CODEWITHSUNDEEP
c++parallel-processingsyclintel-oneapidpc++
sv6
sv6

Reputation: 11

Unexpected/Incorrect Results while running SYCL/DPC++ code

I am a beginner in SYCl/DPC++. I want to print multiples of 10 but, instead of that, I am getting 0's in place of that.

I am using the USM (Unified Shared Memory) and I am checking the data movement in the shared memory and host memory implicitly. So I have created two Arrays and I have initialized and performing the operation on them. I can see the same results for both of them.

Here is my code; I don't understand where I went wrong.

#include <CL/sycl.hpp>
#include<iostream>
using namespace std;
using namespace sycl;
constexpr int n = 10;

int main() {
  queue q;
  int *hostArray = malloc_host<int>(n, q);
  int *sharedArray = malloc_shared<int>(n, q);

  for (int i = 0; i < n; i++)
    hostArray[i] = i;
  q.submit([&](handler &h) {
      h.parallel_for(n, [=](id<1> i) {
          sharedArray[i] = hostArray[i] * 10;
        });
    });

  for (int i = 0; i <  n; i++) {
    hostArray[i] = sharedArray[i];
    cout<<hostArray[i]<<" "<<sharedArray[i];
    cout<<"\n";
  }
  cout<<"\n";
  return 0;
}

Expected Results:

                  0   0
                  10 10
                  20 20
                  30 30
                  40 40
                  50 50
                  60 60
                  70 70
                  80 80
                  90 90

Actual Output:

                  0 0
                  0 0
                  0 0
                  0 0
                  0 0
                  0 0
                  0 0
                  0 0
                  0 0
                  0 0

Upvotes: 0

Views: 280

Answers (1)

Ruyk
Ruyk

Reputation: 795

You are missing a barrier between the submission of the queue and the for loop in the host code.

Although it is true that an USM shared memory allocation is visible on the host and the device, there is no guarantees that the command group you have submitted to the queue will execute before the for loop in the host: Submissions to queues execute asynchronously w.r.t to the calling thread. Updated code below:

    #include <CL/sycl.hpp>
    #include<iostream>
    using namespace std;
    using namespace sycl;
    constexpr int n = 10;
    
    int main() {
      queue q;
      int *hostArray = malloc_host<int>(n, q);
      int *sharedArray = malloc_shared<int>(n, q);
    
      for (int i = 0; i < n; i++)
        hostArray[i] = i;
      q.submit([&](handler &h) {
          h.parallel_for(n, [=](id<1> i) {
              sharedArray[i] = hostArray[i] * 10;
            });
        });
      // Wait for completion of all previously submitted command groups
      q.wait();
    
      for (int i = 0; i <  n; i++) {
        hostArray[i] = sharedArray[i];
        cout<<hostArray[i]<<" "<<sharedArray[i];
        cout<<"\n";
      }
      cout<<"\n";
      return 0;
    }

Upvotes: 2

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