On Nov 23, 2005, at 6:00 AM, Robert Ramey wrote:
Ian McCulloch wrote:
You need to construct an MPI 'datatype', which is conceptually a record of how the type is laid out as a sequence of (offset, nested_datatype) pairs.
...
The 'usual' way (if there is such a thing; datatypes seem to be not used much in MPI) of constructing datatypes is using offsetof(), or just knowing what the layout is for compiler X, and constructing the datatype by hand.
...
I don't know what mechanism Dave is thinking of to construct the datatype, it sounds unlikely that it could be done via a usual serialization function (but maybe if you could do member pointer arithmetic to replace offsetof() ?).
Sounds like a lot of work. But if one want's to use all this stuff - then what's the point of using the serialization library at all? I mean I can see using one or the other - but what is the point of both?
Indeed this sounds like a lot of work and that's why this mechanism for message passing was rarely used in the past. The hard part is to manually build up the custom MPI datatype, i.e. to inform MPI about what the offsets and types of the various data members in a struct are. This is where the serialization library fits in and makes the task extraordinarily easy. Saving a data member with such an MPI archive will register its address, type (as well as the number of identical consecutive elements in an array) with the MPI library. Thus the serialization library does all the hard work already! As Dave mentioned earlier, this information can then used by the MPI library (and network hardware) to directly serialize the data into the I/O buffers of the network interconnect, without ever creating a copy in memory, and automatically taking care of potential endianness and format issues on heterogeneous networks. To answer your question: one wants to use both since the serialization library is used to create the information which the MPI library needs to efficiently send the data. Matthias