how to use c++ window thread - c++

Question

i would like to encrypt the file with multiple threads in order to reduce the time taken. im running on intel i5 processor, 4 GB memory, visual c++ 2008. the problem is when i run below code in debug mode (visual c++ 2008), the time taken is longer, example if i use one thread to encrypt 3 mb file, time taken is 5 seconds but when i use two threads, time taken is 10 seconds. The time is supposed to be short when using 2 threads in debug mode. but in release mode, there is no problem, time taken is short using multiple threads.
is it possible to run the code in debug mode with shorter time taken? is there setting to change in visual c++ 2008?
void load()
{
ifstream readF ("3mb.txt");
string output; string out;
if(readF.is_open())
{
while(!readF.eof())
{
getline(readF,out);
output=output+'\n'+out;
}
readF.close();
//cout<<output<<endl;
//cout<<output.size()<<endl;
text[0]=output;
}
else
cout<<"couldnt open file!"<<endl;
}
unsigned Counter;
unsigned __stdcall SecondThreadFunc( void* pArguments )
{
cout<<"encrypting..."<<endl;
Enc(text[0]);
_endthreadex( 0 );
return 0;
}
unsigned __stdcall SecondThreadFunc2( void* pArguments )
{
cout<<"encrypting..."<<endl;
//Enc(text[0]);
_endthreadex( 0 );
return 0;
}
int main()
{
load();
HANDLE hThread[10];
unsigned threadID;
time_t start, end;
start =time(0);
hThread[0] = (HANDLE)_beginthreadex( NULL, 0, &SecondThreadFunc, NULL, 0, &threadID);
hThread[1] = (HANDLE)_beginthreadex( NULL, 0, &SecondThreadFunc2, NULL, 0, &threadID );
WaitForSingleObject( hThread[0], INFINITE );
WaitForSingleObject( hThread[1], INFINITE );
CloseHandle( hThread[0] );
end=time(0);
cout<<"Time taken : "<<difftime(end, start) << "second(s)" << endl;
system("pause");
}

Answer 1

A potential reason it may be slower is that multiple threads will need to load data from memory into the cpu cache. In debug mode, there may be extra padding around data structures etc which is intended to catch buffer overflows. That might mean when the cpu switches from one thread to the other, it needs to flush out the cache and reload all data from ram. But, in release mode where there is no padding, enough data for both thread does fit into the cache, so it will run quicker.
You will find even in release mode if you add more threads you will reach a point where adding more threads gives diminishing returns and then actually starts to go slower than less threads.