Department of Computer Science & Engineering, BLDEACET, Vijayapura
13 Lab Manual : PARALLEL COMPUTING (BCS702)
Program 6: Write a MPI program to demonstration of deadlock using point to point communication and avoidance of deadlock by altering the call sequence
Objective: Demonstrate deadlock and its avoidance using MPI.
Part A: Deadlock Example
Code (Deadlock-prone)
// mpi_deadlock.c
#include <stdio.h>
#include <mpi.h>
int main(int argc, char* argv[]) {
int rank, size, data;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
if (rank == 0) {
int msg = 100;
MPI_Recv(&data, 1, MPI_INT, 1, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
MPI_Send(&msg, 1, MPI_INT, 1, 0, MPI_COMM_WORLD);
} else if (rank == 1) {
int msg = 200;
MPI_Recv(&data, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
MPI_Send(&msg, 1, MPI_INT, 0, 0, MPI_COMM_WORLD);
}
MPI_Finalize();
return 0;
}
Explanation:
#include <stdio.h>
#include <mpi.h>
int main(int argc, char* argv[]) {
int rank, size, data;
MPI_Init(&argc, &argv); // Start MPI environment
MPI_Comm_rank(MPI_COMM_WORLD, &rank); // Get process rank (0, 1, ...)
MPI_Init → Initializes MPI.
-
MPI_Comm_rank → Gives each process a unique ID (
rank).-
Example: If you run with 2 processes → one will have
rank=0, the otherrank=1.
-
Process 0 (rank = 0)
-
Creates an integer message
msg = 100. -
First action →
MPI_Recv: process 0 waits to receive an integer from process 1. -
Only after receiving, it will send its own message (100) to process 1.
👀 Process 1 (rank = 1)
-
Creates an integer message
msg = 200. -
First action →
MPI_Recv: process 1 waits to receive an integer from process 0. -
Only after receiving, it will send its own message (200) to process 0.
❌ The Problem (Deadlock)
-
Process 0 → waiting for data from Process 1 (via
MPI_Recv). -
Process 1 → waiting for data from Process 0 (via
MPI_Recv).
👉 Both are stuck waiting forever.
Since neither sends before receiving, no data is sent, so both processes are blocked.
This situation is called a deadlock.
Code (Deadlock-Free)
-
MPI_Init → starts MPI.
-
MPI_Comm_rank → gives each process a unique rank (0 or 1 here).
👀 Process 0 (rank = 0)
-
Creates message
msg = 100. -
First action →
MPI_Send: sends100to process 1. -
Then it waits to receive an integer from process 1.
-
Finally prints:
👀 Process 1 (rank = 1)
-
Creates message
msg = 200. -
First action →
MPI_Send: sends200to process 0. -
Then it waits to receive an integer from process 0.
-
Finally prints:
✅ Why This Code Does NOT Deadlock
-
In Part A, both processes did
MPI_Recvfirst → they blocked forever. -
In Part B, both processes do
MPI_Sendfirst → message is sent immediately and stored in MPI’s buffer. -
Then when they call
MPI_Recv, the matching message is already available → they succeed.
Thus, no process gets stuck. 🎯
🔑 Key Takeaway
-
Ordering matters in MPI.
-
If you do
Recvfirst on both sides → ❌ deadlock. -
If you do
Sendfirst → ✅ works fine (because MPI buffers the outgoing message until the other side receives it).
