OpenMP

Question 1

When was OpenMP first introduced for Fortran and C/C++?

Answer 1

• Fortran: October 1997
• C/C++: October 1998

Question 2

What is the latest version of OpenMP? When was it released?

Answer 2

Version 6.0, released in November 2024.

Question 3

What are the three main ways OpenMP extends C, C++, and Fortran?

Answer 3

1. Compiler Directives
2. Environment Variables
3. Runtime Library Routines

Question 4

Describe the OpenMP fork-join execution model.

Answer 4

Execution starts with a single master thread, which forks worker threads in parallel regions and joins them back at the end.

Question 5

How do you include an OpenMP directive in C/C++?

Answer 5

#pragma omp directive [clause1, ...]

Question 6

How do you create a parallel region in OpenMP?

Answer 6

#pragma omp parallel

Question 7

What compiler flag is required for OpenMP in GCC?

Answer 7

-fopenmp

Question 8

How can you control the number of threads used in OpenMP?

Answer 8

Set the OMP_NUM_THREADS environment variable (e.g., export OMP_NUM_THREADS=4).

Question 9

What happens if the number of threads is not specified?

Answer 9

The implementation chooses a default value.

Question 10

What does the OpenMP runtime library provide?

Answer 10

Functions for:
- Execution Environment Control
- Timing
- Device Memory Management
- Locking

Question 11

Name three OpenMP functions that query execution environment details.

Answer 11

1. omp_get_thread_num(): Returns the thread number
2. omp_get_num_threads(): Returns the total number of threads
3. omp_get_num_procs(): Returns the number of available processors

Question 12

What header file must be included to use OpenMP functions in C?

Answer 12

#include <omp.h>

Question 13

Why would conditional compilation be needed for OpenMP programs?

Answer 13

To allow compilation without OpenMP support, enabling serial execution.

Question 14

What preprocessor directive is used for conditional compilation in OpenMP?

Answer 14

#ifdef _OPENMP

Question 15

How can you conditionally include OpenMP-specific code?

Answer 15

#ifdef _OPENMP
include <omp.h>
endif

Question 16

What does the _OPENMP macro indicate?

Answer 16

It is defined only when OpenMP is enabled during compilation.

Question 17

Why is loop parallelisation important in OpenMP?

Answer 17

It allows work to be divided among threads, improving performance.

Question 18

What directive is used to parallelise a loop inside a parallel region?

Answer 18

#pragma omp for

Question 19

How do you combine parallel region creation and loop parallelisation?

Answer 19

#pragma omp parallel for

Question 20

What are the two types of variable scoping in OpenMP?

Answer 20

1. Shared: A variable is accessible by all threads
2. Private: Each thread has its own copy of the variable

Question 21

How can you explicitly define variable scope in OpenMP?

Answer 21

Use the default, private, and shared clauses in OpenMP directives.

Question 22

What does default(none) do in OpenMP?

Answer 22

It forces all variables to be explicitly declared as private or shared.

Question 23

What issue occurs when multiple threads modify a shared variable simultaneously?

Answer 23

A data race, which can lead to incorrect results.

Question 24

How can you safely sum values in parallel?

Answer 24

Use a reduction variable:
#pragma omp for reduction(+ : sum)

Question 25

What is a data dependency?

Answer 25

A situation where the execution of one statement depends on another, potentially causing issues in parallel execution.

Question 26

What are the three types of loop-carried dependencies?

Answer 26

1. **Flow Dependency (Read after Write):** Later iterations depend on earlier ones. 2. **Anti-dependency (Write after Read):** Later iterations overwrite values needed by earlier ones. 3. **Output Dependency (Write after Write):** Multiple iterations write to the same memory location.

Question 27

What OpenMP feature helps retain the last computed value in a parallel loop?

Answer 27

The `lastprivate` clause

Question 28

What does `lastprivate(x)` do?

Answer 28

Ensures that the value of `x` from the last loop iteration is preserved after the parallel region ends.