Quiz: Using Cache and Unified Memory in OpenMP Offloading¶

1. What does the use_device_ptr clause in OpenMP Offloading achieve?

• A. It maps data from the GPU back to the CPU at the end of the target region.
• B. It enables the direct use of a pointer in device memory, reducing the need for data copying.
• C. It creates a shared memory cache on the device for faster data access.
• D. It automatically manages data transfers between the CPU and GPU.

2. True or False: The use_device_ptr clause can provide cache-like behavior in OpenMP Offloading by reducing data transfer overhead.

3. In OpenMP Offloading, which type of memory management allows the CPU and GPU to share a single address space, automatically handling data migration?

• A. Shared Memory
• B. Mapped Memory
• C. Unified Memory
• D. Distributed Memory

4. Which of the following best describes unified memory in OpenMP?

• A. A memory model where the programmer must explicitly manage data transfers between CPU and GPU.
• B. A feature that automatically manages data location and movement between CPU and GPU.
• C. A type of memory exclusively for temporary storage on the GPU.
• D. A memory cache available only in OpenACC, not OpenMP.

5. In the following C/C++ code snippet, what is the role of use_device_ptr(a)?

#pragma omp target data map(to: a[0:n], b[0:n]) map(from: c[0:n])
{
  #pragma omp target teams distribute parallel for
    for (int i = 0; i < n-2; i++) {
        #pragma omp use_device_ptr(a)
        c[i] = a[i] + a[i+1] + a[i+2] + b[i];
          }
}

• A. It allows direct access to array a in device memory without copying it to the GPU.
• B. It maps the array a back to the CPU at the end of the target region.
• C. It ensures a is private for each thread.
• D. It copies the data in a from the device to the host.

6. True or False: Unified memory eliminates the need for explicit map clauses in OpenMP Offloading.

7. Which compiler flag is typically used to enable unified memory in NVIDIA’s OpenMP compiler?

• A. -gpu=unified
• B. -mp=shared
• C. -gpu=managed
• D. -mem=unified

8. In the following Fortran code, why are target enter data and target exit data directives not needed?

subroutine Vector_Addition(a, b, c, n)
  real(8), intent(in) :: a(:), b(:)
  real(8), intent(out) :: c(:)
  integer :: i, n

  !$omp target teams distribute parallel do
  do i = 1, n
     c(i) = a(i) + b(i)
  end do
end subroutine Vector_Addition

• A. Because the variables a, b, and c are only read on the device.
• B. Because target enter data and exit data are redundant when teams is used.
• C. Because unified memory automatically handles data synchronization between CPU and GPU.
• D. Because Fortran doesn’t support data mapping in OpenMP.

9. In which scenario is unified memory most beneficial in OpenMP Offloading?

• A. When data needs to be accessed frequently by both the CPU and GPU.
• B. When data is accessed only once by the GPU.
• C. When the dataset is too large for device memory.
• D. When specific regions of arrays need direct access in device memory.

10. True or False: use_device_ptr is used in OpenMP to allow data to persist across multiple target regions on the GPU.