Question

Currently I’m working on a small program that reads large files and sorts them. After some benchmarking I stumbled upon a weird performance issue. When the input file got to large the writing of the output file took longer than the actual sorting. So I went deeper into the code and finally realized that the fputs-function might be the problem. So I wrote this little benchmarking programm.

#include "stdio.h"
#include "ctime"

int main()
{
    int i;
    const int linecount = 50000000;
    //Test Line with 184 byte
    const char* dummyline = "THIS IS A LONG TEST LINE JUST TO SHOW THAT THE WRITER IS GUILTY OF GETTING SLOW AFTER A CERTAIN AMOUNT OF DATA THAT HAS BEEN WRITTEN. hkgjhkdsfjhgk jhksjdhfkjh skdjfhk jshdkfjhksjdhf\r\n";
    clock_t start = clock();
    clock_t last = start;

    FILE* fp1 = fopen("D:\\largeTestFile.txt", "w");
    for(i=0; i<linecount; i++){
        fputs(dummyline, fp1);
        if(i%100000==0){
            printf("%i Lines written.\r", i);
            if(i%1000000 == 0){
                clock_t ms = clock()-last;
                printf("Writting of %i Lines took %i ms\n", i, ms);
                last = clock();
            }
        }
    }
    printf("%i Lines written.\n", i);
    fclose(fp1);
    clock_t ms = clock()-start;
    printf("Writting of %i Lines took %i ms\n", i, ms);

}

When you execute the programm, you can see a clear drop of performance after about 14 to 15 mio lines which is about 2.5GB of data. The writing takes about 3 times as long as before. The threshold of 2GB indicate a 64bit issue, but I haven’t found anything about that in the web. I also tested if there is a difference between binary and character-mode (e.g. “wb” and “w”), but there is none. I also tried to preallocate the filesize (to avoid file fragmentation) by seeking to the expected end and writing a zerobyte, but that had also little to no effect.

I’m running a Windows 7 64bit machine but I’ve tested it on a Windows Server 2008 64bit R1 machine as well. Currently I’m testing on a NTFS filesystem with more than 200GB of free space. My system has 16GB of RAM so that shouldn’t be a problem either. The testprogram only uses about 700KB. The page faults, which I suspected earlier, are also very low (~400 page faults during whole runtime).

I know that for such large data the fwrite()-function would suite the task better, but at the moment I’m interested if there is another workaround and why this is happening. Any help would be highly appreciated.

Answer 1

The main reason for all this is a Windows disk cache. Then your program eats all RAM for it, then swapping begins, and thus, slowdowns. To fight these you need to:

1) Open file in commit mode using c flag:

FILE* fp1 = fopen("D:\\largeTestFile.txt", "wc");

2) Periodically write buffer to disk using flush function:

if(i%1000000 == 0)
{
    // write content to disk
    fflush(fp1);

    clock_t ms = clock()-last;
    printf("Writting of %i Lines took %i ms\n", i, ms);
    last = clock();
}

This way you will use reasonable amount of disk cache. Speed will be basically limited by the speed of your hard drive.