Broadcast

Table of contents

1. Introduction
2. Connection to Modules
3. Files Required
4. Part 1: Broadcast with Send and Recv
5. Part 2: Replacing Send/Recv with Bcast.

---

Introduction

The MPI_Bcast primitive sends data stored in a buffer at the root to all other ranks in the communicator. The syntax is as follows:

int MPI_Bcast(void *buffer, 
	int count, 
	MPI_Datatype datatype, 
	int root, 
	MPI_Comm comm)

Connection to Modules

• This module is particularly useful for the communication module.

Files Required

• broadcast_starter.c

Part 1: Broadcast with Send and Recv

In this exercise, we assume that there are \(p\geq1\) ranks.

• Rank 0 will store in a buffer a value of 100.
• Rank 0 will send this value in the buffer to all other ranks in the communicator.
• Ranks in the range \([1, p-1]\) will receive the value send by rank 0.
• In this exercise, we will only use MPI_Send and MPI_Recv.
• To get started, use the file broadcast_starter.c.

Figure 1 shows an example with \(p=5\) ranks.

Figure 1: Example of sending the value 100 from rank 0 to ranks 1–4.

A code snippet from broadcast_starter.c is shown below. Write the code to send the buffer storing 100 at rank 0 to the other ranks.

  int buffer;

  //Initialize the value to be broadcast
  if(my_rank==0)
  {
    buffer=100;
  }

  //Write code here

Part 2: Replacing Send/Recv with Bcast.

Using broadcast_starter.c, write the same program as outlined above, except use MPI_Bcast instead of MPI_Send and MPI_Recv.