Guidebook: Fix recommendation for MPI Tags for Reductions
When implementing an own MPI Reductions plotter, the guidebook tells us we should write something like
void finishRow() {
#ifdef Parallel
// Question: Do we really reserve a free tag and release on each function call?
const int reductionTag = tarch::parallel::Node::getInstance().reserveFreeTag(
std::string("TimeSeriesReductions(") + filename + ")::finishRow()" );
if(master) {
double recieved[LEN];
for (int rank=1; rank<tarch::parallel::Node::getInstance().getNumberOfNodes(); rank++) {
if(!tarch::parallel::NodePool::getInstance().isIdleNode(rank)) {
MPI_Recv( &recieved[0], LEN, MPI_DOUBLE, rank, reductionTag, tarch::parallel::Node::getInstance().getCommunicator(), MPI_STATUS_IGNORE );
addValues(recieved);
}
}
} else {
for (int rank=1; rank<tarch::parallel::Node::getInstance().getNumberOfNodes(); rank++) {
if(!tarch::parallel::NodePool::getInstance().isIdleNode(rank)) {
MPI_Send( &data[0], LEN, MPI_DOUBLE, tarch::parallel::Node::getGlobalMasterRank(), reductionTag, tarch::parallel::Node::getInstance().getCommunicator());
}
}
}
tarch::parallel::Node::getInstance().releaseTag(reductionTag);
#endif
if(master) {
TimeSeriesReductions::finishRow();
writeRow();
}
}
However I assume this is not that good, looking at the output:
First question: Does every plotter need it's own tag? In principal only if they would be called in parallel, isn't it?
In any case, I think the tags should'nt be created so frequently, isn't it? Instead, only once on startup time?
This should be fixed in the guidebook and my code :D