Multithreaded storing into file - c++

Question

I have code
const int N = 100000000;
int main() {
FILE* fp = fopen("result.txt", "w");
for (int i=0; i<N; ++i) {
int res = f(i);
fprintf (fp, "%d\t%d\n", i, res);
}
return 0;
}
Here f averagely run for several milliseconds in single thread.
To make it faster I'd like to use multithreading.
What provides a way to get the next i? Or do I need to lock, get, add and unlock?
Should writing be proceeded in a separated thread to make things easier?
Do I need a temporary memory in case f(7) is worked out before f(3)?
If 3, is it likely that f(3) is not calculated for long time and the temporary memory is filled?
I'm currently using C++11, but requiring higher version of C++ may be acceptable

Answer 1

General rule how to improve performance:
Find way to measure performance (automated test)
Do profiling of existing code (find bottlenecks)
Understanding findings in point 2 and try to fix them (without mutilating)
Do a measurement from point 1. and decide if change provided expected improvement.
go back to point 2 couple times
Only if steps 1 to 5 didn't help try use muti threading. Procedure is same as in points 2 - 5, but you have to think: can you split large task to couple smaller one? If yest do they need synchronization? Can you avoid it?
Now in your example just split result to 8 (or more) separate files and merge them at the end if you have to.
This can look like this:
#include <vector>
#include <future>
#include <fstream>
std::vector<int> multi_f(int start, int stop)
{
std::vector<int> r;
r.reserve(stop - start);
for (;start < stop; ++start) r.push_back(f(start));
return r;
}
int main()
{
const int N = 100000000;
const int tasks = 100;
const int sampleCount = N / tasks;
std::vector<std::future<std::vector<int>>> allResults;
for (int i=0; i < N; i += sampleCount) {
allResults.push_back(std::async(&multi_f, i, i + sampleCount));
}
std::ofstream f{ "result.txt" }; // it is a myth that printf is faster
int i = 0;
for (auto& task : allResults)
{
for (auto r : task.get()) {
f << i++ << '\t' << r << '\n';
}
}
return 0;
}