MPI_Scatterv: segmentation fault 11 on process 0 only - c++

Question

I'm trying to scatter values among processes belonging to an hypercube group (quicksort project).
Depending on the amount of processes I either create a new communicator excluding excessive processes, or I duplicate MPI_COMM_WORLD if it fits exactly any hypercube (power of 2).
In both cases, processes other than 0 receive their data, but:
- On first scenario, process 0 throws a segmentation fault 11
- On second scenario, nothing faults, but process 0 received values are gibberish.
NOTE: If I try a regular MPI_Scatter everything works well.
//Input
vector<int> LoadFromFile();
int d; //dimension of hypercube
int p; //active processes
int idle; //idle processes
vector<int> values; //values loaded
int arraySize; //number of total values to distribute
int main(int argc, char* argv[])
{
int mpiWorldRank;
int mpiWorldSize;
int mpiRank;
int mpiSize;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &mpiWorldRank);
MPI_Comm_size(MPI_COMM_WORLD, &mpiWorldSize);
MPI_Comm MPI_COMM_HYPERCUBE;
d = log2(mpiWorldSize);
p = pow(2, d); //Number of processes belonging to the hypercube
idle = mpiWorldSize - p; //number of processes in excess
int toExclude[idle]; //array of idle processes to exclude from communicator
int sendCounts[p]; //array of values sizes to be sent to processes
//
int i = 0;
while (i < idle)
{
toExclude[i] = mpiWorldSize - 1 - i;
++i;
}
//CREATING HYPERCUBE GROUP: Group of size of power of 2 -----------------
MPI_Group world_group;
MPI_Comm_group(MPI_COMM_WORLD, &world_group);
// Remove excessive processors if any from communicator
if (idle > 0)
{
MPI_Group newGroup;
MPI_Group_excl(world_group, 1, toExclude, &newGroup);
MPI_Comm_create(MPI_COMM_WORLD, newGroup, &MPI_COMM_HYPERCUBE);
//Abort any processor not part of the hypercube.
if (mpiWorldRank > p)
{
cout << "aborting: " << mpiWorldRank <<endl;
MPI_Finalize();
return 0;
}
}
else
{
MPI_Comm_dup(MPI_COMM_WORLD, &MPI_COMM_HYPERCUBE);
}
MPI_Comm_rank(MPI_COMM_HYPERCUBE, &mpiRank);
MPI_Comm_size(MPI_COMM_HYPERCUBE, &mpiSize);
//END OF: CREATING HYPERCUBE GROUP --------------------------
if (mpiRank == 0)
{
//STEP1: Read input
values = LoadFromFile();
arraySize = values.size();
}
//Transforming input vector into an array
int valuesArray[values.size()];
if(mpiRank == 0)
{
copy(values.begin(), values.end(), valuesArray);
}
//Broadcast input size to all processes
MPI_Bcast(&arraySize, 1, MPI_INT, 0, MPI_COMM_HYPERCUBE);
//MPI_Scatterv: determining size of arrays to be received and displacement
int nmin = arraySize / p;
int remainingData = arraySize % p;
int displs[p];
int recvCount;
int k = 0;
for (i=0; i<p; i++)
{
sendCounts[i] = i < remainingData
? nmin+1
: nmin;
displs[i] = k;
k += sendCounts[i];
}
recvCount = sendCounts[mpiRank];
int recvValues[recvCount];
//Following MPI_Scatter works well:
// MPI_Scatter(&valuesArray, 13, MPI_INT, recvValues , 13, MPI_INT, 0, MPI_COMM_HYPERCUBE);
MPI_Scatterv(&valuesArray, sendCounts, displs, MPI_INT, recvValues , recvCount, MPI_INT, 0, MPI_COMM_HYPERCUBE);
int j = 0;
while (j < recvCount)
{
cout << "rank " << mpiRank << " received: " << recvValues[j] << endl;
++j;
}
MPI_Finalize();
return 0;
}

Answer 1

First of all, you are supplying wrong arguments to MPI_Group_excl:
MPI_Group_excl(world_group, 1, toExclude, &newGroup);
// ^
The second argument specifies the number of entries in the exclusion list and should therefore be equal to idle. Since you are excluding a single rank only, the resulting group has mpiWorldSize-1 ranks and hence MPI_Scatterv expects that both sendCounts[] and displs[] have that many elements. Of those only p elements are properly initialised and and the rest are random, therefore MPI_Scatterv crashes in the root.
Another error is the code that aborts the idle processes: it should read if (mpiWorldRank >= p).
I would recommend that the entire exclusion code is replaced by a single call to MPI_Comm_split instead:
MPI_Comm comm_hypercube;
int colour = mpiWorldRank >= p ? MPI_UNDEFINED : 0;
MPI_Comm_split(MPI_COMM_WORLD, colour, mpiWorldRank, &comm_hypercube);
if (comm_hypercube == MPI_COMM_NULL)
{
MPI_Finalize();
return 0;
}
When no process supplies MPI_UNDEFINED as its colour, the call is equivalent to MPI_Comm_dup.
Note that you should avoid using in your code names starting with MPI_ as those could clash with symbols from the MPI implementation.
Additional note: std::vector<T> uses contiguous storage, therefore you could do without copying the elements into a regular array and simply provide the address of the first element in the call to MPI_Scatter(v):
MPI_Scatterv(&values[0], ...);