This example shows how to overlap computation and output. The computation is performed by the function compute_buffer().
/*=========================================================================
*
* Function: double_buffer
*
* Synopsis:
* void double_buffer(
* MPI_File fh, ** IN
* MPI_Datatype buftype, ** IN
* int bufcount ** IN
* )
*
* Description:
* Performs the steps to overlap computation with a collective write
* by using a double-buffering technique.
*
* Parameters:
* fh previously opened MPI file handle
* buftype MPI datatype for memory layout
* (Assumes a compatible view has been set on fh)
* bufcount # buftype elements to transfer
*------------------------------------------------------------------------*/
/* this macro switches which buffer "x" is pointing to */
#define TOGGLE_PTR(x) (((x)==(buffer1)) ? (x=buffer2) : (x=buffer1))
void double_buffer( MPI_File fh, MPI_Datatype buftype, int bufcount)
{
MPI_Status status; /* status for MPI calls */
float *buffer1, *buffer2; /* buffers to hold results */
float *compute_buf_ptr; /* destination buffer */
/* for computing */
float *write_buf_ptr; /* source for writing */
int done; /* determines when to quit */
/* buffer initialization */
buffer1 = (float *)
malloc(bufcount*sizeof(float));
buffer2 = (float *)
malloc(bufcount*sizeof(float));
compute_buf_ptr = buffer1; /* initially point to buffer1 */
write_buf_ptr = buffer1; /* initially point to buffer1 */
/* DOUBLE-BUFFER prolog:
* compute buffer1; then initiate writing buffer1 to disk
*/
compute_buffer(compute_buf_ptr, bufcount, &done);
MPI_File_write_all_begin(fh, write_buf_ptr, bufcount, buftype);
/* DOUBLE-BUFFER steady state:
* Overlap writing old results from buffer pointed to by write_buf_ptr
* with computing new results into buffer pointed to by compute_buf_ptr.
*
* There is always one write-buffer and one compute-buffer in use
* during steady state.
*/
while (!done) {
TOGGLE_PTR(compute_buf_ptr);
compute_buffer(compute_buf_ptr, bufcount, &done);
MPI_File_write_all_end(fh, write_buf_ptr, &status);
TOGGLE_PTR(write_buf_ptr);
MPI_File_write_all_begin(fh, write_buf_ptr, bufcount, buftype);
}
/* DOUBLE-BUFFER epilog:
* wait for final write to complete.
*/
MPI_File_write_all_end(fh, write_buf_ptr, &status);
/* buffer cleanup */
free(buffer1);
free(buffer2);
}