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Today we continue with the Parallel Programming series about the OpenMP API. I highly suggest you to go read the previous articles of the series, that you can find by the end of this one. Today we will get into how we define Parallel Sections.
So, without further ado, let's get straight into it!A parallel region is defined using a parallel region construct:
#pragma omp parallel
{
/* This code runs in parallel */
}int i;
#pragma omp parallel for private(i)
for(i = 0; i < N; i++){
...
}
With for loops we saw how we can divide the various iterations among threads to execute them faster when parallelization is possible.
What if there are specific sections of code that can be run in parallel whilst other can't?
Let's take the following flowchart for example:
[Custom Figure using draw.io]
From the chart we understand that B and C have to be executed in sequential order (B → C).
If we somehow could indicate that B and C should be executed sequentially, then A, B → C and D could be executed in parallel!
That's were sections come into play...
A sections construct (with s!) is a directive that is used to define non-iterative work-sharing among threads in a team (that is already defined using a parallel section).
Independent section constructs (without s!) are nested within the sections construct.
Each of these sections is executed once by a thread in the team and different sections might be executed by different threads.
Its a matter of how quickly a thread manages to execute a section and how the implementation defines such behavior.
A sections construct with nested section directives is defined as:
#pragma omp sections [clause ...]
{
/* run in parallel by all threads of the team */
#pragma omp section
{
/* run once by one thread */
}
#pragma omp section
{
/* run once by one thread */
}
...
}#pragma omp parallel sections#pragma omp parallel sections num_threads(3)
{
#pragma omp section
{
/* Code for Work A */
}
#pragma omp section
{
/* Code for Work B */
/* Code for Work C */
}
#pragma omp section
{
/* Code for Work D */
}
}
A similar construct that is quite useful when only one section inside a parallel region has to be executed by one thread.
Instead of creating a nested section inside of sections to write that code, we can just use a single construct.
The syntax of such a construct is simply:
#pragma omp single#pragma omp masterThe following clauses can be used to configure sections:
Let's execute a calculation of the Fibonacci series and the Factorial (!) in parallel by using sections.
The Fibonacci series is defined as:
The Factorial is calculated as:
It's worth noting that 0! = 1.
To calculate the Fibonacci series in C we write the following function:
void fibonacci(int A[]){
int i;
A[0] = 0;
A[1] = 1;
for(i = 2; i < N; i++){
A[i] = A[i - 1] + A[i - 2];
}
}To calculate the Factorial in C we write the following function:
void factorial(int A[]){
int i;
A[0] = A[1] = 1;
for(i = 2; i < M; i++){
A[i] = i * A[i - 1];
}
}
In the main function we simply define two arrays with size N and M (global #define) and create a parallel section to execute the two functions in.
#include <omp.h>
#include <stdio.h>
#include <stdlib.h>
#define N 45 /* Fibonacci limit */
#define M 15 /* Factorial limit */
... functions ...
int main(){
unsigned int fib[N];
unsigned int fac[M];
int i;
/* Parallel Sections */
#pragma omp parallel sections
{
/* Calculate Fibonacci Series */
#pragma omp section
{
fibonacci(fib);
}
/* Calculate Factorial */
#pragma omp section
{
factorial(fac);
}
}
/* print arrays */
printf("Fibonacci Series:\n");
for(i = 0; i < N; i++){
printf("%u ", fib[i]);
}
printf("\n");
printf("Factorial:\n");
for(i = 0; i < M; i++){
printf("%u ", fac[i]);
}
printf("\n");
return 0;
}
Running the program for N = 45 and M = 15 we get:
which are the values that we expected...
There are of course better ways to use sections, but running two different operations in parallel is also not that bad(!)
Keep on drifting!