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

fixing unmatched int type

fixing indent

trying to fix the broadcast error

Trying to fix broadcast error II

adding _c to write_at functions

fixing rank issue

fixing footer_offset

last fix on rank

indent

removing some unused line

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(
@@ -7688,8 +7690,8 @@ DataOutInterface<dim, spacedim>::write_vtu_in_parallel(
   write_vtu(f);
 #else
 
-  const int myrank = Utilities::MPI::this_mpi_process(comm);
-
+  const unsigned int myrank = Utilities::MPI::this_mpi_process(comm);
+  const unsigned int n_ranks = Utilities::MPI::n_mpi_processes(comm);
   MPI_Info info;
   int ierr = MPI_Info_create(&info);
   AssertThrowMPI(ierr);
@@ -7707,21 +7709,24 @@ 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;
+  std::uint64_t footer_offset;
 
   // write header
   if (myrank == 0)
     {
       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 = Utilities::MPI::LargeCount::MPI_File_write_at_c(
+        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();
@@ -7741,21 +7746,47 @@ 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);
+
+    if (myrank == n_ranks - 1)
+      {
+        // Locating Footer with offset on last rank.
+        footer_offset = size_on_proc + offset;
+
+        std::stringstream ss;
+        DataOutBase::write_vtu_footer(ss);
+        const unsigned int footer_size = ss.str().size();
+
+        // Writing Footer.
+        ierr =
+          Utilities::MPI::LargeCount::MPI_File_write_at_c(fh,
+                                                          footer_offset,
+                                                          ss.str().c_str(),
+                                                          footer_size,
+                                                          MPI_CHAR,
+                                                          MPI_STATUS_IGNORE);
+        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);
-      AssertThrowMPI(ierr);
-    }
+
   ierr = MPI_File_close(&fh);
   AssertThrowMPI(ierr);
 #endif