why parallel_for use time more than serial use time

#include <windows.h>
#include <ppl.h>
#include <vector>
using namespace concurrency;
using namespace std;

void DumpDataToFile(LPCTSTR lpszFile, LPVOID lpData, LONGLONG llDataLen, BOOL bAppend = FALSE);
static void calc(const double b[120], const double x[163970], double y[163970])
{
	int k, j;
	memset(&y[0], 0, 163970U * sizeof(double));
	for (k = 0; k < 120; k++) {
		for (j = k; j + 1 < 163971; j++) {
			y[j] += b[k] * x[j - k];
		}
	}
}

static void calc_ex(const double b[120], const double x[163970], double y[163970])
{
	int k, j;
	memset(&y[0], 0, 163970U * sizeof(double));
	for (k = 0; k < 120; k++) {
		parallel_for(int(k), 163970, [&](int j) {
			y[j] += b[k] * x[j - k];
		});
	}
}

template <class Function>
__int64 time_call(Function&& f)
{
	__int64 begin = GetTickCount();
	f();
	return GetTickCount() - begin;
}


int _tmain(int argc, _TCHAR* argv[])
{
	double *pb = new double[120];
	double *px = new double[163970];
	double *py = new double[163970];

	auto InitData = [&]() {
		for (int i = 0; i < 120; ++i) {
			pb[i] = i;
		}

		for (int i = 0; i < 163970; ++i) {
			px[i] = i;
		}
	};

	
	InitData();
	printf("serial time:%d\n", time_call([&](){		
		calc(pb, px, py);
	}));
	
	DumpDataToFile(_T("E:\\1.dat"), py, 163970 * sizeof(double));
	
	InitData();
	printf("parallel time:%d\n", time_call([&](){
		calc_ex(pb, px, py);
	}));
	DumpDataToFile(_T("E:\\2.dat"), py, 163970 * sizeof(double));

	delete[]pb;
	delete[]px;
	delete[]py;

	system("pause>nul");

	return 0;
}

void DumpDataToFile(LPCTSTR lpszFile, LPVOID lpData, LONGLONG llDataLen, BOOL bAppend)
{
	HANDLE hFile = CreateFile(lpszFile, GENERIC_WRITE, FILE_SHARE_READ, NULL, bAppend ? OPEN_ALWAYS : CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);
	if (hFile == INVALID_HANDLE_VALUE) return;

	DWORD		dwWriteLen = 2 * 1024 * 1024;
	DWORD		dwWritten = 0;
	LONGLONG	llOffset = 0;

	if (llDataLen < (LONGLONG)dwWriteLen) {
		dwWriteLen = (DWORD)llDataLen;
	}

	if (bAppend) {
		SetFilePointer(hFile, 0, 0, FILE_END);
	}

	while (llDataLen>0)
	{
		if (!WriteFile(hFile, (byte*)lpData + llOffset, dwWriteLen, &dwWritten, NULL)) {
			printf("%d\n", GetLastError());
		}
		llOffset += dwWritten;
		llDataLen -= dwWritten;

		if (llDataLen < (LONGLONG)dwWriteLen) {
			dwWriteLen = (DWORD)llDataLen;
		}
	}

	CloseHandle(hFile);
}