, i, h_rbuf_r[idx]);
+          fflush(nullptr);
+        }
+      }
+      */
+      copy(q, h_rbuf_r, rbuf_r);
+    }
+    // d_z.view(_all, _s(-n_bnd, _)) = rbuf_r;
+    copy_dest_slice(q, rbuf_r, d_z, 0, -n_bnd, 0);
+  }
+
+  q.wait();
+}
+
+int main(int argc, char** argv)
+{
+  using T = double;
+
+  static_assert(
+    std::is_trivially_copyable_v<span2d<T, sycl::usm::alloc::device>>,
+    "span2d device not trivial");
+  static_assert(std::is_trivially_copyable_v<span2d<T, sycl::usm::alloc::host>>,
+                "span2d host not trivial");
+
+  // sycl::queue q2{};
+  // test_buf_view(q2, 6);
+  // return EXIT_SUCCESS;
+
+  // Note: domain will be n_global x n_global plus ghost points in one dimension
+  int n_global = 8 * 1024;
+  bool stage_host = false;
+  int n_iter = 100;
+  int n_warmup = 5;
+
+  if (argc > 1) {
+    n_global = std::atoi(argv[1]) * 1024;
+  }
+  if (argc > 2) {
+    if (argv[2][0] == '1') {
+      stage_host = true;
+    }
+  }
+  if (argc > 3) {
+    n_iter = std::atoi(argv[3]);
+  }
+
+  int n_sten = 5;
+  int n_bnd = (n_sten - 1) / 2;
+  int world_size, world_rank, device_id;
+  uint32_t vendor_id;
+
+  MPI_Init(NULL, NULL);
+
+  MPI_Comm_size(MPI_COMM_WORLD, &world_size);
+  MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
+
+  if (n_global % world_size != 0) {
+    printf("%d nmpi (%d) must be divisor of domain size (%d), exiting\n",
+           world_rank, world_size, n_global);
+    exit(1);
+  }
+
+  const int n_local = n_global / world_size;
+  const int n_local_with_ghost = n_local + 2 * n_bnd;
+
+  sycl::queue q = get_rank_queue(world_size, world_rank);
+
+  if (world_rank == 0) {
+    printf("n procs    = %d\n", world_size);
+    printf("rank       = %d\n", world_rank);
+    printf("n_global   = %d\n", n_global);
+    printf("n_local    = %d\n", n_local);
+    printf("n_iter     = %d\n", n_iter);
+    printf("n_warmup   = %d\n", n_warmup);
+    printf("stage_host = %d\n", stage_host);
+  }
+
+  int z_size = n_local_with_ghost * n_global;
+  int dzdx_size = n_local * n_global;
+
+  array2d<T, sycl::usm::alloc::host> h_z{q, n_local_with_ghost, n_global};
+  array2d<T, sycl::usm::alloc::device> d_z{q, n_local_with_ghost, n_global};
+
+  array2d<T, sycl::usm::alloc::host> h_dzdx_actual{q, n_local, n_global};
+  array2d<T, sycl::usm::alloc::host> h_dzdx_numeric{q, n_local, n_global};
+  array2d<T, sycl::usm::alloc::device> d_dzdx_actual{q, n_local, n_global};
+  array2d<T, sycl::usm::alloc::device> d_dzdx_numeric{q, n_local, n_global};
+
+  double lx = 8;
+  double dx = lx / n_global;
+  double lx_local = lx / world_size;
+  double scale = n_global / lx;
+  auto fn = [](double x, double y) { return x * x * x + y * y; };
+  auto fn_dzdx = [](double x, double y) { return 3 * x * x; };
+
+  struct timespec start, end;
+  double iter_time = 0.0;
+  double total_time = 0.0;
+
+  double x_start = world_rank * lx_local;
+  for (int j = 0; j < n_global; j++) {
+    double ytmp = j * dx;
+    for (int i = 0; i < n_local; i++) {
+      double xtmp = x_start + i * dx;
+      h_z[idx2(n_local_with_ghost, i + n_bnd, j)] = fn(xtmp, ytmp);
+      h_dzdx_actual[idx2(n_local, i, j)] = fn_dzdx(xtmp, ytmp);
+    }
+  }
+
+  // fill boundary points on ends
+  if (world_rank == 0) {
+    for (int j = 0; j < n_global; j++) {
+      double ytmp = j * dx;
+      for (int i = 0; i < n_bnd; i++) {
+        double xtmp = (i - n_bnd) * dx;
+        h_z[idx2(n_local_with_ghost, i, j)] = fn(xtmp, ytmp);
+      }
+    }
+  }
+  if (world_rank == world_size - 1) {
+    for (int j = 0; j < n_global; j++) {
+      double ytmp = j * dx;
+      for (int i = 0; i < n_bnd; i++) {
+        double xtmp = lx + i * dx;
+        h_z[idx2(n_local_with_ghost, n_bnd + n_local + i, j)] = fn(xtmp, ytmp);
+      }
+    }
+  }
+
+  /*
+  for (int i = 0; i < 5; i++) {
+    int idx = idx2(n_global, 1, i);
+    printf("%d row1-l %f\n", world_rank, h_z[idx]);
+  }
+  for (int i = 0; i < 5; i++) {
+    int idx = idx2(n_global, 1, n_local_with_ghost - 1 - i);
+    printf("%d row1-r %f\n", world_rank, h_z[idx]);
+  }
+  */
+
+  copy(q, h_z, d_z);
+
+  for (int i = 0; i < n_warmup + n_iter; i++) {
+    clock_gettime(CLOCK_MONOTONIC, &start);
+    boundary_exchange_x(MPI_COMM_WORLD, world_size, world_rank, q, n_bnd, d_z,
+                        stage_host);
+    clock_gettime(CLOCK_MONOTONIC, &end);
+    iter_time =
+      ((end.tv_sec - start.tv_sec) + (end.tv_nsec - start.tv_nsec) * 1.0e-9);
+
+    if (i >= n_warmup) {
+      total_time += iter_time;
+    }
+
+    // do some calculation, to try to more closely simulate what happens in GENE
+    auto e = stencil2d_1d_5(q, d_dzdx_numeric, d_z, scale);
+    e.wait();
+  }
+  printf("%d/%d exchange time %0.8f ms\n", world_rank, world_size,
+         total_time / n_iter * 1000);
+
+  copy(q, d_dzdx_numeric, h_dzdx_numeric).wait();
+
+  /*
+  for (int i = 0; i < 5; i++) {
+    int idx = idx2(n_global, 8, i);
+    printf("%d la %f\n%d ln %f\n", world_rank, h_dzdx_actual[idx], world_rank,
+           h_dzdx_numeric[idx]);
+  }
+  for (int i = 0; i < 5; i++) {
+    int idx = idx2(n_global, 8, n_local - 1 - i);
+    printf("%d ra %f\n%d rn %f\n", world_rank, h_dzdx_actual[idx], world_rank,
+           h_dzdx_numeric[idx]);
+  }
+  */
+
+  double err_norm = diff_norm(q, h_dzdx_numeric.size(), h_dzdx_numeric.data(),
+                              h_dzdx_actual.data());
+
+  printf("%d/%d [%d:0x%08x] err_norm = %.8f\n", world_rank, world_size,
+         device_id, vendor_id, err_norm);
+
+  MPI_Finalize();
+
+  return EXIT_SUCCESS;
+}