Multithreaded File I/O (Reflections on Dr. Dobb’s article by Stefan Wörthmüller)

Thoughts on the results that Stefan Wörthmüller reports in his article on Dr. Dobb’s Journal.

I ran across an interesting article on Multi-Threaded File I/O in Dr. Dobb’s today. You can read the article at

I was particularly intrigued by the statements on variability,

I repeated the entire test suite three times. The values I present here are the average of the three runs. The standard deviation in most cases did not exceed 10-20%. All tests have been also run three times with reboots after every run, so that no file was accessed from cache.

Initially, I thought 10-20% was a bit much; this seemed like a relatively straightforward test and variability should be low. Then I looked at the source code for the test and I’m now even more puzzled about the variability.

Get a copy of the sources here. It is a single source file and in the only case of randomization, it uses rand() to get a location into the file.

The code to do the random seek is below

      // Seek new position for Random access
      if(i >= maxCount)
      long pos = (rand() * fileSize) / RAND_MAX - BlockSize;
      fseek(file, pos, SEEK_SET);

While this is a multi-threaded program, I see no calls to srand() anywhere in the program. Just to be sure, I modified Stefan’s program as attached here. (My apologies, the file has an extension of .jpg because I can’t upload a .cpp or .zip onto this free wordpress blog. The file is a Windows ZIP file, just rename it).

// mtRandom.cpp   Amrith Kumar 2009 (amrith (dot) kumar (at) gmail (dot) com
// This program is adapted from the program FileReadThreads.cpp by Stefan Woerthmueller
// No rights reserved. Feel Free to do what ever you like with this code
// but don't blame me if the world comes to an end.

#include "Windows.h"
#include "stdio.h"
#include "conio.h"


// Worker Thread Function

DWORD WINAPI threadEntry(LPVOID lpThreadParameter)

    int index = (int)lpThreadParameter;
        FILE * fp;
        char filename[32];

        sprintf ( filename, "file-%d.txt", index );

        fprintf ( stderr, "Thread %d startedn", index );
        if ((fp = fopen ( filename, "w" )) == (FILE * ) NULL)
                fprintf (stderr, "Error opening file %sn", filename );
                for (int i = 0; i < 10; i ++)
                        fprintf ( fp, "%un", rand());

                fclose (fp);

        fprintf ( stderr, "Thread %d donen", index );

    return 0;

#define MAX_THREADS (5)

int main(int argc, char* argv[])

    HANDLE h_workThread[MAX_THREADS];

    for(int i = 0; i < MAX_THREADS; i++)
        h_workThread[i] = CreateThread(NULL, 0, threadEntry, (LPVOID) i, 0, NULL );

    WaitForMultipleObjects(MAX_THREADS, h_workThread, TRUE, INFINITE);
    printf ( "All done. Good byen" );
    return 0;

So, I confirmed that Stefan will be getting the same sequence of values from rand() over and over again, across reboots.

Why then is he still seeing 10-20% variability? Beats me, something smells here … I would assume that from run to run, there should be very little variability.


2 thoughts on “Multithreaded File I/O (Reflections on Dr. Dobb’s article by Stefan Wörthmüller)”

  1. You’re actually right. Calling srand() would be more accurate, as every thread should access different positions. I inserted such a statement now and fixed also a overflow when calculating the random position.

    Inserting srand interestingly mostly influences the performance for single random read. The other tests do not differ much from the results published.

    The variability in some cases was even stronger than 20%. This absolutely mirrors my experience with disk drives. Continuous as well as random access to files is interfered by a number of factors. You may encounter that yourself. If you copy a large file 10 times, you will probably encounter 1 or 2 trials that take much longer or shorter than the other runs.


    1. Thanks Stefan,

      I will try the tests out with the amended code on a larger server today (I don’t have access to RAID-5 and SCSI) but I will try with what I have.

      Yesterday, I was testing with a PC and its internal RAID0 SATA disk drive(s) and I was getting about 0.8% variability. I am using Windows XP BUT I have Write Caching disabled.

      Further, my experience (albeit not SATA) has been that over time benchmarks get grooved and response times and performance land in a very narrow band of 0.5% or less.

      At one company where I worked, we did regular benchmarks using an industry standard benchmark (TPC-H) and over a course of months, we would get test results that were predictable, like the swiss trains. And then BANG, something would change and we’d get a huge change. When changes over 2% happened, one of my colleagues would get all hot and bothered 🙂

      So, I’m still puzzled about variability like 20%




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