Project 3: MPI Ping-Pong and Ring Shift

Background

The ping-pong problem is a benchmark often used to evaluate the performance of message passing interfaces (MPI) in parallel computing. In this problem, two processes exchange messages back and forth a specified number of times, with each process sending a message and receiving a message alternately. In the ping-pong, process i sends a message of size m to process j, then receives a message of size m back from j. The values of i, j, and m to use are given below.

The “ring shift” problem is similar to ping-pong. In the MPI ring shift, a group of processes is arranged in a ring, with each process holding a unique subset of a larger array of data. The goal is to shift the data elements by a specified number of positions around the ring, wrapping around the ends of the ring as necessary.

Part 1: Blocking Ping-Pong

Your task is to implement the ping-pong problem using MPI in C or C++ and analyze the behavior and performance of your code. Specifically, you should:

  1. Implement the ping-pong problem using MPI in C or C++. Use blocking MPI_Send() and MPI_Recv() calls. You should define the number of iterations and the size of the message to be exchanged.
  2. Measure the time taken to complete the ping-pong exchange for different message sizes. You should use the MPI_Wtime() function to obtain the time before and after the exchange and calculate the elapsed time. Vary the message size from 2 bytes to 4 kilobytes in powers of 2 (i.e., 2 bytes, 4 bytes, 8 bytes,…, 2048 bytes, 4096 bytes). For each message size, perform 100 iterations of the ping-pong to build up statistical significance.
  3. Record the total amount of data sent and received during the ping-pong exchange for each configuration.
  4. Repeat steps 2 and 3 but ensure that the 2 processes that are communicating reside on different physical hardware nodes on HPCC.
  5. Plot the average communication time of a single exchange (send and receive) as a function of message size for the two cases. Using this plot, estimate the latency and bandwidth for each case. Are they different? Explain your results.
  6. Analyze and discuss your results. Explain the behavior of the resulting curves.

Part 2: Non-block Ping-Pong

Repeat Part 1 using non-blocking MPI communication, i.e., using MPI_Isend() and MPI_Irecv(). You will need to include explicit process synchronization using, e.g., MPI_Wait() calls. Compare the results to the blocking case.

Part 3: MPI Ring Shift

  1. Implement the MPI ring shift in C or C++ for an arbitrary number of processes in the ring and arbitrary message size (i.e., number of elements per process). In your implementation, use MPI_Sendrecv() instead of separate MPI_Send() and MPI_Recv() calls.
  2. As in Parts 1 and 2, vary the message size from 2 bytes to 4 kb, in powers of 2. Also vary the number of processes used from 2 to N, in powers of 2, where N is sufficiently large that rank 0 and rank N-1 are guaranteed to reside on separate nodes (N will depend on which cluster you are using on HPCC).
  3. Compute the bandwidth and latency, as above. Plot the bandwidth as a function of message size. Include separate lines for each number of processes used.
  4. Analyze and discuss your results. Explain the behavior of the resulting curves.

Part 4: Non-blocking MPI Ring Shift

Repeat Part 3 but using non-blocking communication via MPI_Isend() and MPI_Irecv(). Compare the results to the blocking case.

What to turn-in

To your git project repo, commit your final working code for the above exercises and a concise write-up including all plots, and detailed responses to the questions posed concerning your results.