#C++
Copy this code into broken_loopcount.cpp
#include <iostream>
#ifdef _OPENMP
#include <omp.h>
#else
#define omp_get_thread_num() 0
#endif
int main(int argc, char **argv)
{
int global_nloops = 0;
#pragma omp parallel
{
int private_nloops = 0;
int thread_id = omp_get_thread_num();
#pragma omp for
for (int i=0; i<100000; ++i)
{
++private_nloops;
}
std::cout << "Thread " << thread_id << " adding its iterations ("
<< private_nloops << ") to the sum (" << global_nloops
<< ")...\n";
global_nloops += private_nloops;
std::cout << "...total nloops now equals " << global_nloops << ".\n";
}
std::cout << "The total number of loop iterations is " << global_nloops
<< ".\n";
return 0;
}and copy this code into fixed_loopcount.cpp
#include <iostream>
#ifdef _OPENMP
#include <omp.h>
#else
#define omp_get_thread_num() 0
#endif
int main(int argc, char **argv)
{
int global_nloops = 0;
#pragma omp parallel
{
int private_nloops = 0;
int thread_id = omp_get_thread_num();
#pragma omp for
for (int i=0; i<100000; ++i)
{
++private_nloops;
}
#pragma omp critical
{
std::cout << "Thread " << thread_id << " adding its iterations ("
<< private_nloops << ") to the sum (" << global_nloops
<< ")...\n";
global_nloops += private_nloops;
std::cout << "...total nloops now equals " << global_nloops
<< ".\n";
}
}
std::cout << "The total number of loop iterations is " << global_nloops
<< ".\n";
return 0;
}The only new code here is the omp critical section in fixed_loopcount.cpp. The critical section is performed by each thread, but can only be performed by one thread at a time. This ensures that while one thread is updating the global nloops variable (global_nloops) with the thread local value of nloops (private_nloops), that the value of global_nloops is not changed by any other thread.
You can compile these programs using one of these sets of commands (choose one for the compiler you wish to use);
- g++ :
g++ -fopenmp broken_loopcount.cpp -o broken_loopcount ; g++ -fopenmp fixed_loopcount.cpp -o fixed_loopcount - icpc :
icpc -openmp broken_loopcount.cpp -o broken_loopcount -cxxlib-icc ; icpc -openmp fixed_loopcount.cpp -o fixed_loopcount -cxxlib-icc - pgCC :
pgCC -mp broken_loopcount.cpp -o broken_loopcount ; pgCC -mp fixed_loopcount.cpp -o fixed_loopcount
This will produce the executables, broken_loopcount and fixed_loopcount.