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MPI Collective Communication and Comm Split in Psmile

Understand the use of MPI collective calls and the importance of calling MPI_comm_split in Psmile. Learn about MPI_Init, MPI_Comm_Spawn, MPI_Comm_Group, and point-to-point synchronous and asynchronous communications.

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MPI Collective Communication and Comm Split in Psmile

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  1. Collective calls of MPI have to be called by each process included in the communicator appearing in the routine MPI  so when the driver (or the psmile) makes a collective call within the global communicator, a similar call has to be put in the psmile (or the driver), else the processes calling the collective routine will wait (see mpi_allreduce and mpi_bcast in prismdrv_init_appl and Psmile_init_mpi1) MPI_comm_split(comm_old (in), color, key, comm_new(out), ierror) : all procs with same color are in the same comm_new, with rank key (making the key value zero for all processes of a given color means that one doesn't really care about the rank-order of the processes in the new communicator) MPI_comm_split(comm_old, 1, 0, comm_new, ierror) : to create the comm comm_psmile applies only to applications If color = MPI_undefined for one of the processes, it is excluded from the new comm

  2. Include ‘mpif.h’ MPI_Init (ierror) : must be called before any other MPI routine are called except MPI_Initialized that is used to know if a process has called MPI_init. All programs MPI must contain a call to MPI_Init MPI_Comm_Spawn (char*command,char*arg,int*maxprocs,MPI_INFO_NULL,int*root, int*intracomm, int*intercomm, int*errors)

  3. To construct a communicator, you can use another communicator or a group of processors. The communicator created by default is MPI_COMM_WORLD. It is created with MPI_Init and killed with MPI_Finalize. A communicator is associated to a group and MPI_Comm_Group is used to know this group : MPI_comm_group(comm(in), group(out), ierror) : associated with a communicator Is its group identity or handle (here group). MPI_Group_translate_ranks ( MPI_Group group_a, int n, int *ranks_a, MPI_Group group_b, int *ranks_b, ierror ) : (IN) groupa : groupa (handle) (IN) n number of ranks in ranks1 and ranks2 arrays (IN) (integer) ranksa array of zero or more valid ranks in groupa (IN) groupb : groupb (handle) (OUT) ranksb array of corresponding ranks in group2, (MPI_UNDEFINED when No correspondence exists).

  4. A message in MPI == (data + envelope) Point to point synchronous communications, blocking : MPI_SEND(buf,count,DTYPE, dest,tag,comm,ierr) : info on the destination is given MPI_RECV(buf,count,DTYPE, source,tag,comm,status,ierr) : info from which proc it comes is given Définition du mot SYNCHRONE : se dit d'une communication lorsque l'émetteur et le récepteur fonctionnent au rythme d'une horloge commune, réglée au début de la communication.

  5. A message in MPI == (data + envelope) Point to point asynchronous communications, non blocking (I=immediate): MPI_I(R,S,B)SEND(buf,count,DTYPE,dest,tag,comm,request,ierr) : info on the destination is given MPI_IRECV(buf,count,DTYPE,source,tag,comm,request,ierr) : info from which proc it comes is given A receive request can be determined being completed by calling the MPI_Wait, MPI_Waitany, MPI_Test, or MPI_Testany with request returned by this function Définition du mot ASYNCHRONE : désigne un type d’échange de données entre deux machines où les données échangées sont émises et analysée selon une référence de temps différente et un rythme variable.

  6. MPI_REDUCE(sendbuf,recvbuf,count,datatype,operation,root,comm,ierr) : Reduces values on all processes to a single value, following the operation “operation” defined above. The result of the operation on sendbuf is in recvbuf on proc root. MPI_ALLREDUCE(sendbuf,recvbuf,count,datatype,operation,comm,ierr) : Reduces values on all processes to a single value, following the operation “operation” defined above. The result of the operation on sendbuf is in recvbuf redistributed to all procs. MPI_BCAST(buf,count,datatype,root,comm,ierr) : Broadcasts (sends) a message from the process with rank "root" to all other processes of the group in the comm. MPI_GATHER(sndbuf, sndcount, sndtype, rcvbuf, rcvcount,rcvtype, root, comm, ierr) :

  7. MPI_WAITALL(int count,MPI_request, MPI_status, ierr) : blocks until all communications associated with active handles in the list complete, and return the status of all these operations. Both arrays MPI_request and MPI_status have the same number of valid entries

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