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Summary of MPI commands Luis Basurto

Summary of MPI commands Luis Basurto. Large scale systems. Shared Memory systems Memory is shared among processors Distributed memory systems Each processor has its own memory. MPI. Created in 1993 as an open standard by large scale system users and creators.

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Summary of MPI commands Luis Basurto

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  1. Summary of MPI commands Luis Basurto

  2. Large scale systems • Shared Memory systems • Memory is shared among processors • Distributed memory systems • Each processor has its own memory

  3. MPI • Created in 1993 as an open standard by large scale system users and creators. • Each system provider implements MPI for its systems. • Currently at version MPI-2.0 • Other implementations of MPI such as MPICH,OpenMPI.

  4. How many commands? • 130+ commands • 6 basic commands (we will cover 11) • C and Fortran bindings

  5. How does an MPI program work? • Start program on n processors • For i=0 to n-1 • Run a copy of program on processor i • Pass messages between processors • End For • End Program

  6. What are messages? • Simplest message: an array of data of one type. • Predefined types correspond to commonly used types in a given language • MPI_REAL (Fortran), MPI_FLOAT (C) • MPI_DOUBLE_PRECISION (Fortran), MPI_DOUBLE (C) • MPI_INTEGER (Fortran), MPI_INT (C) • User can define more complex types and send packages.

  7. Before we start • Include MPI in our program • In C/C++ #include “mpi.h” • In Fortran include 'mpif.h' • In C MPI calls are functions MPI_Init(); • In Fortran they are subroutines call MPI_Init(ierror)

  8. A note about Fortran • All calls to MPI include an extra parameter, an error code of type integer. • Used to test the success of the function (i.e. The function executed correctly).

  9. Basic Communication • Data values are transferred from one processor to another • One processor sends the data • Another receives the data • Synchronous • Call does not return until the message is sent or received • Asynchronous • Call indicates a start of send or receive, and another call is made to determine if finished

  10. MPI_init() • Initializes the MPI environment • Every MPI program must have this. • C • MPI_Init(); • If using command line arguments • MPI_Init( &argc, &argv ); • Fortran • call MPI_Init(ierror)

  11. MPI_Finalize() • Stops the MPI environment • Every MPI program must have this at the end. • C MPI_Finalize ( ); • Fortran call MPI_Finalize(ierr)

  12. MPI_Comm_size() • Returns the size of the communicator (number of nodes) that we are working with. • C MPI_Comm_size ( MPI_COMM_WORLD, &p ); • Fortran call MPI_COMM_SIZE(MPI_COMM_WORLD, p, ierr )

  13. MPI_Comm_rank() • Return the zero based rank (id number) of the node executing the program. • C MPI_Comm_rank ( MPI_COMM_WORLD, &id ); • Fortran call MPI_COMM_RANK(MPI_COMM_WORLD, my_rank, ierr )

  14. A note con communicators • MPI_COMM_WORLD is the default communicator (all nodes in the cluster) • Communicators can be created dynamically in order to assign certain tasks to certain nodes (processors). • Inter communicator message passing is possible.

  15. MPI_Send() • C MPI_Send(void *buf, int count, MPI_Datatype dtype, int dest, int tag, MPI_Comm comm); • Fortran Call MPI_Send(buffer, count, datatype, destination,tag,communicator, ierr)

  16. MPI_Recv() • C MPI_Recv(void *buf, int count, MPI_Datatype dtype, int src,int tag, MPI_Comm comm, MPI_Status *stat); • Fortran Call MPI_Recv(buffer, count, datatype,source, tag, communicator,status, ierr)

  17. MPI_Bcast() • Send message to all nodes • C MPI Bcast(void * buf, int count, MPI_Datatype dtype, int root, MPI Comm comm); • Fortran CALL MPI_BCAST(buff, count, MPI_TYPE, root, comm, ierr)

  18. MPI_Reduce() • Receive message from all nodes, do operation on every element. • C MPI_Reduce(void *sbuf, void* rbuf, int count, MPI_Datatype dtype, MPI_Op op, int root, MPI Comm comm); • Fortran CALL MPI_REDUCE(sndbuf, recvbuf,count, datatype,operator,root,comm,ierr)

  19. MPI_Barrier() • Used as a synchronization barrier, every node that reaches this point must wait until all nodes reach it in order to proceed. • C MPI_Barrier(MPI_COMM_WORLD); • Fortran call MPI_Barrier(MPI_COMM_WORLD,ierr)

  20. MPI_Scatter() • Parcels out data from the root to every member of the group in linear order by node • C MPI_Scatter(void *sbuf, int scount, MPI_Datatype sdtype,void *rbuf, int rcount, MPI_Datatype rdtype,int root, MPI_Comm comm) • Fortran CALL MPI_SCATTER(sndbuf,scount,datatype, recvbuf,rcount,rdatatype,root,comm, ierr)

  21. MPI_Scatter Node 0 Node 1 Node 2 Node 3

  22. MPI_Gather() • C MPI_Gather(void *sbuf, int scount, MPI_Datatype sdtype,void *rbuf, int rcount, MPI_Datatype rdtype,int root, MPI_Comm comm) Fortran CALL MPI_GATHER(sndbuf,scount,datatype, recvbuf,rcount,rdatatype,root,comm,ierr)

  23. Deadlock • The following code may provoke deadlock if(rank==0) { MPI_COMM_WORLD.Send(vec1,vecsize,MPI::DOUBLE,1,0); MPI_COMM_WORLD.Recv(vec2,vecsize,MPI::DOUBLE,1,MPI::ANY_TAG); } if(rank==1) { MPI_COMM_WORLD.Send(vec3,vecsize,MPI::DOUBLE,0,0); MPI_COMM_WORLD.Recv(vec4,vecsize,MPI::DOUBLE,0,MPI::ANY_TAG); }

  24. Bcast • Must be called by all nodes, the following code will not work if(rank==0) { MPI_Bcast(&value, 1, MPI_int,0, MPI_comm_world); } else { /* Do something else */ }

  25. Questions

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