title: MPI_CART_CREATE period argument
body: <p>I kind of understand what this routine does, in the sense that it creates a Cartesian coordinate system for processors over the actual scheme, but I don't understand what the "periods" argument does. </p>
  
  <p>I'm currently setting it all to 0, but I'd like to know what periods are, can't find anything online on the subject.</p>
  
  <p>Here's the signature : </p>
  
  <pre><code>int MPI_Cart_create(
    MPI_Comm comm_old,
    int ndims,
    int *dims,
    int *periods,
    int reorder,
    MPI_Comm *comm_cart
  );
  </code></pre>