c++ - Multithreaded MD5 Hashing -

i'm trying multithread program wich take word file hash these , write another.

if make without multithreading it's fast, it's able use 15-20% of cpu , has out 300.000line/s

but when tryied multithreading slow down , start hash @ 17000lines/s, can please me?

thanks

#include <iostream>     // std::cout, std::streambuf, std::streamsize #include <fstream>      // std::ifstream #include <string> #include <thread> #include "md5.h"  using namespace std;  static const int num_threads = 10;  void call_from_thread(int tid) {     cout << "launched thread " << tid << std::endl;     int cl = 0;     int uscita = 0;     int parole = 0;     char* contents;     ifstream istr("test.txt");      if (istr) {         streambuf * pbuf = istr.rdbuf();         streamsize size = pbuf->pubseekoff(0, istr.end);         pbuf->pubseekoff(0, istr.beg);       // rewind         contents = new char[size];         pbuf->sgetn(contents, size);         istr.close();          ofstream myfile;         myfile.open("out.txt");          {             string prova("");             uscita = 0;             {                 if (contents[cl] == '\n') {                     uscita = 1;                 }                 prova += contents[cl];                 cl += 1;             } while (uscita != 1);             parole += 1;             //cout << prova << ":" << md5(prova) << endl;             myfile << prova << ":" << md5(prova) << endl;         } while (parole != 9586054);          myfile.close();     } }  int main() {      thread t[num_threads];      //launch group of threads     (int = 0; < num_threads; ++i) {         t[i] = thread(call_from_thread, i);     }      cout << "launched main\n";      //join threads main thread     (int = 0; < num_threads; ++i) {         t[i].join();     }      return 0; } 

what see here threads reading same file , doing same work. results in file has read num_threads times, causing slowdown of num_threads (since file i/o on critical path) or maybe more because of more cache misses.

you try reading entire file memory, splitting between threads , having each thread process subset of files content.

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