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.deven...@yahoo.co.uk>
To: Open MPI Users <us...@open-mpi.org>
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
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