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Subject: [OMPI users] Can we avoid derived datatypes?: Update!
From: devendra rai (rai.devendra_at_[hidden])
Date: 2012-01-05 05:35:14

Dear All, I read a little more about MPI derived data types, and to answer my own question: In general, we cannot assume portability of sending a C/C++ struct as a stream of bytes. There must be a promise that data representation on machines involved in the transmission must perform the same data layout. Also, there was a +1 on using derived data types in terms of clarity of code. But, now that I decided to use it, I run into another problem. I have a function that commits the new datatype: Add_New_MPITypes(). This is called just after MPI_Init(...). After a few subsequent function calls, I am doing MPI-Send/Receive in another function, which looks like: void sendMessagetoSlave(void* Payload, int MESSAGETYPE) {     switch (MESSAGETYPE)     {     case MSGINSTALLP:       {         //Add_MPI_msgInstallP_Type(); /*Was already done in Add_New_MPITypes() */         msgInstallP InstallPMessage;         InstallPMessage = *(msgInstallP*)Payload;         MPI_Ssend(                   (void*)Payload,                       /* Payload */                   sizeof(msgInstallP),                 /* size of the payload */                   MPI_MSGINSTALLP,                      /* MPI Data type */                   InstallPMessage.location,             /* location to which the message is being sent */                   MASTERSLAVECONTROLMESSAGE,            /* Tag */                   MPI_COMM_WORLD                        /* Communicator */                   );       }       break;     default:       break;     } } The linker complains that it does not know MPI_MSGINSTALLP derived datatype. Specifically, the message from the linker is: "‘MPI_MSGINSTALLP’ was not declared in this scope". I have using mpic++ (1.4.2) to compile, and g++ (4.5.3) to link. Can anyone help? Best. Devendra ________________________________ From: devendra rai <rai.devendra_at_[hidden]> To: Open MPI Users <users_at_[hidden]> Sent: Wednesday, 4 January 2012, 17:31 Subject: [OMPI users] Can we avoid derived datatypes? Hello All, I need to send a struct- datatype over MPI. Currently, I send the strcture as a series of MPI_BYTEs and on the other end, I dereference it as though it were a struct- type. Something like this: MPI_Ssend((void*)&MasterSlavePayload, sizeof(MasterSlavePayload), MPI_BYTE, destNode,MASTERSLAVECONTROLMESSAGE,MPI_COMM_WORLD); where MasterSlavePayload is a structure variable. This currently seems to work, where we have a homogenous environment: same hardware configuration, and same operating system. The question is: Is this approach portable? Safe? And whether this will work on a system of nodes with mixed processor types? I read from MPI tutorials "...Primitive data types are contiguous. Derived data types allow you to specify non-contiguous data in a convenient manner and to treat it as though it was contiguous. " So, since I am using a primitive data type, does it mean that the packing of elements is maintained across the MPI_Send/MPI_Recv process? If so, it would mean that the approach that I use should work. Any ideas? Thanks a lot, best Devendra _______________________________________________ users mailing list users_at_[hidden]