Parallel programming using MPI

Parallel Programming
Using MPI
Dr. Ajit K Nayak
SOA University, Odisha, India

Introduction to MPI/Ajit Nayak//2
What is MPI?
 The Message Passing Interface, is a
standardised and portable message passing
system
 It is designed by a group of researchers from
academia and industry to function on a wide
variety of parallel computer
 The standard defines the syntax and semantics
of core library routines useful to a wide range
of users writing portable message-passing
programs in Fortran or C

The Environment
 Clustering is used as a Parallel Computing
Environment
 Clustering is nothing but a network (cluster) of
computers configured to work as one computer
 This type of environment is cheaper (no more
than a general network environment!) than buying
an original parallel computer
 A Parallel computer (of this kind) may be
deployed on the fly

Building the Environment
 Choosing a Network Topology
• Network of workstations (NOW)
 Choosing an Operating System
• Linux
 Installing and configuring the Environment
• The network setup for master and slaves
 Choosing, Installing and Configuring a MPI
implementation
• mpich-1.2.5 (free downloadable from

Network of Workstations
 The cluster should be viewed as a special,
standalone entity.
 It should have its own networking facilities that
are not shared with other systems.
 A cluster should have one Master node that
serves as the central focus of the operational
aspects of the cluster.
 Also it should have some Slave nodes, which
will be used as workstations.

A Pictorial view
Slave Nodes
Master Node
Ethernet Hub
Network of Workstations

Programming using MPI
 MPI Assumes that the no of processes created
(for a program) are independent of processors
(computers) available (for the cluster).
 The number of computers for a cluster is
decided by the network administrator
 The number of processes for a program is
decided by the parallel programmer
 i.e. no processes are created (or no processors
could be attached) during the execution of a
program.

How it works?
 Each process is recognized by a unique integer
id called rank (0 .. p-1).
 Each process is assigned to a computer
(Processor) in a round-robin fashion.
 Process Zero always runs on Master
Process 0
P1
P2
P6
P5
P4 P3

Process Distribution
 4 processors and 8 processes
Process 0 Assigned to Master
Process 1 Assigned to Slave 1
Process 4 Assigned to Master

Master Node
(P0, P4)
Slave 1 Slave 2 Slave 3
(P1, P5)
(P2, P6) (P3)
This programs
are called SPMD
programs!

MPI Program Structure
#include “mpi.h”
main(int argc, char** argv){
/*. . . Declarations . . . */
/*. . . Sequential Algorithms . . . */
MPI_Init(&argc, &argv);
/*. . . Parallel Algorithm . . . */
MPI_Finalize( );
/* . . . Sequential Algorithm . . . */
} // End of main program !

Program Structure contd…
 int MPI_Init( int *argc, char ***argv )
• It initializes the parallel environment.
• It must be called before any other MPI
routine
 int MPI_Finalize( void )
• It cleans-up all MPI state
• After this invocation no MPI routine should
be called.

Sending information
 int MPI_Send( void *buf, int count, MPI_Datatype
datatype, int dest, int tag, MPI_Comm comm)
• It is used for point-to-point communication, i.e. comm. between a
pair of processes. One side sending and other receiving.
•The send call blocks until the send buffer can be reclaimed
buf initial address of send buffer
count number of elements to send
datatype datatype of each entry
dest rank of destination
tag message tag
comm communicator

Receiving information
 int MPI_Recv( void *buf, int count, MPI_Datatype
datatype, int source, int tag, MPI_Comm comm,
MPI_Status *status)
• It performs a blocking receive operation
buf initial address of send buffer
count max number of elements to receive
datatype datatype of each entry
dest rank of destination
tag message tag
comm communicator (defines a communication domain)
status return status (A structure having information
about source, tag and error)

A First Program
 Problem:
• Root process receives a greeting message
from all other processes.
• Each process other than Root send a greeting
message to Root process
• Root process prints messages received from
other processes
 We will use blocking send and recv calls to
perform this task

The Complete Program
#include<stdio.h>
#include<string.h>
#include “mpi.h”
int main (int argc, char *argv[ ]) {
int myRank, numProcs, dest;
int Root=0, tag=0; int i;
char msg[30];
MPI_Status status;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &myRank);
MPI_Comm_size(MPI_COMM_WORLD, &numProcs);
if(myRank !=Root){
strcpy(msg, “Helo World”);

Greeting Program contd..
MPI_Send(msg, strlen(msg)+1, MPI_CHAR, Root, tag,
MPI_COMM_WORLD);
} //end if
else{
for(i=1; i < numProcs; i++){
MPI_Recv(msg, 30, MPI_CHAR, i, tag,
MPI_COMM_WORLD, &status);
printf(“%s From process %dn”, msg, i);
}//end for
} // end else
MPI_Finalize( );
} // End of program !

MPI Datatypes
 MPI datatype C datatype
 MPI_CHAR signed char
 MPI_SHORT signed short int
 MPI_INT signed int
 MPI_LONG signed long int
 MPI_UNSIGNED_CHAR unsigned char
 MPI_UNSIGNED_SHORT unsigned short int
 MPI_UNSIGNED unsigned int
 MPI_UNSIGNED_LONG unsigned long int
 MPI_FLOAT float
 MPI_DOUBLE double
 MPI_LONG_DOUBLE long double

Where /How to get it done?
 Login to a specified user
 Open a file in vi editor (vim greeting.c)
 Write the source code, save and exit
 In command prompt issue following commands
To Compile
mpicc -o <objFileName> <CFileName>
To Execute
mpiexec –n <4> ./<objFileName>

A Sum Program
Problem:
• To find the sum of n integers on p processes
• Where p=n and p=2q

Problem Understanding
0 1 2 3 4 5 6 7
6+220
1+5 9+130 4
No of processes (p)= 8
q=3 (p=2q)
0+1 2+3 4+5 6+70 2 64

Program
#include<stdio.h>
#include<math.h>
#include "mpi.h"
#define IS_INT(X) ( (X) == (int) (X)) ? 1:0
#define LOG2(X) log10(X)/log10(2)
int main(int argc, char *argv[]){
int myRank, numProcs, Root=0;
int source, destination, tag=0;
int iLevel, level, nextLevel , value, sum=0, ans;
float height; //height of the tree
MPI_Status status;

Program contd.
MPI_Init(&argc,&argv);
MPI_Comm_rank(MPI_COMM_WORLD, &myRank);
MPI_Comm_size(MPI_COMM_WORLD, &numProcs);
height=LOG2(numProcs);

Program contd.
/* if p!=2q */
if(!(IS_INT(height))){
if(MyRank==Root)
printf("n Error: Number of processes
should be power of 2...n");
MPI_Finalize();
exit(-1);
}//end error checking
sum=myRank;

Program contd.
//find sender and receiver in each level
for(ilevel=0; ilevel < height; ilevel++){
Level = pow(2, ilevel);
if((MyRank % level)== 0){
NextLevel = pow(2,(ilevel+1));
if((myRank% nextLevel)==0){ //if receiver
source = myRank+Level;
MPI_Recv(&value, 1, MPI_INT,
Source, tag, MPI_COMM_WORLD, &status);
sum += value;
}

Program contd.
else{//if sender
Destination=MyRank - Level;
MPI_Send(&sum, 1, MPI_INT,
destination, tag, MPI_COMM_WORLD);
}//end of else
}
} // end of for loop
if(MyRank==Root)
printf("nnMy Rank is %d and the Final Sum %d
nn",myRank,sum);
MPI_Finalize(); } //end of the program

Collective Communications
Collective communications transmit data
among all processes in a group.
Synchronizes processes without passing data.
MPI provides the following collective
communication
Global Communication Functions
Broadcast
Gather/Scatter etc.
Global Reduction Functions
Reduce

Broadcast
 int MPI_Bcast(void *buf, int count, MPI_Datatype
datatype, int root, MPI_Comm comm);
• It broadcasts a message from root to all processes in the
group (including itself)
Broadcast
Processes
Elements
A0 A0
A0
A0
A0

Broadcasting How to?
. . .
int array[100];
int Root=0;
MPI_Bcast(array, 100, MPI_INT, Root,
MPI_COMM_WORLD);
. . .
 This call will broadcast an array of 100
integers to all processes
 We don‟t require a corresponding receive call in
all processes to receive this broadcast

Gather / Scatter
 int MPI_Gather/Scatter(void *sendbuf, int
sendcount, MPI_Datatype sendtype, void
*recvdbuf, int recvcount, MPI_Datatype
recvtype, int root, MPI_Comm comm);
Processes
Elements
Gather
Scatter
A0
A1
A2
A3
A0 A1 A2 A3

Gather / Scatter
• Each process (including Root) sends the
contents of its send buffer to the root
process
• The root process receives the messages and
stores them in rank order
• recvcount argument at the root indicates the
no of items it receives from each process
100
Proc 0
100
Proc 1
100
Proc 2
Proc 0 100 100 100
Gather
Scatter

Gather / Scatter contd.
if (myRank==0)
rbuf=(int *) malloc (size*100*sizeof(int));
MPI_Gather(sendArray, 100, MPI_INT, rbuf,
100, MPI_INT, root, MPI_COMM_WORLD);
• Similarly in case of Scatter, Root process
distributes items to all processes in rank order
rbuf = ( int *) malloc (recvCount * sizeof(int));
if (myRank==0)
MPI_Scatter(sendbuf, sendCount, MPI_INT, rbuf,
recvCount, MPI_INT, root, MPI_COMM_WORLD);

Gather to All
 This is like gather, except all process receives the
same result.
Processes
Elements
D0
C0
B0
A0
D0C0B0A0
D0C0B0A0
D0C0B0A0
D0C0B0A0
AllGather
 The data received from jth process is placed at jth
block of rbuf.
int MPI_Allgather(void *sbuf, int scount, MPI_Datatype stype, void
*rbuf, int rcount, MPI_Datatype rtype, MPI_COMM_WORLD);

All to All
 Each process sends distinct data to each of the
receivers.
Processes
Elements
D3D2D1D0
C3C2C1C0
B3B2B1B0
A3A2A1A0
D3C3B3A3
D2C2B2A2
D1C1B1A1
D0C0B0A0
Alltoall
 The jth block sent from process i is received by
process j and is placed in the ith block of rbuf.
int MPI_Alltoall(void *sbuf, int scount, MPI_Datatype stype, void
*rbuf, int rcount, MPI_Datatype rtype, MPI_COMM_WORLD);

Next Program
 Problem
• Matrix Vector Product ( A X = B )
Input: A [ 1..m, 1..n ], X[ 1..n ]
Output: B[ 1..m ]
Algorithm:
1. an 8x8 matrix has been divided into 4 (2x8) sub-matrices.
2. Each sub-matrix is stored in the local memory of each
process.
3. A local matrix-vector product is performed at each
process.
4. Then the results are assembled from each process onto
Root process.

The Program
#include <stdio.h>
#include "mpi.h"
int main( int argc, char **argv ){
int a[2][8], b[8], cpart[2], ctotal[8];
int rank, size, i, k;
MPI_Init( &argc, &argv );
MPI_Comm_rank( MPI_COMM_WORLD, &rank);
MPI_Comm_size( MPI_COMM_WORLD, &size );

Matrix vector Prog contd..
if (size != 4) {
printf("Error!:# of processors must be equal to 4n");
printf("Programm aborting....n");
MPI_Abort(MPI_COMM_WORLD, 1); }
for (i=0;i<2;i++) // setting values for a[1..2, 1..8]
for (k=0;k<8;k++) a[i][k] = rank*(k+1);
printf("process %d:n",rank); //printing values of „a‟
for (i=0;i<2;i++){
for (k=0;k<8;k++)printf("%dt",a[i][k]);
printf("n");
}
printf("n");

Matrix vector prod. Prog. contd..
for (k=0;k<8;k++) b[k] = k+1; // value of „b‟
for (i=0;i<2;i++){
cpart[i] = 0;
for (k=0;k<8;k++) cpart[i] = cpart[i] + a[i][k] * b[k];
}
printf("After multiplication process %d:n",rank);
for (k=0;k<2;k++)
printf("%dt",cpart[k]);
printf("n");

Matrix vector prod Prog contd..
MPI_Gather(cpart, 2, MPI_INT, ctotal, 2,
MPI_INT,0, MPI_COMM_WORLD);
if(rank==0){
printf("Vector b:n");
for (k=0;k<8;k++) printf("%dt",b[k]);
printf(“n result isn");
for (k=0;k<8;k++)
printf("%dt",ctotal[k]);
printf("n");
} // end of if
MPI_Finalize();
} // end of main program

Matrix vector prod Prog Output
process 0:
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
process 1:
1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8
process 2:
2 4 6 8 10 12 14 16
2 4 6 8 10 12 14 16
process 3:
3 6 9 12 15 18 21 24
3 6 9 12 15 18 21 24

Matrix vector prod Prog Output
After multiplication process 0: 0 0
Vector b:
1 2 3 4 5 6 7 8
result vector:
0 0 204 204 408 408 612 612

Reduce
 int MPI Reduce(void* sendbuf, void* recvbuf,
int count, MPI Datatype datatype, MPI Op op,
int root, MPI Comm comm)
Reduce
(+)
Processes
Elements
C2A2
B2
C1B1A1
C0B0
A0 C0+ C1+ C2B0+ B1+ B2
A0+ A1+ A2

Other Reduce functions
All Reduce
(+)
Processes
Elements
C2A2
B2
C1B1A1
C0B0
A0
C0+ C1+ C2A0+ A1+ A2
B0+ B1+ B2
C0+ C1+ C2B0+ B1+ B2A0+ A1+ A2
C0+ C1+ C2B0+ B1+ B2
A0+ A1+ A2
Reduce
Scatter
(+)
Processes
Elements
C2A2
B2
C1B1A1
C0B0
A0
C0+ C1+ C2
B0+ B1+ B2
A0+ A1+ A2

Predefined Operations
 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

Practice Programs
 Find sum of integers in the range(0:500000) using
blocking calls (send, recv)
• Method 1: (simple)
 Divide different ranges in in different processes
 Process with rank greater than 0, sends the value of its
result to root process. Process 0 calculates and prints
final sum
• Method 2: (Linear array)
 Process n-1 sends its sum to process n-2, n-2 adds its
own sum with received sum and sends the resulting sum
to n-3.
 This process continues till it reaches at process 0.

Practice Programs
• Method 3: (hypercube) use the hypercube algorithm.
 Find sum of above range of integers using non-
blocking calls (reduce)
 Modify Matrix vector product program to work with
any size Matrix.

Other Utility-Calls
 double MPI_Wtime(void)
• returns a double floating point number of
seconds, since some arbitrary point of time in
the past.
• It can be used to find the execution-time of
programs or ‘section of programs’. The time
interval can be measured by calling this
routine at the beginning and at the end of
program/section and subtracting the values
returned.

Other Utility-Calls
 int MPI_Barrier (MPI_Comm comm)
• MPI_Barrier blocks the calling process until all processes in
communicator have entered the function.
 int MPI_Sendrecv (void *sendbuf, int sendcount,
MPI_Datatype sendtype, int dest, int sendtag, void
*recvbuf , int recvcount, MPI_Datatype recvtype, int
source, int recvtag, MPI_Comm comm, MPI_Status
*status)
• performs both a send and a receive. Can be matched with
ordinary send and recv.
• No deadlock arises

References
The standardization forum:
• http://mpi-forum.org/
Software
• https://www.mpich.org/
Books
 Peter. S. Pacheco – Parallel Programming with MPI,
Morgan Kaufmann Publishers
 M. Snir, S. Otto, S H-Lederman, D Walker, J Dongarra
– MPI: The Complete Reference, The MIT Press,
Cambridge, Massachusetts, London

Thank You

Parallel programming using MPI

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