PCA 32: MPI Reduce

In order to successfully complete this assignment you must do the required reading, watch the provided videos and complete all instructions. The embedded survey form must be entirely filled out and submitted on or before 11:59pm. Students must come to class the next day prepared to discuss the material covered in this assignment.

PCA 32: MPI Reduce#

Goals for today’s pre-class assignment#

Monte Carlo Method for Estimating \(\pi\)
MPI Reduce Options
Review Pi Program Serial Code
Assignment wrap-up

1. Monte Carlo Method (Another way for Estimating \(\pi\))#

In the next class we are going to implement a different kind of \(\Pi\) estimator using the algorithm described in the following video:

from IPython.display import YouTubeVideo
YouTubeVideo("M34TO71SKGk",width=640,height=360)

2. MPI Reduce Options#

Like OpenMP there are some special MPI messages that are specifically designed to help with common types of algorithms. These are:

MPI_Allreduce - Combines values from all processes and distributes the result back to all processes.

int MPI_Allreduce(const void *sendbuf, 
                  void *recvbuf, 
                  int count,
                  MPI_Datatype datatype, 
                  MPI_Op op, MPI_Comm comm)

MPI_reduce - Reduces values on all processes to a single value

int MPI_Reduce(const void *sendbuf, 
               void *recvbuf, 
               int count, 
               MPI_Datatype datatype,
               MPI_Op op, 
               int root, 
               MPI_Comm comm)

The MPI_Op is a reduce operator that can have one of the following values.

Name	Meaning
MPI_MAX	maximum
MPI_MIN	minimum
MPI_SUM	sum
MPI_PROD	product
MPI_LAND	logical and
MPI_BAND	bit-wise and
MPI_LOR	logical or
MPI_BOR	bit-wise or
MPI_LXOR	logical xor
MPI_BXOR	bit-wise xor
MPI_MAXLOC	max value and location
MPI_MINLOC	min value and location

Above table from: https://www.mpi-forum.org/docs/mpi-1.1/mpi-11-html/node78.html

✅ QUESTION: Think about the Pi program described in the video above and show in the algorithm below. Which reduce operator(s) do you think may be helpful in implementing this algorithm in MPI?

Put your answer here.

3. Review Pi Program Serial Code#

Review the following serial code implementation of the Pi program described in the above video.

✅ DO THIS: Lets check our instincs. Without doing too much googling, include comments in the following code on where you think you would need to make modifications for the code to run in MPI.

For example, You may want to include /** Inicialize MPI here **/, /** Clean up MPI Here , /** Have each process send X to master process **/, /** have master rpocess recicve X from all other processes and combine them to do Y **/

Put as much or as little detail as you want. You are not going to turn this in but I will expect everyone to get this working as a class.

#include <stdlib.h>
#include <stdio.h>
#define sqr(x)	((x)*(x))
long random(void);

double dboard(int darts)
     {
     double x_coord,       /* x coordinate, between -1 and 1  */
            y_coord,       /* y coordinate, between -1 and 1  */
            pi,            /* pi  */
            r;             /* random number between 0 and 1  */
     int score,            /* number of darts that hit circle */
         n;
     long rd;
     unsigned long cconst; /* used to convert integer random number */
                           /* between 0 and 2^31 to double random number */
                           /* between 0 and 1  */

     cconst = 2 << (31 - 1); 
     cconst = RAND_MAX;
     score = 0;

     /* "throw darts at board" */
     for (n = 1; n <= darts; n++)  {
          /* generate random numbers for x and y coordinates */
          rd = random();
	  //printf("Rand - %ld\t",rd);
          r = (double)rd/cconst;
          //printf("%10.8f\n",r);
          x_coord = (2.0 * r) - 1.0;
          r = (double)random()/cconst;
          y_coord = (2.0 * r) - 1.0;

          /* if dart lands in circle, increment score */
          if ((sqr(x_coord) + sqr(y_coord)) <= 1.0)
               score++;
          }

     /* calculate pi */
     pi = 4.0 * (double)score/(double)darts;
     return(pi);
     } 

✅ QUESTION: Please share notes with the instructor. Did you find this activity easy/hard. Where did you get stuck?