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将元素分配给数组不起作用（使用 OpenMP 并行合并排序）

[英]Assignment of an element to an array does not work (Parallel Mergesort with OpenMP)

 原文  2021-07-24 18:33:57  5   3    c++/ arrays/ algorithm/ sorting/ mergesort

问题描述

我想并行化合并排序。 我的想法是查看第一个子数组中间的值，然后对另一个数组执行二分搜索以找到一个上限，其索引应作为枢轴在那里拆分第二个数组。然后我对前半部分进行排序第一个阵列的第一部分与第二个阵列的第一部分按顺序排列，第一个阵列的后半部分与第二个阵列的后半部分。 我想在 open mp 中使用任务，这样一个线程执行第一次合并，另一个线程执行第二次合并。

在我的代码中，赋值产生了垃圾值。 即out[a]=in[b]; 然后当我检查printf("%d%",out[a]); 我得到一个不在in[b}的值c 。 当我打印出数组时，值out[i]有时与打印函数显示的值不同，即out[a]=d和d既不是c也不是in[b]

我已经准备了一些打印语句的代码和一些输出，这些语句显示了in[b]和in[a] 。

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/time.h>

#include <iostream>
#include <algorithm>

#include <cstdlib>
#include <cstdio>

#include <cmath>
#include <ctime>
#include <cstring>
#include <omp.h>
// Constants.h
#if !defined(MYLIB_CONSTANTS_H)
#define MYLIB_CONSTANTS_H 1

const int CUTOFF =11;

#endif



int binarysearchfinduperrank(int *in,int n,int value, int projection){

    int* array= in+projection;
    int L=0;
    int R=n;

      printf("\nUpperBinarysearchrankfinder [");
      for(int i=0;i<n; i++){
          printf("%d,",array[i]);

     }
     printf("]\n");
    while(R-L>1){
        int middle = (R+L)/2;
         printf("L:%d,middle:%d,R:%d,value:%d\n",L,middle,R,value);
        if(array[middle]==value){
            while(array[middle]==value&&middle<n){
                middle=middle+1;

            }
             printf("L:%d,R:%d,result:%d,index:%d\n",L,R,middle,middle+projection);
            return middle;

        }
        if(array[middle]<value){
            L=middle;

        }
        if(array[middle]>value){
            R=middle;

        }
    }

      printf("L:%d,R:%d,value:%d\n",L,R,value);

     if(n==1){
         if(array[0]> value){
              printf(" result:0\n,index:%d\n",0+projection);
             return 0;

        }
        else{
             printf(" result:1,index:%d\n",1+projection);
            return 1;

        }

    }

    if(L==0&&array[L]>value){
         printf("L:%d,R:%d,result:%d,index:%d\n",L,R,0,projection);
        return 0;

    }
    if(R==n && array[R-1]<= value){
          printf("L:%d,R:%d,result:%d,index:%d\n",L,R,n,n+projection);
        return n;

    }
    if(R==n&& array[R-1]>value){
          printf("L:%d,R:%d,result:%d,index:%d\n",L,R,R-1,((R-1)+projection));
        return R-1;

    }
    if(array[R]<=value){
          printf("L:%d,R:%d,result:%d,index:%d\n",L,R,R+1,(R+1+projection));
        return R+1;

    }
    if(array[L]<=value){
         printf("L:%d,R:%d,result:%d,index:%d\n",L,R,R,R+projection);
        return R;

    }

     printf("L:%d,R:%d,result:%d,index:%d\n",L,R,L,L+projection);
    return L;


}

void printarray(int *array,int n){
    printf("\n[");
    for(int i=0;i<n;i++){
        printf("%d,",array[i]);
    }
    printf("]\n");
}


void MsMergeSequential(int *out, int *in, int begin1, int end1, int begin2, int end2, int outBegin) {
    int left = begin1;
    int right = begin2;
    int idx = outBegin;
    printf("Inputarray1:");
    printarray(in+begin1,(end1-begin1));

    printf("Inputarray2:");
    printarray(in+begin2,(end2-begin2));


    while (left < end1 && right < end2) {
        if (in[left] <= in[right]) {
            out[idx] = in[left];
            printf("! %d \n",in[left]);
            printf(" %d \n",out[idx]);
            left++;
        } else {

            out[idx] = in[right];
            printf("!! %d \n",in[left]);
            printf(" %d \n",out[idx]);
            right++;
        }
        idx++;
        }

    while (left < end1) {

        out[idx] = in[left];
        printf("!!! %d \n",in[left]);

        printf(" %d \n",out[idx]);
        left++, idx++;
    }
    while (right < end2) {
        out[idx] = in[right];
        printf(" !!!!%d \n",in[right]);
        printf(" %d \n",out[idx]);
        right++, idx++;
    }

    printf("Outputarray:\n[");
    printarray(out+begin1,(end1-begin1)+(end2-begin2));
    printf("]\n");
}


bool myfunc (long i , long j){return (i<j);}

void MsSequential(int *array, int *tmp, bool inplace, long begin, long end) {
  if( end <= begin + CUTOFF -1){

    std::sort(array+begin,array + end, myfunc);
  }
  else  if (begin < (end - 1)) {
           long half =(begin+end) / 2;
           int halffirst= (half+begin)/2;


            #pragma omp taskgroup
         {
           #pragma omp task shared(array) untied if(end-begin >= (1<<15))

             MsSequential(array, tmp, !inplace, begin, half);

             MsSequential(array, tmp, !inplace, half, end);
              }
        if (inplace){
            printf("Originalinputarray-tmp:");
            printarray(tmp+begin,(end-begin));
            int halfsecond=binarysearchfinduperrank(tmp,(end-half),tmp[halffirst],half)+half+begin;
            printf("%d,%d\n",halffirst,halfsecond);
            printf("Halffirst:%d,Halfsecond:%d\n,",tmp[halffirst],tmp[halfsecond]);
            #pragma omp taskgroup
         {
           #pragma omp task shared(tmp) untied if(end-begin >= (1<<15))

             MsMergeSequential(array, tmp, begin, halffirst, half, halfsecond, begin);


            MsMergeSequential(array, tmp, halffirst, half,halfsecond, end, begin+(halffirst-begin)+(halfsecond-half));
              }



        }
        else {
            int halfsecond=binarysearchfinduperrank(array,(end-half),array[halffirst],half)+half;
            printf("%d,%d\n",halffirst,halfsecond);
            printf("Originalinputarray-arr:");
            printarray(array+begin,(end-begin));
            printf("Halffirst:%d,Halfsecond:%d\n,",array[halffirst],array[halfsecond]);


             #pragma omp taskgroup
         {
           #pragma omp task shared(array) untied if(end-begin >= (1<<15))

             MsMergeSequential(tmp, array, begin, halffirst, half,halfsecond, begin);

            MsMergeSequential(tmp, array, halffirst, half,halfsecond, end, begin+halffirst+(halfsecond-half));
              }




        }

    } else if (!inplace) {

        tmp[begin] = array[begin];
    }
}

void MsParallel(int *array, int *tmp, const size_t size) {

    MsSequential(array, tmp, true, 0, size);
}




bool isSorted(int ref[], int data[], const size_t size){
    std::sort(ref, ref + size);
    for (size_t idx = 0; idx < size; ++idx){
        if (ref[idx] != data[idx]) {
            printf("\nFalscher Index:%d\n",idx);
            return false;
        }
    }
    return true;
}

/**

/**
  * @brief program entry point
  */
int main(int argc, char* argv[]) {
    // variables to measure the elapsed time
    struct timeval t1, t2;
    double etime;

    // expect one command line arguments: array size
    if (argc != 2) {
        printf("Usage: MergeSort.exe <array size> \n");
        printf("\n");
        return EXIT_FAILURE;
    }
    else {
        const size_t stSize = strtol(argv[1], NULL, 10);
        int *data = (int*) malloc(stSize * sizeof(int));
        int *tmp = (int*) malloc(stSize * sizeof(int));
        int *ref = (int*) malloc(stSize * sizeof(int));
        printf("Initialization...\n");

        srand(95);

        #pragma omp parallel for num_threads(100) schedule(static)
        {
        for (size_t idx = 0; idx < stSize; ++idx){
            data[idx] = (int) (stSize * (double(rand()) / RAND_MAX));
        }
        }
        std::copy(data, data + stSize, ref);
        double dSize = (stSize * sizeof(int)) / 1024 / 1024;
        printf("Sorting %u elements of type int (%f MiB)...\n", stSize, dSize);
        gettimeofday(&t1, NULL);
        // Mergesort starts
        #pragma omp parallel num_threads(80)
        {
        #pragma omp single
        {
        MsParallel(data, tmp, stSize);
        }
        }



        gettimeofday(&t2, NULL);
        etime = (t2.tv_sec - t1.tv_sec) * 1000 + (t2.tv_usec - t1.tv_usec) / 1000;
        etime = etime / 1000;
        printf("done, took %f sec. Verification...", etime);
        if (isSorted(ref, data, stSize)) {
            printf(" successful.\n");
        }
        else {
            printf(" FAILED.\n");
        }
        free(data);
        //delete[] data;
        free(tmp);
        //delete[] tmp;
        free(ref);
        //delete[] ref;
    }
    return EXIT_SUCCESS;
}

输出：（! 标记用于定位代码工具中赋值的位置）

Initialization...
Sorting 40 elements of type int (0.000000 MiB)...

UpperBinarysearchrankfinder [0,8,8,15,26,29,30,38,39,39,]
L:0,middle:5,R:10,value:15
L:0,middle:2,R:5,value:15
L:2,middle:3,R:5,value:15
L:2,R:5,result:4,index:14
5,14
Originalinputarray-arr:
[0,4,6,7,11,15,17,19,29,33,0,8,8,15,26,29,30,38,39,39,]
Halffirst:15,Halfsecond:26
,Inputarray1:
[0,4,6,7,11,]
Inputarray2:
[0,8,8,15,]
! 0
 0
!! 4
 0
! 4
 4
! 6
 6
! 7
 7
!! 11
 8
!! 11
 8
! 11
 11
 !!!!15
 15
Outputarray:
[
[0,0,4,6,7,8,8,11,15,]
]
Inputarray1:
[15,17,19,29,33,]
Inputarray2:
[26,29,30,38,39,39,]
! 15
 15
! 17
 17
! 19
 19
!! 29
 26
! 29
 29
!! 33
 29
!! 33
 30
! 33
 33
 !!!!38
 38
 !!!!39
 39
 !!!!39
 39
Outputarray:
[
[8,8,11,15,15,17,19,26,29,29,30,]
]

UpperBinarysearchrankfinder [1,5,7,11,16,26,27,30,32,36,]
L:0,middle:5,R:10,value:26
L:0,R:10,result:6,index:36
25,36
Originalinputarray-arr:
[3,4,5,7,10,26,29,33,34,35,1,5,7,11,16,26,27,30,32,36,]
Halffirst:26,Halfsecond:27
,Inputarray1:
[3,4,5,7,10,]
Inputarray2:
[1,5,7,11,16,26,]
!! 3
 1
! 3
 3
! 4
 4
! 5
 5
!! 7
 5
! 7
 7
!! 10
 7
! 10
 10
 !!!!11
 11
 !!!!16
 16
 !!!!26
 26
Outputarray:
[
[1,3,4,5,5,7,7,10,11,16,26,]
]
Inputarray1:
[26,29,33,34,35,]
Inputarray2:
[27,30,32,36,]
! 26
 26
!! 29
 27
! 29
 29
!! 33
 30
!! 33
 32
! 33
 33
! 34
 34
! 35
 35
 !!!!36
 36
Outputarray:
[
[7,7,10,11,16,26,-1751738988,-1684366952,-6382180,]
]
Originalinputarray-tmp:
[0,0,4,6,7,8,8,11,15,15,17,19,26,29,29,30,33,38,39,39,1,3,4,5,5,7,7,10,11,16,26,-1751738988,-1684366952,-6382180,-1549622880,-1482250844,-1414878808,-1347506772,-1280134736,-1212762700,]

UpperBinarysearchrankfinder [1,3,4,5,5,7,7,10,11,16,26,-1751738988,-1684366952,-6382180,-1549622880,-1482250844,-1414878808,-1347506772,-1280134736,-1212762700,]
L:0,middle:10,R:20,value:17
L:0,middle:5,R:10,value:17
L:5,middle:7,R:10,value:17
L:7,middle:8,R:10,value:17
L:8,middle:9,R:10,value:17
L:9,R:10,value:17
L:9,R:10,result:10,index:30
10,30
Halffirst:17,Halfsecond:26
,Inputarray1:
[0,0,4,6,7,8,8,11,15,15,]
Inputarray2:
[1,3,4,5,5,7,7,10,11,16,]
! 0
 0
! 0
 0
!! 4
 1
!! 4
 3
! 4
 4
!! 6
 4
!! 6
 5
!! 6
 5
! 6
 6
! 7
 7
!! 8
 7
!! 8
 7
! 8
 8
! 8
 8
!! 11
 10
! 11
 11
!! 15
 11
! 15
 15
! 15
 15
 !!!!16
 16
Outputarray:
[
[0,0,1,3,4,4,5,5,6,7,7,7,8,8,10,11,11,15,15,16,]
]
Inputarray1:
[17,19,26,29,29,30,33,38,39,39,]
Inputarray2:
[26,-1751738988,-1684366952,-6382180,-1549622880,-1482250844,-1414878808,-1347506772,-1280134736,-1212762700,]
! 17
 17
! 19
 19
! 26
 26
!! 29
 26
!! 29
 -1751738988
!! 29
 -1684366952
!! 29
 -6382180
!! 29
 -1549622880
!! 29
 -1482250844
!! 29
 -1414878808
!! 29
 -1347506772
!! 29
 -1280134736
!! 29
 -1212762700
!!! 29
 29
!!! 29
 29
!!! 30
 30
!!! 33
 33
!!! 38
 38
!!! 39
 39
!!! 39
 39
Outputarray:
[
[7,7,8,8,10,11,11,15,15,16,17,19,26,26,-1751738988,-1684366952,-6382180,-1549622880,-1482250844,-1414878808,]
]
done, took 0.046000 sec. Verification...
Falscher Index:1
 FAILED.

3 个解决方案

解决方案1
 2  2021-07-24 20:20:28

如果要并行化它，则应使用线程并行执行排序部分。 我在您的代码中没有看到任何并行化。

正常的合并排序是：

mergesort(arr, l, r):
1. middle = (l + r)/2
2. mergesort(arr, l, middle)
3. mergesort(arr, middle + 1, r)
4. merge(arr, l, middle, r) //merges the subarray

如果你想并行化，你可以创建一个线程来做 2 和另一个线程来做 3 然后加入它们，一旦它们完成，你就进行合并。

mergesort(arr, l, r):
1. middle = (l + r)/2
2. create thread to execute mergesort(arr, l, middle)
3. create thread to execute mergesort(arr, middle + 1, r)
4. wait until both threads are done
5. merge(arr, l, middle, r) //merges the subarray

时间复杂度为 T(n) = T(n/2) + O(n)，如果您求解，它将是 n + n/2 + n/4 + ... 1 = O(n)。

解决方案2
 1  2021-07-24 21:13:22

我创建了一个使用二进制搜索进行合并的自顶向下合并排序的简化顺序示例。 我使用修改后的版本验证了该代码是一种稳定的类型。 MergeSortAtoA 和 MergeSortAtoB 是相互调用的相互递归函数，而不是传递一个就地标志。 其要点是函数 BinSrchLo、BinSrchHi 和 Merge。 BinSrchxx(a, ll, rr, v) 返回将 v 插入到 a[ll,rr] 中的索引（就好像 a[] 扩展了 1 个元素以为插入的元素腾出空间）。 例如，如果 v < a[ll]，则返回 ll；如果 v > a[rr-1]，则返回 rr。 请注意，merge 在两个 for 循环中都分配了 b[i+j-rr] = a[i]。 检查您的代码与此工作示例之间的差异。

void MergeSortAtoA(int a[], int b[], size_t ll, size_t ee);
void MergeSortAtoB(int a[], int b[], size_t ll, size_t ee);
void Merge(int a[], int b[], size_t ll, size_t rr, size_t ee);

void MergeSort(int a[], int b[], size_t n)
{
    MergeSortAtoA(a, b, 0, n);
}

void MergeSortAtoA(int a[], int b[], size_t ll, size_t ee)
{
    if (ee-ll > 1) {
        size_t rr = (ll + ee)>>1;       /* midpoint, start of right half */
        MergeSortAtoB(a, b, ll, rr);
        MergeSortAtoB(a, b, rr, ee);
        Merge(b, a, ll, rr, ee);        /* merge b to a */
    }
}

void MergeSortAtoB(int a[], int b[], size_t ll, size_t ee)
{
    if (ee-ll > 1) {
        size_t rr = (ll+ee)/2;          /* midpoint, start of right half */
        MergeSortAtoA(a, b, ll, rr);
        MergeSortAtoA(a, b, rr, ee);
        Merge(a, b, ll, rr, ee);        /* merge a to b */
    } else if (ee-ll == 1) {
        b[ll] = a[ll];
    }
}

size_t BinSrchLo(int a[], size_t ll, size_t rr, int v)
{
    while(ll < rr){
        size_t i = (ll+rr)/2;
        if(a[i] < v)
            ll = i+1;
        else
            rr = i;
    }
    return ll;
}

size_t BinSrchHi(int a[], size_t ll, size_t rr, int v)
{
    while(ll < rr){
        size_t i = (ll+rr)/2;
        if(a[i] <= v)
            ll = i+1;
        else
            rr = i;
    }
    return ll;
}

void Merge(uint64_t a[], uint64_t b[], size_t ll, size_t rr, size_t ee)
{
    for(size_t i = ll; i < rr; i++){
        size_t j = BinSrchLo(a, rr, ee, a[i]);
        b[i+j-rr] = a[i];
    }
    for(size_t i = rr; i < ee; i++){
        size_t j = BinSrchHi(a, ll, rr, a[i]);
        b[i+j-rr] = a[i];
    }
}

解决方案3
 0  2021-07-25 18:33:53

代码中有一些错误，因此变量 halffirst 没有分配给它应该分配的索引。 底部的叶子数组也充满了垃圾值。 固定代码在这里。 大小为 10^7 的数组的运行时间为 0.2 秒，大小为 10^8 的数组为 2.4 秒，假设计算机有 256 个线程

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/time.h>

#include <iostream>
#include <algorithm>

#include <cstdlib>
#include <cstdio>

#include <cmath>
#include <ctime>
#include <cstring>
#include <omp.h>
// Constants.h
#if !defined(MYLIB_CONSTANTS_H)
#define MYLIB_CONSTANTS_H 1

const int CUTOFF =11;

#endif



int binarysearchfinduperrank(int *in,int n,int value, int projection){

    int* array= in+projection;
    int L=0;
    int R=n;

      /*printf*/("\nUpperBinarysearchrankfinder [");
//       for(int i=0;i<n; i++){
//           printf("%d,",array[i]);

//      }
//      printf("]\n");
    while(R-L>1){
        int middle = (R+L)/2;
//          printf("L:%d,middle:%d,R:%d,value:%d\n",L,middle,R,value);
        if(array[middle]==value){
            while(array[middle]==value&&middle<n){
                middle=middle+1;

            }
//              printf("L:%d,R:%d,result:%d,index:%d\n",L,R,middle,middle+projection);
            return middle;

        }
        if(array[middle]<value){
            L=middle;

        }
        if(array[middle]>value){
            R=middle;

        }
    }

//       printf("L:%d,R:%d,value:%d\n",L,R,value);

     if(n==1){
         if(array[0]> value){
//               printf(" result:0\n,index:%d\n",0+projection);
             return 0;

        }
        else{
//              printf(" result:1,index:%d\n",1+projection);
            return 1;

        }

    }

    if(L==0&&array[L]>value){
//          printf("L:%d,R:%d,result:%d,index:%d\n",L,R,0,projection);
        return 0;

    }
    if(R==n && array[R-1]<= value){
//           printf("L:%d,R:%d,result:%d,index:%d\n",L,R,n,n+projection);
        return n;

    }
    if(R==n&& array[R-1]>value){
//           printf("L:%d,R:%d,result:%d,index:%d\n",L,R,R-1,((R-1)+projection));
        return R-1;

    }
    if(array[R]<=value){
//           printf("L:%d,R:%d,result:%d,index:%d\n",L,R,R+1,(R+1+projection));
        return R+1;

    }
    if(array[L]<=value){
//          printf("L:%d,R:%d,result:%d,index:%d\n",L,R,R,R+projection);
        return R;

    }

//      printf("L:%d,R:%d,result:%d,index:%d\n",L,R,L,L+projection);
    return L;


}

void printarray(int *array,int n){
    printf("\n[");
    for(int i=0;i<n;i++){
        printf("%d,",array[i]);
    }
    printf("]\n");
}


void MsMergeSequential(int *out, int *in, int begin1, int end1, int begin2, int end2, int outBegin) {
    int left = begin1;
    int right = begin2;
    int idx = outBegin;
//     printf("Inputarray1:");
//     printarray(in+begin1,(end1-begin1));

//     printf("Inputarray2:");
//     printarray(in+begin2,(end2-begin2));


    while (left < end1 && right < end2) {
        if (in[left] <= in[right]) {
            out[idx] = in[left];
//             printf("! %d \n",in[left]);
//             printf(" %d \n",out[idx]);
            left++;
        } else {

            out[idx] = in[right];
//             printf("!! %d \n",in[left]);
//             printf(" %d \n",out[idx]);
            right++;
        }
        idx++;
        }

    while (left < end1) {

        out[idx] = in[left];
//         printf("!!! %d \n",in[left]);

//         printf(" %d \n",out[idx]);
        left++, idx++;
    }
    while (right < end2) {
        out[idx] = in[right];
//         printf(" !!!!%d \n",in[right]);
//         printf(" %d \n",out[idx]);
        right++, idx++;
    }

//  printf("Outputarray:\n[");
//     printarray(out+outBegin,end2-outBegin);
//     printf("]\n");
}


bool myfunc (long i , long j){return (i<j);}

void MsSequential(int *array, int *tmp, bool inplace, long begin, long end) {
  if( end <= begin + CUTOFF -1){

    std::sort(array+begin,array + end, myfunc);
    std::copy(array+begin,array+end,tmp+begin);
  }
  else  if (begin < (end - 1)) {
           long half =(begin+end) / 2;
           int halffirst= (half+begin)/2;


            #pragma omp taskgroup
         {
           #pragma omp task shared(array) untied if(end-begin >= (1<<15))

             MsSequential(array, tmp, !inplace, begin, half);

             MsSequential(array, tmp, !inplace, half, end);
              }
        if (inplace){
//             printf("Originalinputarray-tmp:");
//             printarray(tmp+begin,(end-begin));
            int halfsecond=binarysearchfinduperrank(tmp,(end-half),tmp[halffirst],half)+half;
//             printf("%d,%d\n",halffirst,halfsecond);
//             printf("Halffirst:%d,Halfsecond:%d\n,",tmp[halffirst],tmp[halfsecond]);
            #pragma omp taskgroup
         {
           #pragma omp task shared(tmp) untied if(end-begin >= (1<<15))

             MsMergeSequential(array, tmp, begin, halffirst, half, halfsecond, begin);


            MsMergeSequential(array, tmp, halffirst, half,halfsecond, end, begin+(halffirst-begin)+(halfsecond-half));
              }



        }
        else {
            int halfsecond=binarysearchfinduperrank(array,(end-half),array[halffirst],half)+half;
//             printf("%d,%d\n",halffirst,halfsecond);
//             printf("Originalinputarray-arr:");
//             printarray(array+begin,(end-begin));
//             printf("Halffirst:%d,Halfsecond:%d\n,",array[halffirst],array[halfsecond]);


             #pragma omp taskgroup
         {
           #pragma omp task shared(array) untied if(end-begin >= (1<<15))

             MsMergeSequential(tmp, array, begin, halffirst, half,halfsecond, begin);

            MsMergeSequential(tmp, array, halffirst, half,halfsecond, end, begin+(halffirst-begin)+(halfsecond-half));
              }




        }

    } else if (!inplace) {

        tmp[begin] = array[begin];
    }
}

void MsParallel(int *array, int *tmp, const size_t size) {

    MsSequential(array, tmp, true, 0, size);
}




bool isSorted(int ref[], int data[], const size_t size){
    std::sort(ref, ref + size);
    for (size_t idx = 0; idx < size; ++idx){
        if (ref[idx] != data[idx]) {
//             printf("\nFalscher Index:%d\n",idx);
            return false;
        }
    }
    return true;
}

/**

/**
  * @brief program entry point
  */
int main(int argc, char* argv[]) {
    // variables to measure the elapsed time
    struct timeval t1, t2;
    double etime;

    // expect one command line arguments: array size
    if (argc != 2) {
//         printf("Usage: MergeSort.exe <array size> \n");
//         printf("\n");
        return EXIT_FAILURE;
    }
    else {
        const size_t stSize = strtol(argv[1], NULL, 10);
        int *data = (int*) malloc(stSize * sizeof(int));
        int *tmp = (int*) malloc(stSize * sizeof(int));
        int *ref = (int*) malloc(stSize * sizeof(int));
        printf("Initialization...\n");

        srand(95);

        #pragma omp parallel for num_threads(100) schedule(static)
        {
        for (size_t idx = 0; idx < stSize; ++idx){
            data[idx] = (int) (stSize * (double(rand()) / RAND_MAX));
        }
        }
        std::copy(data, data + stSize, ref);
        double dSize = (stSize * sizeof(int)) / 1024 / 1024;
        printf("Sorting %u elements of type int (%f MiB)...\n", stSize, dSize);
        gettimeofday(&t1, NULL);
        // Mergesort starts
        #pragma omp parallel num_threads(80)
        {
        #pragma omp single
        {
        MsParallel(data, tmp, stSize);
        }
        }



        gettimeofday(&t2, NULL);
        etime = (t2.tv_sec - t1.tv_sec) * 1000 + (t2.tv_usec - t1.tv_usec) / 1000;
        etime = etime / 1000;
        printf("done, took %f sec. Verification...", etime);
        if (isSorted(ref, data, stSize)) {
            printf(" successful.\n");
        }
        else {
            printf(" FAILED.\n");
        }
        free(data);
        //delete[] data;
        free(tmp);
        //delete[] tmp;
        free(ref);
        //delete[] ref;
    }
    return EXIT_SUCCESS;
}

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