Windows Threads: Parallel Mergesort - c++

Question

I have what is hopefully a very easy question, I just cant find the answer online. I made a merge sort function ( which im sure has inefficiencies), but im here to ask about the threads. I'm using Windows' CreateThread function to spawn threads to sort intervals of a given array. Once all the threads are finished, I will merge the segments together for the final result. I havent implemented the final merge yet because im getting strange errors which im sure is from a dumb mistake in the threads. I'll post my code, if you could kindly look at paraMSort. Ill post the whole MergeSort.h file so you can see the helper functions as well. Sometimes the code will compile and run perfectly. Sometimes the console will abruptly close with no errors/exceptions. There shouldnt be mutex issues because im doing operations on different segments of the array (Different memory locations altogether). Does anyone see something wrong? Thanks so much.
PS. Are Windows CreateThread's kernel level? In other words, if I make 2 threads on a dual core computer, may they run simultaneously on separate cores? Im thinking yes, since on this computer I can do the same work in 1/2 the time with 2 threads (with another test example).
PPS. I also saw some parallelism answers solved using Boost.Thread. Should I just use boost threads instead of windows threads? I don't have experience with Boost.
#include "Windows.h"
#include <iostream>
using namespace std;
void insert_sort(int* A, int sA, int eA, int* B, int sB, int eB)
{
int value;
int iterator;
for(int i = sA + 1; i < eA; i++)
{
value = A[i]; // Grab the next value in the array
iterator = i - 1;
// Move this value left up the list until its in the right spot
while(iterator >= sA && value < A[iterator])
A[iterator + 1] = A[iterator--];
A[iterator + 1] = value; // Put value in its correct spot
}
for(int i = sA; i < eB; i++)
{
B[i] = A[i]; // Put results in B
}
}
void merge_func(int* a, int sa, int ea, int* b, int sb, int eb, int* c, int sc)
{
int i = sa, j = sb, k = sc;
while(i < ea && j < eb)
c[k++] = a[i] < b[j] ? a[i++] : b[j++];
while(i < ea)
c[k++] = a[i++];
while(j < eb)
c[k++] = b[j++];
}
void msort_big(int* a, int* b, int s, int e, bool inA)
{
if(e-s < 4)
{
insert_sort(a, s, e, b, s, e);
return; // We sorted (A,s,e) into (B,s,e).
}
int m = (s + e)/2;
msort_big(a, b, s, m, !inA);
msort_big(a, b, m, e, !inA);
// If we want to merge in A, do it. Otherwise, merge in B
inA ? merge_func(b, s, m, b, m, e, a, s) : merge_func(a, s, m, a, m, e, b, s);
}
void msort(int* toBeSorted, int s, int e)
// Sorts toBeSorted from [s, e+1), so just enter [s, e] and
// the call to msort_big adds one.
{
int* b = new int[e - s + 1];
msort_big(toBeSorted, b, s, e+1, true);
delete [] b;
}
template <class T>
struct SortData_Send
{
T* data;
int start;
int end;
};
DWORD WINAPI msort_para_callback(LPVOID lpParam)
{
SortData_Send<int> dat = *(SortData_Send<int>*)lpParam;
msort(dat.data, dat.start, dat.end);
cout << "done! " << endl;
}
int ceiling_func(double num)
{
int temp = (int)num;
if(num > (double)temp)
{
return temp + 1;
}
else
{
return temp;
}
}
void paraMSort(int* toBeSorted, int s, int e, int numThreads)
{
HANDLE threads[numThreads];
DWORD threadIDs[numThreads];
SortData_Send<int>* sent[numThreads];
for(int i = 0; i < numThreads; i++)
{
// So for each thread, make an interval and pass the pointer to the array of ints.
// So for numThreads = 3 and array size of 0 to 99 (100), we have 0-32, 33-65, 66-100.
// 100 because sort function takes [start, end).
sent[i] = new SortData_Send<int>;
sent[i]->data = toBeSorted;
sent[i]->start = s + ceiling_func(double(i)*(double)e/double(numThreads));
sent[i]->end = ceiling_func(double(i+1)*double(e)/double(numThreads)) + ((i == numThreads-1) ? 1 : -1);
threads[i] = CreateThread(NULL, 0, msort_para_callback, sent[i], 0, &threadIDs[i]);
}
WaitForMultipleObjects(numThreads, threads, true, INFINITE);
cout << "Done waiting!" <<endl;
}

Answer 1

Assuming 's' is your starting point and 'e' is your ending point for a thread shouldn't your code be something like
sent[i]->start = s + ceiling_func(double(i)*(double)(e-s)/double(numThreads));
sent[i]->end = (i == numThreads-1) ? e : (s - 1 + ceiling_func(double(i+1)*(double)(e-s)/double(numThreads)));
This is in case your function void paraMSort(int* toBeSorted, int s, int e, int numThreads) is being called with a value of 's' not equal to 0? This could cause you to read wrong sections of memory.