How to parallelize an array shift with OpenMP? - openmp

Question

How can I parallelize an array shift with OpenMP?
I've tryed a few things but didn't get any accurate results for the following example (which rotates the elements of an array of Carteira objects, for a permutation algorithm):
void rotaciona(int i)
{
Carteira aux = this->carteira[i];
for(int c = i; c < this->size - 1; c++)
{
this->carteira[c] = this->carteira[c+1];
}
this->carteira[this->size-1] = aux;
}
Thank you very much!

Answer 1

This is an example of a loop with loop-carried dependencies, and so can't be easily parallelized as written because the tasks (each iteration of the loop) aren't independent. Breaking the dependency can vary from a trivial modification to the completely impossible
(eg, an iteration loop).
Here, the case is somewhat in between. The issue with doing this in parallel is that you need to find out what your rightmost value is going to be before your neighbour changes the value. The OMP for construct doesn't expose to you which loop iterations values will be "yours", so I don't think you can use the OpenMP for worksharing construct to break up the loop. However, you can do it yourself; but it requires a lot more code, and it won't nicely reduce to the serial case any more.
But still, an example of how to do this is shown below. You have to break the loop up yourself, and then get your rightmost value. An OpenMP barrier ensures that no one starts modifying values until all the threads have cached their new rightmost value.
#include <stdio.h>
#include <stdlib.h>
#include <omp.h>
int main(int argc, char **argv) {
int i;
char *array;
const int n=27;
array = malloc(n * sizeof(char) );
for (i=0; i<n-1; i++)
array[i] = 'A'+i;
array[n-1] = '\0';
printf("Array pre-shift = <%s>\n",array);
#pragma omp parallel default(none) shared(array) private(i)
{
int nthreads = omp_get_num_threads();
int tid = omp_get_thread_num();
int blocksize = (n-2)/nthreads;
int start = tid*blocksize;
int end = start + blocksize - 1;
if (tid == nthreads-1) end = n-2;
/* we are responsible for values start...end */
char rightval = array[end+1];
#pragma omp barrier
for (i=start; i<end; i++)
array[i] = array[i+1];
array[end] = rightval;
}
printf("Array post-shift = <%s>\n",array);
return 0;
}

Answer 2

Though your sample doesn't show any explicit openmp pragma's, I don't think it could work easily:
you are doing an in-place operation with overlapping regions.
If you split the loop in chunks, you'll have race conditions at the boundaries (because el[n] gets copied from el[n+1], which might already have been updated in another thread).
I suggest that you do manual chunking (which can be done), but I suspect that openmp parallel for is not flexible enough (haven't tried), so you could just have a parallell region that does the work in chunks, and fixup the boundary elements after a thread barrier/end of parallel block
Other thoughts:
if your values are POD, you can use memmove instead
if you can, simply switch to a list
.
std::list<Carteira> items(3000);
// rotation is now simply:
items.push_back(items.front());
items.erase(items.begin());