Upgrade write_vtu_in_parallel based on mpi large IO update

Updating author name

fixing work

update indent

Remove the broadcast and fix some comment and fix author name

updates

updates!

last fix

fixing indent

source/base/data_out_base.cc

@@ -34,6 +34,7 @@
 #include <deal.II/base/data_out_base.h>
 #include <deal.II/base/memory_consumption.h>
 #include <deal.II/base/mpi.h>
+#include <deal.II/base/mpi_large_count.h>
 #include <deal.II/base/parameter_handler.h>
 #include <deal.II/base/thread_management.h>
 #include <deal.II/base/utilities.h>
@@ -7673,6 +7674,7 @@ DataOutInterface<dim, spacedim>::write_svg(std::ostream &out) const
                          out);
 }
 
+
 template <int dim, int spacedim>
 void
 DataOutInterface<dim, spacedim>::write_vtu_in_parallel(
@@ -7689,7 +7691,7 @@ DataOutInterface<dim, spacedim>::write_vtu_in_parallel(
 #else
 
   const int myrank = Utilities::MPI::this_mpi_process(comm);
-
+  const int n_ranks = Utilities::MPI::n_mpi_processes(comm);
   MPI_Info info;
   int ierr = MPI_Info_create(&info);
   AssertThrowMPI(ierr);
@@ -7707,6 +7709,7 @@ DataOutInterface<dim, spacedim>::write_vtu_in_parallel(
   ierr = MPI_Info_free(&info);
   AssertThrowMPI(ierr);
 
+  // Define header size so we can broadcast later.
   unsigned int header_size;
 
   // write header
@@ -7715,13 +7718,15 @@ DataOutInterface<dim, spacedim>::write_vtu_in_parallel(
       std::stringstream ss;
       DataOutBase::write_vtu_header(ss, vtk_flags);
       header_size = ss.str().size();
-      // Write the header on rank 0 and automatically move the
-      // shared file pointer to the location after header;
-      ierr = MPI_File_write_shared(
-        fh, ss.str().c_str(), header_size, MPI_CHAR, MPI_STATUS_IGNORE);
+      // Write the header on rank 0 at the starting of a file, i.e., offset 0.
+      ierr = MPI_File_write_at(
+        fh, 0, ss.str().c_str(), header_size, MPI_CHAR, MPI_STATUS_IGNORE);
       AssertThrowMPI(ierr);
     }
 
+  ierr = MPI_Bcast(&header_size, 1, MPI_UNSIGNED, 0, comm);
+  AssertThrowMPI(ierr);
+
 
   {
     const auto &patches = get_patches();
@@ -7741,19 +7746,58 @@ DataOutInterface<dim, spacedim>::write_vtu_in_parallel(
                                   vtk_flags,
                                   ss);
 
-    ierr = MPI_File_write_ordered(
-      fh, ss.str().c_str(), ss.str().size(), MPI_CHAR, MPI_STATUS_IGNORE);
+    // using prefix sum to find specific offset to write at.
+    const std::uint64_t size_on_proc = ss.str().size();
+    std::uint64_t prefix_sum = 0;
+    ierr =
+      MPI_Exscan(&size_on_proc, &prefix_sum, 1, MPI_UINT64_T, MPI_SUM, comm);
+    AssertThrowMPI(ierr);
+
+    // Locate specific offset for each processor.
+    const MPI_Offset offset = static_cast<MPI_Offset>(header_size) + prefix_sum;
+
+    ierr =
+      Utilities::MPI::LargeCount::MPI_File_write_at_all_c(fh,
+                                                          offset,
+                                                          ss.str().c_str(),
+                                                          ss.str().size(),
+                                                          MPI_CHAR,
+                                                          MPI_STATUS_IGNORE);
     AssertThrowMPI(ierr);
+
+    // Sending the offset for writing the footer to rank 0.
+    if (myrank == n_ranks - 1)
+      {
+        const std::uint64_t footer_offset = size_on_proc + offset;
+        ierr = MPI_Send(&footer_offset, 1, MPI_UINT64_T, 0, 0, comm);
+        AssertThrowMPI(ierr);
+      }
   }
 
   // write footer
   if (myrank == 0)
     {
       std::stringstream ss;
       DataOutBase::write_vtu_footer(ss);
-      unsigned int footer_size = ss.str().size();
-      ierr = MPI_File_write_shared(
-        fh, ss.str().c_str(), footer_size, MPI_CHAR, MPI_STATUS_IGNORE);
+      const unsigned int footer_size = ss.str().size();
+
+      std::uint64_t footer_offset;
+      ierr = MPI_Recv(&footer_offset,
+                      1,
+                      MPI_UINT64_T,
+                      n_ranks - 1,
+                      0,
+                      comm,
+                      MPI_STATUS_IGNORE);
+      AssertThrowMPI(ierr);
+
+      // Writing Footer.
+      ierr = MPI_File_write_at(fh,
+                               footer_offset,
+                               ss.str().c_str(),
+                               footer_size,
+                               MPI_CHAR,
+                               MPI_STATUS_IGNORE);
       AssertThrowMPI(ierr);
     }
   ierr = MPI_File_close(&fh);