MPI Scatter error on communicator MPI_COMM_WORLD

Question

The following code fails during runtime because of the MPI Scatter Error which I am not able to fix. When following documentation and other similar error pages, I didn't see any issue. Please help. I am using openmpi/4.0.5-gcc.

#include <stdio.h> #include <stdlib.h> #include "mpi.h" #define UPPER_LIMIT 4 #define master 0 int main(int argc, char *argv[]) { int *data; int process_id, total_process, temp_result; int i, tag, final_result; MPI_Init(&argc, &argv); MPI_Comm_rank(MPI_COMM_WORLD, &process_id); MPI_Comm_size(MPI_COMM_WORLD, &total_process); MPI_Status status; if ( total_process > UPPER_LIMIT ) { if ( process_id == 0 ) printf("max allowed processes limit [%d] exceeded.\n", UPPER_LIMIT); exit(0); } final_result = 0; for ( i = 0; i < total_process; i++){ data[i] = (int)i; } int j; for(j = 0; j < total_process; j++) { printf("%d ", data[j]); if(j==total_process-1) printf("*** %d\n", process_id); } MPI_Scatter(data, total_process, MPI_INT, &temp_result, 1, MPI_INT, 0, MPI_COMM_WORLD); if(process_id!=master){ temp_result = temp_result/process_id; MPI_Reduce(&temp_result, &final_result, 1, MPI_INT, MPI_SUM, 1, MPI_COMM_WORLD); } if(final_result>0){ tag = process_id; MPI_Send(&final_result, 1, MPI_INT, 0, tag, MPI_COMM_WORLD); } if(process_id==master){ MPI_Recv(&final_result, 1, MPI_INT, MPI_ANY_SOURCE, MPI_ANY_TAG, MPI_COMM_WORLD, &status); } MPI_Finalize(); return 0; }

Error Log

0 1 2 3 *** 0 0 1 2 3 *** 1 0 1 2 3 *** 2 0 1 2 3 *** 3 *** An error occurred in MPI_Scatter *** reported by process [1855324161,0] *** on communicator MPI_COMM_WORLD *** MPI_ERR_TRUNCATE: message truncated *** MPI_ERRORS_ARE_FATAL (processes in this communicator will now abort, *** and potentially your MPI job)

MPI_Scatter Documentation

MPI_Scatter Sends data from one process to all other processes in a communicator

Synopsis int MPI_Scatter(const void *sendbuf, int sendcount, MPI_Datatype sendtype, void *recvbuf, int recvcount, MPI_Datatype recvtype, int root, MPI_Comm comm)

Input Parameters

sendbuf - address of send buffer (choice, significant only at root)

sendcount - number of elements sent to each process (integer, significant only at root)

sendtype - data type of send buffer elements (significant only at root) (handle)

recvcount - number of elements in receive buffer (integer)

recvtype - data type of receive buffer elements (handle)

root - rank of sending process (integer)

comm - communicator (handle)

Output Parameters

recvbuf - address of receive buffer (choice)

Update

After updating the MPI_Scatter send count to 1, The above-mentioned error goes away but the program stays ideal and it didn't print anything that was placed after the MPI_Scatter line.

MPI_Scatter(data, 1, MPI_INT, &temp_result, 1, MPI_INT, 0, MPI_COMM_WORLD);

Solution:

MPI_Reduce need to be called by all processor. So, removing it from the conditional block fixed the issue.

This site can be used to execute the above code online. codingame.com/playgrounds/349/introduction-to-mpi/hello-world — Lionel
– Lionel, Commented Nov 23, 2021 at 5:28
I can't see where you allocate data. Did I miss the call to malloc? — Jeff Hammond
– Jeff Hammond, Commented Jun 17, 2022 at 11:43

Victor Eijkhout · Accepted Answer · 2021-11-23 05:43:10Z

2

You quote the relevant line: "sendcount - number of elements sent to each process (integer". So if you send 1 element to each process, you need to set the sendcount to 1, not total_process.

answered Nov 23, 2021 at 5:43

Victor Eijkhout

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5 Comments

Lionel Over a year ago

Thank you very much for your answer. I did put 1 and it get past the Scatter block but the program became idle. Do you have any idea why that happens?

Victor Eijkhout Over a year ago

Do you have a deadlock with your sends and receives?

Lionel Over a year ago

I put a print statement after scatter line and even commented send and receive code. but I didn't see anything printed after the scatter line and it still stays ideal. I think the problem is still coming from the scattering line. I am able to figure it out.

Victor Eijkhout Over a year ago

You have an MPI_Reduce inside a conditional. That doesn't work. All processes need to call that.

Lionel Over a year ago

It works. Thank you very much. I don't know how I missed that line.

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