diff --git a/Parallel_mpi/Parallel.ml b/Parallel_mpi/Parallel.ml
index 5641272..bf81735 100644
--- a/Parallel_mpi/Parallel.ml
+++ b/Parallel_mpi/Parallel.ml
@@ -83,6 +83,8 @@ module Node = struct
     | None -> assert false
 
   let broadcast x = broadcast_generic Mpi.broadcast x
+
+  let barrier () = (* Mpi.barrier comm *) ()
 end
 
 
diff --git a/Parallel_mpi/Parallel.mli b/Parallel_mpi/Parallel.mli
index 19d1f23..9a74928 100644
--- a/Parallel_mpi/Parallel.mli
+++ b/Parallel_mpi/Parallel.mli
@@ -30,6 +30,7 @@ val broadcast_float_array : float array -> float array
 val broadcast_vec : Lacaml.D.vec -> Lacaml.D.vec
 (** Broadcasts a Lacaml vector to all processes. *)
 
+
 (** {5 Intra-node  operations} *)
 module Node : sig
   val name : string
@@ -46,10 +47,14 @@ module Node : sig
 
   val broadcast : 'a lazy_t -> 'a
   (** Broadcasts data to all the processes of the current node. *)
+
+  val barrier : unit -> unit
+  (** Wait for all processes among the node to reach this point. *)
 end
 
+
 (*
-(** {5 Intra-node  operations} *)
+(** {5 Inter-node  operations} *)
 module InterNode : sig
 
   val comm : Mpi.communicator
diff --git a/Utils/FourIdxStorage.ml b/Utils/FourIdxStorage.ml
index b1e45a2..078ffca 100644
--- a/Utils/FourIdxStorage.ml
+++ b/Utils/FourIdxStorage.ml
@@ -14,7 +14,6 @@ type storage_t =
 type t =
   {
     size         : int ;
-    filename     : string ;
     four_index   : storage_t ;
   }
 
@@ -128,41 +127,18 @@ let set ~r1 ~r2 ~value =
 let increment ~r1 ~r2 = 
   increment_four_index ~r1 ~r2 
 
-let create ~size ?temp_dir sparsity =
+let create ~size ?(temp_dir="/dev/shm") sparsity =
   assert (size < max_index);
-  let filename = Parallel.broadcast (lazy (Filename.temp_file ?temp_dir "4idx." ".tmp")) in
-  at_exit (fun () -> try Sys.remove filename with _ -> ());
   let four_index = 
     match sparsity with
-    | `Dense ->
-      let result = 
-        if Parallel.master then
-          begin
-Printf.printf "filename: %s\n%!" filename;
-            let fd = Unix.openfile filename [Unix.O_RDWR] 0o600 in
-            let result = 
-              Unix.map_file fd Float64 Bigarray.fortran_layout  true
-                [| size*size ; size*size |]
-              |> Bigarray.array2_of_genarray
-            in Bigarray.Array2.fill result 0.;
-            Parallel.barrier ();
-            result
-          end
-        else
-          begin
-            Parallel.barrier ();
-            let fd = Unix.openfile filename [Unix.O_RDONLY] 0o600 in
-            Unix.map_file fd Float64 Bigarray.fortran_layout false
-              [| size*size ; size*size |]
-            |> Bigarray.array2_of_genarray
-          end
-      in
-      Dense result
-    | `Sparse -> 
-      let result = Hashtbl.create (size*size+13) in
+    | `Dense  -> let result =
+                     SharedMemory.create Float64 [| size*size ; size*size |]
+                     |> Bigarray.array2_of_genarray
+                 in Dense result
+    | `Sparse -> let result = Hashtbl.create (size*size+13) in
       Sparse result
   in
-  { size ; filename ; four_index }
+  { size ; four_index }
 
 
 let size t = t.size