Hello,
I have a custom datatype MPI_EVENTDATA (created with MPI_Type_create_struct)
which is a struct with some fixed size fields and a variable sized array of
ints (data). I want to collect a variable number of these types (Events) from
all ranks at rank 0. My current version is working for a fixed size custom
datatype:
void collect()
{
int rank;
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
size_t globalSize = events.size();
// Get total number of Events that are to be received
MPI_Allreduce(MPI_IN_PLACE, &globalSize, 1, MPI_INT, MPI_SUM, MPI_COMM_WORLD);
std::vector<MPI_Request> requests(globalSize);
std::vector<MPI_EventData> recvEvents(globalSize);
if (rank == 0) {
for (size_t i = 0; i < globalSize; i++) {
MPI_Irecv(&recvEvents[i], 1, MPI_EVENTDATA, MPI_ANY_SOURCE, MPI_ANY_TAG,
MPI_COMM_WORLD, &requests[i]);
}
}
for (const auto & ev : events) {
MPI_EventData eventdata;
assert(ev.first.size() < 255);
strcpy(eventdata.name, ev.first.c_str());
eventdata.rank = rank;
eventdata.dataSize = ev.second.data.size();
MPI_Send(&eventdata, 1, MPI_EVENTDATA, 0, 0, MPI_COMM_WORLD);
}
if (rank == 0) {
MPI_Waitall(globalSize, requests.data(), MPI_STATUSES_IGNORE);
for (const auto & evdata : recvEvents) {
// Save in a std::multimap with evdata.name as key
globalEvents.emplace(std::piecewise_construct,
std::forward_as_tuple(evdata.name),
std::forward_as_tuple(evdata.name, evdata.rank));
}
}
}
Obviously, next step would be to allocate a buffer of size evdata.dataSize,
receive it, add it to globalEvents multimap<Event> and be happy. Questions I
have:
* How to correlate the received Events in the first step, with the received
data vector in the second step?
* Is there a way to use a variable sized compononent inside a custom MPI
datatype?
* Or dump the custom datatype and use MPI_Pack instead?
* Or somehow group two succeeding messages together?
I'm open to any good and elegant suggestions!
Thanks,
Florian
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