openmp parallel sections benchmark - c++

Question

I'm trying to benchmark my implementation of merge sort using openmp. I have written the following code.
#include <iostream>
#include <vector>
#include <cstdlib>
#include <ctime>
#include <omp.h>
using namespace std;
class Sorter {
private:
int* data;
int size;
bool isSorted;
public:
Sorter(int* data, int size){
this->data = data;
this->size = size;
this->isSorted = false;
}
void sort(){
vector<int> v(data,data+size);
vector<int> ans = merge_sort(v);
copy(ans.begin(),ans.end(),data);
isSorted = true;
}
vector<int> merge_sort(vector<int>& vec){
if(vec.size() == 1){
return vec;
}
std::vector<int>::iterator middle = vec.begin() + (vec.size() / 2);
vector<int> left(vec.begin(), middle);
vector<int> right(middle, vec.end());
#pragma omp parallel sections
{
#pragma omp section
{left = merge_sort(left);}
#pragma omp section
{right = merge_sort(right);}
}
return merge(vec,left, right);
}
vector<int> merge(vector<int> &vec,const vector<int>& left, const vector<int>& right){
vector<int> result;
unsigned left_it = 0, right_it = 0;
while(left_it < left.size() && right_it < right.size()) {
if(left[left_it] < right[right_it]){
result.push_back(left[left_it]);
left_it++;
}else{
result.push_back(right[right_it]);
right_it++;
}
}
while(left_it < left.size()){
result.push_back(left[left_it]);
left_it++;
}
while(right_it < right.size()){
result.push_back(right[right_it]);
right_it++;
}
return result;
}
int* getSortedData(){
if(!isSorted){
sort();
}
return data;
}
};
void printArray(int* array, int size){
for(int i=0;i<size;i++){
cout<<array[i]<<", ";
}
cout<<endl;
}
bool isSorted(int* array, int size){
for(int i=0;i<size-1;i++){
if(array[i] > array[i+1]) {
cout<<array[i]<<" > "<<array[i+1]<<endl;
return false;
}
}
return true;
}
int main(int argc, char** argv){
if(argc<3){
cout<<"Specify size and threads"<<endl;
return -1;
}
int size = atoi(argv[1]);
int threads = atoi(argv[2]);
//omp_set_nested(1);
omp_set_num_threads(threads);
cout<<"Merge Sort of "<<size<<" with "<<omp_get_max_threads()<<endl;
int *array = new int[size];
srand(time(NULL));
for(int i=0;i<size;i++){
array[i] = rand() % 100;
}
//printArray(array,size);
Sorter* s = new Sorter(array, size);
cout<<"Starting sort"<<endl;
double start = omp_get_wtime();
s->sort();
double stop = omp_get_wtime();
cout<<"Time: "<<stop-start<<endl;
int* array2 = s->getSortedData();
if(size<=10)
printArray(array2,size);
cout<<"Array sorted: "<<(isSorted(array2,size)?"yes":"no")<<endl;
return 0;
}
The program runs correctly, but when i specify the number of threads to be, say 4, the program still creates only 2 threads. I tried using omp_set_nested(1) before omp_set_num_threads(threads) but that hands the whole terminal until the program crashes and says "libgomp: Thread creation failed: Resource temporarily unavailable" I think because too many threads are created? I haven't found a work around it yet.
Edit:
After the program crashes, I check the system load and it shows the load to be over 1000!
I have a 4-core AMD A8 CPU and 10GB RAM
If I uncomment omp_set_nested(1) and run the program
$ ./mergeSort 10000000 4
Merge Sort of 10000000 with 4
Starting sort
libgomp: Thread creation failed: Resource temporarily unavailable
libgomp: Thread creation failed: Resource temporarily unavailable
$ uptime
02:14:12 up 1 day, 11:13, 4 users, load average: 482.21, 522.87, 338.75
Watching the processes, I can spot 4 threads being launched. If I comment out the omp_set_nested(1) the program runs normally but only uses 2 threads
Edit:
If i use tasks and remove omp_set_nested then it launches the threads correctly, but it doesn't speed up. Execution with 1 thread becomes faster than with 4. With sections, it speeds up. but only by a factor less than two (as it launches only 2 threads at a time)

Answer 1

I tested your code and it did create 4 or more threads, didn't get what you meant exactly. Also I suggest you to change omp section to omp task, as by definition in a section only 1 thread handles a given section and in your recursive call you would never utilize your idle threads.