Invalid node count in MPI pi calculation

Question

I am trying to parallelize the following code for calculation of pi.

My approach is to use scatter to parallelize the for, and then use a reduce to calculate the sum value and finally show pi.

My code is the following

#include <stdio.h>
#include <mpi.h> 

long num_steps = 100000;
double step = 1.0/100000.0;

int main() {

int i, myid, size;
double x, pi, local_sum = 0.0, sum=0.0;
double send_vec[num_steps], recv_vect[num_steps]; 

// Initialize the MPI environment 
MPI_Init(NULL, NULL); 
MPI_Comm_size(MPI_COMM_WORLD, &size); 
MPI_Comm_rank(MPI_COMM_WORLD,&myid);

if (myid ==0){
    int i=0; 
    for (i=0; i<num_steps;i++){
        send_vec[i]=i;
    }
}

MPI_Scatter(send_vec, num_steps/size, MPI_INT, recv_vect, 
    num_steps, MPI_INT, 0, MPI_COMM_WORLD);

for(i = 0; i < num_steps; ++i) {
    x = (recv_vect[i]-0.5)*step;
    local_sum += 4.0/(1.0+x*x);
}

MPI_Reduce(&local_sum, &sum, 1, MPI_DOUBLE, MPI_SUM, 0, MPI_COMM_WORLD);

if (myid == 0){
    pi = step*sum;
    printf("PI value = %f\n", pi);  
 }

// Finalize the MPI environment. 
MPI_Finalize();
}

The thing is when I run the program with the option -np 1 and 2 I do get the desired result.

Yet when I run with 3, 4 and higher I get the following error:

PIC_Send(284).........: Negative count, value is -240000
Fatal error in PMPI_Scatter: Invalid count, error stack

francis · Accepted Answer · 2016-10-22 12:34:18Z

2

The call to MPI_Scatter() is to be corrected:

MPI_Scatter(send_vec, num_steps/size, MPI_INT, recv_vect, 
num_steps, MPI_INT, 0, MPI_COMM_WORLD);

To send double, use the datatype MPI_DOUBLE as you did in the MPI_Reduce()
Since the sendtype is similar to the recvtype, the number of item sent to each process sendcount must be equal to the number of item received by each process recvcount. In the present case, it's num_steps/size.

Finally, the call to MPI_Scatter() will look like:

MPI_Scatter(send_vec, num_steps/size, MPI_DOUBLE, recv_vect, 
num_steps/size, MPI_DOUBLE, 0, MPI_COMM_WORLD);

Lastly, dynamic memory allocation can be used to avoid using the stack for storing large arrays. Moreover, the allocated space can be decreased so as to reduce the memory footprint:

num_steps=(num_steps/size)*size;
double* send_vec=NULL;
double* recv_vec=NULL;
if(rank==0){
   send_vec=malloc((num_steps/size)*sizeof(double));
   if(send_vec==NULL){fprintf(stderr,"malloc failed\n");exit(1);}
}
recv_vec=malloc(num_steps*sizeof(double));
if(recv_vec==NULL){fprintf(stderr,"malloc failed\n");exit(1);}

...

if(rank==0){
   free(send_vec);
}
free(recv_vec);

answered Oct 22, 2016 at 12:34

francis

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3 Comments

Alessandroempire Over a year ago

I works. Thank you. Could you explain why the receive size has to be: num_steps/size? Is it because I am receiving everything I sent?

francis Over a year ago

As specified in the documentation of openmpi about MPI_Scatter(), the argument sendcount must be the "Number of elements sent to each process" and recvcount must be "Number of elements in receive buffer". If num_steps/size elements of type double are sent to each process, each process will receive num_steps/size of type double.

francis Over a year ago

See also p160 of the MPI standards 3.1 about MPI_Scatter():

The type signature associated with sendcount,  sendtype at the root must be equal to the type signature associated with recvcount,  recvtype at all processes (however, the type maps may be different).  This implies that the amount of data sent must be equal to the amount of data received, pairwise between each process and the root.  Distinct type maps between sender and receiver are still allowed.

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