A little benchmark to show the low performance of the std.stdio.File().writeln() with numbers. The programs accept N from the command line, and just save a textual file (opened as binary file) that contains numbers from 0 to N-1. 

The write1 program is the basic one that uses File.writeln(). The successive programs are for comparison.

The write2 program is equally basic, but it uses C functions, using the same D compiler and compilation arguments, and it's almost 12 times faster.

The write3 program is a basic Java program, it's more than 17 times faster than write1.

The write4 is a test to write a very fast D program, it's more than 30 times faster than write2 (it's not the fastest possible program because the DMD compiler is currently not able to perform the small dividend optimization. GCC and Clang are able to replace the small dividend with some faster operations).


DMD 2.060alpha, Windows, -O -release -inline
Java 1.7.0_03-b05

Timings, N = 2_000_000, best of 3, seconds:
  write1: 9.67
  write2: 0.83 
  write3: 0.56 (Java)
  write4: 0.32

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// write1
import std.stdio, std.conv;

void main(string[] args) {
    auto f = File("numbers.txt", "wb");
    foreach (i; 0 .. args[1].to!int())
        f.writeln(i);
    f.close();
}

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// write2
import core.stdc.stdio, std.conv;

void main(string[] args) {
    auto f = fopen("numbers.txt", "wb");
    foreach (i; 0 .. args[1].to!int())
        fprintf(f, "%d\n", i);
    fclose(f);
}

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// write3
import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;

class write3 {
    public static void main(String args[]) {
        if (args.length != 1)
            System.exit(-1);
        int count = Integer.parseInt(args[0]);

        try {
            BufferedWriter bw = new BufferedWriter(new FileWriter("numbers.txt"));

            for (int i = 0; i < count; i++) {
                bw.write(i);
                bw.write("\n");
            }

            bw.close();
        } catch (IOException e) {
            System.exit(-2);
        }
    }
}

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// write4
import core.stdc.stdio, std.conv, std.typetuple;

void saveNumbers(uint buf_size)(FILE* f, in uint n, in uint start) nothrow {
    __gshared static char[buf_size] buffer = void;
    __gshared static immutable string digits = "0123456789";
    char* ptr = buffer.ptr;

    foreach (k; start .. start + n) {
        uint v = k;
        if (v >= 0 && v < 10) {
            *ptr = digits[v];
            ptr++;
        } else {
            if (v < 0) {
                v = -v;
                *ptr = '-';
                ptr++;
            }

            char* start_ptr = ptr;

            foreach (_; TypeTuple!(0,1,2,3,4,5,6,7,8,9)) {
                // DMD doesn't know how to perform a fast division
                // or modulus when the quotient is a small integer
                *ptr = (v % 10) + '0';
                v /= 10;
                ptr++;
                if (!v) break;
            }

            immutable size_t i = ptr - start_ptr;
            for (size_t j = 0; j < i / 2; ++j) {
                immutable aux = start_ptr[j];
                start_ptr[j] = start_ptr[i - j - 1];
                start_ptr[i - j - 1] = aux;
            }
        }
        *ptr = '\n';
        ptr++;
    }

    *ptr = '\0';
    fputs(buffer.ptr, f);
}

void main(in string[] args) {
    auto f = fopen("numbers.txt", "wb");
    if (args.length != 2 || f == null) return;
    immutable uint N = to!uint(args[1]);

    enum uint max_int_len = 12;
    enum uint buf_size = 1_000_000;
    enum uint M = buf_size / max_int_len;

    uint count;
    foreach (_0; 0 .. N / M) {
        saveNumbers!buf_size(f, M, count);
        count += M;
    }
    saveNumbers!buf_size(f, N % M, count);

    fclose(f);
}

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