1. Collectives
Reduce
Scatter
Gather
Many more

2. Prototype of MPI_Reduce()
int MPI_Reduce ( void *operand,
/* addr of 1st reduction element */ void *result,
/* addr of 1st reduction result */ int count,
/* reductions to perform */ MPI_Datatype type,
/* type of elements */ MPI_Op operator,
/* reduction operator */
int root,
/* process getting result(s) */
MPI_Comm comm
/* communicator */ )

3. MPI_Datatype Options MPI_CHAR MPI_DOUBLE MPI_FLOAT MPI_INT MPI_LONG
MPI_LONG_DOUBLE
MPI_SHORT
MPI_UNSIGNED_CHAR
MPI_UNSIGNED
MPI_UNSIGNED_LONG
MPI_UNSIGNED_SHORT

4. MPI_Op Options MPI_BAND MPI_BOR MPI_BXOR MPI_LAND MPI_LOR MPI_LXOR
MPI_MAX
MPI_MAXLOC
MPI_MIN
MPI_MINLOC
MPI_PROD
MPI_SUM

5. Our Call to MPI_Reduce()
MPI_Reduce (&count,
&global_count, 1,
MPI_INT,
MPI_SUM, 0,
MPI_COMM_WORLD);
Only process 0
will get the result
if (!id) printf ("There are %d different solutions\n",
global_count);

6. MPI_Allgatherv

7. MPI_Allgatherv int MPI_Allgatherv ( void *send_buffer, int send_cnt,
MPI_Datatype send_type,
void *receive_buffer,
int *receive_cnt,
int *receive_disp,
MPI_Datatype receive_type,
MPI_Comm communicator)

8. MPI_Allgatherv in Action

9. Reading a Block-Column Matrix
File

10. MPI_Scatterv

11. Header for MPI_Scatterv
int MPI_Scatterv (
void *send_buffer,
int *send_cnt,
int *send_disp,
MPI_Datatype send_type,
void *receive_buffer,
int receive_cnt,
MPI_Datatype receive_type,
int root,
MPI_Comm communicator)

12. Printing a Block-Column Matrix
Data motion opposite to that we did when reading the matrix
Replace “scatter” with “gather”
Use “v” variant because different processes contribute different numbers of elements

13. Function MPI_Gatherv

14. Header for MPI_Gatherv
int MPI_Gatherv (
void *send_buffer,
int send_cnt,
MPI_Datatype send_type,
void *receive_buffer,
int *receive_cnt,
int *receive_disp,
MPI_Datatype receive_type,
int root,
MPI_Comm communicator)

15. Function MPI_Alltoallv

16. Header for MPI_Alltoallv
int MPI_Gatherv (
void *send_buffer,
int *send_cnt,
int *send_disp,
MPI_Datatype send_type,
void *receive_buffer,
int *receive_cnt,
int *receive_disp,
MPI_Datatype receive_type,
MPI_Comm communicator)

17. Count/Displacement Arrays
create_mixed_xfer_arrays
builds these
MPI_Alltoallv requires two pairs of count/displacement arrays
First pair for values being sent
send_cnt: number of elements
send_disp: index of first element
Second pair for values being received
recv_cnt: number of elements
recv_disp: index of first element