Reduction

Reduction

Motivation

Let’s review sum_series_openmp_for.c:

c linenums="1" --8<-- "docs/csc466/lectures/data/openmp/sum_series_openmp_for.c"
We used a shared data array for partial sums from the threads, this is to prevent data races.
Another solution: reduction clause.

c linenums="1" --8<-- "docs/csc466/lectures/data/openmp/sum_series.c"

More reduction

Can be done over section

c linenums="1" --8<-- "docs/csc466/lectures/data/openmp/function_sections.c"
Can be done without for

c linenums="1" --8<-- "docs/csc466/lectures/data/openmp/reduction_rand.c"

Hands-on

Implement the parallel dot product for Lab 1 using reduction

Reduction operators

Built-in reductions

Arithmetic reductions: +, *, max, min.
- - is deprecated as of OpenMP 5.2
Logical reductions: &, &&, |, ||, ^

Custom reductions

Formula:

 #pragma omp declare reduction (identifier : typelist : combiner) initializer (initializer expression)
 

This is all written on a single line!!!
To write this over multiple lines, a \ is needed
initializer (initializer expression) is optional

 #pragma omp declare reduction ( \
    identifier : typelist : combiner) \
    initializer (initializer expression)
 

Example code for sum series:

c linenums="1" --8<-- "docs/csc466/lectures/data/openmp/sum_series_custom.c"

Midterm Exam

Midterm review: Monday March 03
- Quiz retaking: Monday March 03
Midterm exam: Wednesday March 05
Debrief/In-class Assignment: Friday March 07