Speed-up a single task using multi-threading in c++

Question

I'm sorry if this is a repeat question but I already tried to search for an answer and came up empty handed. I have a code to transfer data (189156 numbers) from txt file (input.txt) to another file (test.txt), after executing the program, the process takes about 23 seconds to transfer all data from the file: input.txt to the file : test.txt. I wanted to speed up the process, so I divided the process into multiple threads (4 threads), each thread process 1/4 of the data, After executing the program, there was no difference in the time it took to transfer all the data. here is my codes:

// This program reads data from a file into an array.

#include <iostream>
#include <fstream> // To use ifstream
#include <vector>
#include <thread>
using namespace std;

void test(int start, int end)
{
    std::vector<int> numbers;
    ifstream inputFile("input.txt");        // Input file stream object

    // Check if exists and then open the file.
    if (inputFile.good()) {
        // Push items into a vector
        int current_number = 0;
        while (inputFile >> current_number) {
            numbers.push_back(current_number);

        }

        // Close the file.
        inputFile.close();

        // Display the numbers read:
        cout << "The numbers are: ";
        for (int count = start ; count < end; count++) {
           cout << numbers[count] << " " ;


            std::ofstream ofs;
            ofs.open("test.txt", std::ofstream::out | std::ofstream::app);

            ofs << numbers[count] << endl;

            ofs.close();

        }

        cout << endl;
    }
    else {
        cout << "Error!";
        _exit(0);
    }

  
}


int main() {

    std::thread worker1(test, 0, 50000); 
    std::thread worker2(test, 50000, 100000);
    std::thread worker3(test, 100000, 150000);
    std::thread worker4(test, 150000, 189156);
    worker1.join();
    worker2.join();
    worker3.join();
    worker4.join();
   


    return 0;
}

I am a beginner, I do not know if it is correct to use multi-threads in such a case, please, if so, where is my mistake and if not, what is the correct way to speed up the process.

Answer 1

There is a big race condition in the code that not only prevent the code to be fast, but also should produce wrong results (possibly non-deterministically). Indeed, all threads can write in the same file "test.txt" simultaneously. While this operation may be thread safe on the target system, the order in which the threads append data in the target file is undefined and thus the result can be shuffled. The file appending have to be serialized and this when this processes is thread safe, it is typically protected with a lock that prevent any parallel execution.

Additionally, the open+write+close should be extremely slow since it results in 3 system calls per line and system calls are generally slow, especially IO ones.

That being said, you cannot use one ofstream object with multiple thread without protection since it would cause a bigger undefined behaviour. Indeed, here is what the C++ standard explicitly states:

Concurrent access to a stream object [string.streams, file.streams], stream buffer object [stream.buffers], or C Library stream [c.files] by multiple threads may result in a data race [intro.multithread] unless otherwise specified [iostream.objects]. [Note: Data races result in undefined behavior [intro.multithread]. --end note]

An efficient solution is to do a inner-thread reduction: all threads append data to a thread-local ostringstream so to perform the integer to string serialization in a big buffer and then write data in a serialized way (so for the order to be the same than the sequential program). The serialization should be speed up by the use of multiple thread while the IO part will still be sequential. In practice, the serialization should be pretty slow so the use of multiple thread should help to significantly reduce the execution time.

There is another big issue: the input file is entirely read by each thread! This means the 4 threads overall compute 4 time more work than using just 1 thread. This completely defeat the benefit of using multiple threads. You need to split the input file in relatively equal parts and then perform the computation. This is not so easy since the line delimiter should be taken into account.

One solution to this problem is to first retrieve the size of the file and then divide the 0..size range in N parts, where N is the number of workers. The split ranges then need to be corrected so to reference the begining of a line. You can do this correction by reading a line in the file at the starting location of each range and then adapt the start/end location of each range consequently (you just need to add the size of the line read). Once corrected, each worker can operate on a completely independent part of the file and read it in parallel (using a different ifstream object like you did).