MPI Chapter 3 More Beginning MPI

MPI Philosopy One program for all processes – Starts with init – Get my process number Process 0 is usually the “Master” node (One process to bind them all – apologies to J.R.R. Tolkien.) – Big if/else statement to do master stuff verses slave stuff. Master could also do some slave stuff – Load balancing issues

C++ MPI at WU on Herot #include “mpi.h” int main(int argc, char *argv[]) MPI::Init(argc, argv) – Typically –np # to set up COMM_WORLD mpic++ - to compile mpi programs mpirun –np # Plus stuff in Josh’s handout about system stuff

Bcast MPI::COMM_WORLD.Bcast(buf, count, datatype, root) – EVERY PROCESS executes this function. It is BOTH a send and receive. – Root is the “sender”, all other processes are receivers.

Reduce MPI::COMM_WORLD.Reduce(sendbuf, recvbuf, count, datatype, op, root) Executed by ALL processes (somewhat of a send and receive). EVERYONE sends sendbuf where op is performed on all those items and the answer appears in recvbuf of process root. Op is specified by one of many constants (ex. MPI::SUM, MPI::PROD, MPI::MAX, MPI::MIN)

Timing MPI Programs double MPI::Wtime() – Time in seconds since some arbitrary point in time – Call twice, once at beginning, once at end of code to time – Difference is elapsed time double MPI::Wtick() – Granularity, in seconds, of MPI Wtime function

Receive revisited Recall – MPI::COMM_WORLD.Recv(buf, count, datatype, source, tag, status) – Source and/or tag could be a wildcard (MPI::ANY_TAG, MPI::ANY_SOURCE) – Status type MPI::Status int MPI::Status.Get_source() int MPI::Status.Get_tag()

Communicators MPI_COMM_WORLD – has everything Can create different communicators so can do operations with subgroups of processors

Creating Communicators MPI_Comm – data type for a communicator MPI_Group – data type for a group Can assign communicators (com1=com2) Use a group of processors to create a communicator. MPI_Comm_Group – gets the group from a communicator MPI_Comm_create – create communicator from a group

Communicator Manipulation MPI_Group_excl – exclude a process from a group MPI_Comm_free – releases communicator MPI_Group_free

Allreduce Equivalent to reduce + Bcast.