topic Re:Parallel result is different with serial result in Intel® oneAPI DPC++/C++ Compiler

Parallel result is different with serial result

Siqiao_Fu — Thu, 10 Jun 2021 10:06:41 GMT

#include <CL/sycl.hpp> #include <iostream> #include <ctime> using namespace std; using namespace cl::sycl; constexpr long num_steps = 10000000; double without_oneapi() { double step = 1.0 / (double)num_steps; double x = 0.0; double sum = 0.0; clock_t start = clock(); for (int i = 0; i < num_steps; i++) { x = (i + 0.5) * step; sum += 4.0 / (1.0 + x * x); } clock_t end = clock(); std::cout << "Without oneapi cost " << (double)(end - start) / CLOCKS_PER_SEC << " second" << std::endl; double res = step * sum; return res; } double with_oneapi_buffer() { queue q; //std::cout << "Device: " << q.get_device().get_info<info::device::name>() << std::endl; double step = 1.0 / (double)num_steps; double data[2] = {step, 0.0}; buffer buf(data, range<1>(2)); clock_t start = clock(); q.submit([&](handler& h) { accessor a(buf, h); h.parallel_for(range<1>(num_steps), [=](auto i) { double temp = ((double)i + 0.5) * a[0]; a[1] += 4.0 / (1.0 + temp * temp); }); }).wait(); clock_t end = clock(); std::cout << "With oneapi buffer cost " << (double)(end - start) / CLOCKS_PER_SEC << " second" << std::endl; double res = step * data[1]; return res; } double with_oneapi_usm() { queue q; //std::cout << "Device: " << q.get_device().get_info<info::device::name>() << std::endl; double step = 1.0 / (double)num_steps; double* data = malloc_shared<double>(2, q); data[0] = step; data[1] = 0.0; clock_t start = clock(); q.parallel_for(range<1>(num_steps), [=](id<1> i){ double temp = ((double)i + 0.5) * data[0]; data[1] += 4.0 / (1.0 + temp * temp); }).wait(); clock_t end = clock(); std::cout << "With oneapi usm cost " << (double)(end - start) / CLOCKS_PER_SEC << " second" << std::endl; double res = step * data[1]; free(data, q); return res; } int main() { clock_t start, end; double PI1 = without_oneapi(); double PI2 = with_oneapi_buffer(); double PI3 = with_oneapi_usm(); std::cout << "Without oneapi result:" << PI1 << std::endl; std::cout << "With oneapi buffer result:" << PI2 << std::endl; std::cout << "With oneapi usm result:" << PI3 << std::endl; return 0; }

I tried this simple test and the result is weird. When num_steps is larger than 10000. Both of the results of using usm and using buffer becomes smaller than they should be which is PI.

Why does this happened? Did I use oneAPI in the right way?

Re:Parallel result is different with serial result

RahulV_intel — Fri, 11 Jun 2021 12:15:06 GMT

Hi,

The source code provided by you is not thread-safe.

Specifically, this line of code:

a[1] += 4.0 / (1.0 + temp * temp);

The DPC++ runtime launches as many threads as specified in the parallel_for region. In your case, the runtime launches 10000000 threads (as specified by the num_steps variable). Since all these threads are writing to the same memory location (a[1]), the results can go wrong.

To avoid this, you could create an array to store all the partial results and then perform a reduction sum operation on the array to get the final result.

Alternatively, you might want to check out the Montecarlo PI approximation sample in the link below:

https://github.com/oneapi-src/oneAPI-samples/tree/master/Libraries/oneMKL/monte_carlo_pi

Regards,

Rahul

Re: Re:Parallel result is different with serial result

Siqiao_Fu — Fri, 11 Jun 2021 12:47:56 GMT

Really helpful! Thank you Rahul!

Re:Parallel result is different with serial result

RahulV_intel — Mon, 14 Jun 2021 10:26:51 GMT

Hi,

Thanks for the confirmation. Intel will no longer monitor this thread. Further discussions on this thread will be considered community only.

Regards,

Rahul