Introduction to Parallel Programming with C and MPI at MCSR Part 2 Broadcast/Reduce

Collective Message Passing Broadcast –Sends a message from one to all processes in the group Scatter –Distributes each element of a data array to a different process for computation Gather –The reverse of scatter…retrieves data elements into an array from multiple processes

Collective Message Passing w/MPI MPI_Bcast() Broadcast from root to all other processes MPI_Gather() Gather values for group of processes MPI_Scatter() Scatters buffer in parts to group of processes MPI_Alltoall() Sends data from all processes to all processes MPI_Reduce() Combine values on all processes to single val MPI_Reduce_Scatter() Broadcast from root to all other processes MPI_Bcast() Broadcast from root to all other processes

Log in to mimosa & get workshop files A. Use secure shell to login to mimosa using your assigned training account: ssh ssh See lab instructor for password. B. Copy workshop files into your home directory by running: /usr/local/apps/ppro/prepare_mpi_workshop

Examine, compile, and execute add_mpi.c

Examine add_mpi.pbs

Submit PBS Script: add_mpi.pbs

Examine Output and Errors add_mpi.c

Determine Speedup

Determine Parallel Efficiency

How Could Speedup/Efficiency Improve?

What Happens to Results When MAXSIZE Not Evenly Divisible by n?

Exercise 1: Change Code to Work When MAXSIZE is Not Evenly Divisible by n

Exercise 2: Change Code to Improve Speedup