Introduction
The MPI_Scatter
primitive is sends chunks of data stored in an array to several process ranks, where each rank receives different data elements. It is similar to MPI_Broadcast
, except that MPI_Broadcast
sends the same data to process ranks.
The syntax is as follows:
int MPI_Scatter(const void *sendbuf,
int sendcount,
MPI_Datatype sendtype,
void *recvbuf,
int recvcount,
MPI_Datatype recvtype,
int root,
MPI_Comm comm)
The MPI_Gather
primitive receives data at the root from other processes. The syntax is as follows:
int MPI_Gather(const void *sendbuf,
int sendcount,
MPI_Datatype sendtype,
void *recvbuf,
int recvcount,
MPI_Datatype recvtype,
int root,
MPI_Comm comm)
Connection to Modules
- This module may be useful for the distance matrix and K-means modules.
Files Required
Sum of an Array Part 1: Implementing Scatter and Gather using Send and Recv
In this exercise, rank 0 will generate an array of size $N$. There are $p$ process ranks. We assume $N~\mathrm{mod}~p=0$, i.e., $N/p$ divides evenly.
We will compute the total sum of the values in the array of size $N$ using the $p$ ranks. Using MPI_Send
you will send chunks of size $N/p$ to each process rank from rank 0. Each rank will add the values in the array together ($N/p$ elements) and will send the data back to rank 0 using MPI_Send
. At the end, rank 0 will add all of the values from each of the ranks together and output the total sum.
In this exercise, we will only use MPI_Send
and MPI_Recv
.
To get started, use the file scatter_starter.c
.
Figure 1 shows an example of the sending the data, where $N=12$ and $p=4$. Each rank is sent $N/p=3$ elements to compute the local sums.
Figure 2 shows an example of sending the data from each rank to rank 0.
Sum of an Array Part 2: Using Scatter and Gather
Copy your code from Part 1. This time, implement the program using MPI_Scatter
and MPI_Gather
without using MPI_Send
or MPI_Recv
.