Issue #
Unused data should never be copied to or from the GPU to prevent unnecessary data movements between the CPU and the GPU, which impacts performance.
Relevance #
One of the key challenges when offloading work to the GPU is minimizing the data transfers between CPU memory and GPU memory. These transfers can greatly affect performance and should be performed only when needed. Thus, only the strictly required data should be copied to or from the GPU memory.
Actions #
Restrict the array range to be copied to the GPU to the range strictly required.
Code example #
The following code performs the sum of two arrays:
void foo() {
int A[100], B[100], sum[100];
#pragma omp target map(to: A[0:100], B[0:100]) map(from: sum[0:100])
#pragma omp parallel for
for (int i = 0; i < 50; i++) {
sum[i] = A[i] + B[i];
}
}
However, only half of the total array elements are actually being used. Thus, there is no need to transfer the entire arrays:
void foo() {
int A[100], B[100], sum[100];
#pragma omp target map(to: A[0:50], B[0:50]) map(from: sum[0:50])
#pragma omp parallel for
for (int i = 0; i < 50; i++) {
sum[i] = A[i] + B[i];
}
}
