Hi, I'm developing a causality chain tracking library and need a mechanism to attach an extra data to every MPI message, so called piggyback mechanism.
As far as I know there are a few solutions to this problem from which the two fundamental ones are the following: - Dynamic datatype wrapping - if a user MPI_Send, let's say 1024 doubles, the wrapped send call implementation dynamically creates a derived datatype that is a structure composed of a pointer to 1024 doubles and extra fields to be piggybacked. The datatype is constructed with absolute addresses to avoid copying the original buffer. The receivers side creates the equivalent datatype to receive the original data and extra data. The performance of this solution depends on the how good is derived data type handling, but seems to be lightweight. - Sending extra data in a separate message -- seems this can have much more significant overhead Do you know any other portable solution? I have implemented the first solution for P2P operations and it works pretty well. However there are problems with collective operations. There are 2 classes of collective calls that are problematic: 1. Single receiver calls, like MPI_Gather. The sender tasks in gather can be handled in the same way as a normal send, a data item is wrapped and extra data is piggybacked with the message. The problem is at the receiver side when a root gathers N data items that must be received in an array big enough to receive all items strided by datatype extent. In particular, it seems impossible to construct a datatype that contains data item and extra data (i.e. structure type with absolute addresses) AND make an array of these datatypes separated by a fixed extent. For example: data item to receive from every process is a vector of 1024 doubles. Extra data is a single integer. User provides a receive buffer with place for N * 1024 * double. The library allocates an array of N integers to receive piggybacked data. How to construct a datatype that can be used to receive data in MPI_Gather? 2. MPI_Reduce calls. There is no problem with datatypes as the receiver gets the single data item and not an array as in previous case. The problem is the reduction operator itself (MPI_Op) because these operators do not work with wrapped data types. So I can create a new operator to recognize the wrapped data type that extracts the original data (skipping extra data) and performs the original reduction. The point is how to invoke the original reduction on an existing datatype. I have found that Open MPI calls internally ompi_op_reduce(op, inbuf, rbuf, count, dtype) this solves a problem. However this makes the code MPI-implementation dependent. Any idea on more portable options? Thank you in advance for any comment. --Oleg
