Performance of Computing CRC

Question

#include "windows.h"

#include <string>
#include <iostream>

unsigned long   crc32_table[] = {
    0x00000000, 0x77073096, 0xEE0E612C, 0x990951BA, 0x076DC419, 
    0x706AF48F, 0xE963A535, 0x9E6495A3, 0x0EDB8832, 0x79DCB8A4, 
    0xE0D5E91E, 0x97D2D988, 0x09B64C2B, 0x7EB17CBD, 0xE7B82D07, 
    0x90BF1D91, 0x1DB71064, 0x6AB020F2, 0xF3B97148, 0x84BE41DE, 
    0x1ADAD47D, 0x6DDDE4EB, 0xF4D4B551, 0x83D385C7, 0x136C9856, 
    0x646BA8C0, 0xFD62F97A, 0x8A65C9EC, 0x14015C4F, 0x63066CD9, 
    0xFA0F3D63, 0x8D080DF5, 0x3B6E20C8, 0x4C69105E, 0xD56041E4, 
    0xA2677172, 0x3C03E4D1, 0x4B04D447, 0xD20D85FD, 0xA50AB56B, 
    0x35B5A8FA, 0x42B2986C, 0xDBBBC9D6, 0xACBCF940, 0x32D86CE3, 
    0x45DF5C75, 0xDCD60DCF, 0xABD13D59, 0x26D930AC, 0x51DE003A, 
    0xC8D75180, 0xBFD06116, 0x21B4F4B5, 0x56B3C423, 0xCFBA9599, 
    0xB8BDA50F, 0x2802B89E, 0x5F058808, 0xC60CD9B2, 0xB10BE924, 
    0x2F6F7C87, 0x58684C11, 0xC1611DAB, 0xB6662D3D, 0x76DC4190, 
    0x01DB7106, 0x98D220BC, 0xEFD5102A, 0x71B18589, 0x06B6B51F, 
    0x9FBFE4A5, 0xE8B8D433, 0x7807C9A2, 0x0F00F934, 0x9609A88E, 
    0xE10E9818, 0x7F6A0DBB, 0x086D3D2D, 0x91646C97, 0xE6635C01, 
    0x6B6B51F4, 0x1C6C6162, 0x856530D8, 0xF262004E, 0x6C0695ED, 
    0x1B01A57B, 0x8208F4C1, 0xF50FC457, 0x65B0D9C6, 0x12B7E950, 
    0x8BBEB8EA, 0xFCB9887C, 0x62DD1DDF, 0x15DA2D49, 0x8CD37CF3, 
    0xFBD44C65, 0x4DB26158, 0x3AB551CE, 0xA3BC0074, 0xD4BB30E2, 
    0x4ADFA541, 0x3DD895D7, 0xA4D1C46D, 0xD3D6F4FB, 0x4369E96A, 
    0x346ED9FC, 0xAD678846, 0xDA60B8D0, 0x44042D73, 0x33031DE5, 
    0xAA0A4C5F, 0xDD0D7CC9, 0x5005713C, 0x270241AA, 0xBE0B1010, 
    0xC90C2086, 0x5768B525, 0x206F85B3, 0xB966D409, 0xCE61E49F, 
    0x5EDEF90E, 0x29D9C998, 0xB0D09822, 0xC7D7A8B4, 0x59B33D17, 
    0x2EB40D81, 0xB7BD5C3B, 0xC0BA6CAD, 0xEDB88320, 0x9ABFB3B6, 
    0x03B6E20C, 0x74B1D29A, 0xEAD54739, 0x9DD277AF, 0x04DB2615, 
    0x73DC1683, 0xE3630B12, 0x94643B84, 0x0D6D6A3E, 0x7A6A5AA8, 
    0xE40ECF0B, 0x9309FF9D, 0x0A00AE27, 0x7D079EB1, 0xF00F9344, 
    0x8708A3D2, 0x1E01F268, 0x6906C2FE, 0xF762575D, 0x806567CB, 
    0x196C3671, 0x6E6B06E7, 0xFED41B76, 0x89D32BE0, 0x10DA7A5A, 
    0x67DD4ACC, 0xF9B9DF6F, 0x8EBEEFF9, 0x17B7BE43, 0x60B08ED5, 
    0xD6D6A3E8, 0xA1D1937E, 0x38D8C2C4, 0x4FDFF252, 0xD1BB67F1, 
    0xA6BC5767, 0x3FB506DD, 0x48B2364B, 0xD80D2BDA, 0xAF0A1B4C, 
    0x36034AF6, 0x41047A60, 0xDF60EFC3, 0xA867DF55, 0x316E8EEF, 
    0x4669BE79, 0xCB61B38C, 0xBC66831A, 0x256FD2A0, 0x5268E236, 
    0xCC0C7795, 0xBB0B4703, 0x220216B9, 0x5505262F, 0xC5BA3BBE, 
    0xB2BD0B28, 0x2BB45A92, 0x5CB36A04, 0xC2D7FFA7, 0xB5D0CF31, 
    0x2CD99E8B, 0x5BDEAE1D, 0x9B64C2B0, 0xEC63F226, 0x756AA39C, 
    0x026D930A, 0x9C0906A9, 0xEB0E363F, 0x72076785, 0x05005713, 
    0x95BF4A82, 0xE2B87A14, 0x7BB12BAE, 0x0CB61B38, 0x92D28E9B, 
    0xE5D5BE0D, 0x7CDCEFB7, 0x0BDBDF21, 0x86D3D2D4, 0xF1D4E242, 
    0x68DDB3F8, 0x1FDA836E, 0x81BE16CD, 0xF6B9265B, 0x6FB077E1, 
    0x18B74777, 0x88085AE6, 0xFF0F6A70, 0x66063BCA, 0x11010B5C, 
    0x8F659EFF, 0xF862AE69, 0x616BFFD3, 0x166CCF45, 0xA00AE278, 
    0xD70DD2EE, 0x4E048354, 0x3903B3C2, 0xA7672661, 0xD06016F7, 
    0x4969474D, 0x3E6E77DB, 0xAED16A4A, 0xD9D65ADC, 0x40DF0B66, 
    0x37D83BF0, 0xA9BCAE53, 0xDEBB9EC5, 0x47B2CF7F, 0x30B5FFE9, 
    0xBDBDF21C, 0xCABAC28A, 0x53B39330, 0x24B4A3A6, 0xBAD03605, 
    0xCDD70693, 0x54DE5729, 0x23D967BF, 0xB3667A2E, 0xC4614AB8, 
    0x5D681B02, 0x2A6F2B94, 0xB40BBE37, 0xC30C8EA1, 0x5A05DF1B, 
    0x2D02EF8D
};

unsigned long   repeat = 0;
unsigned long GetCRC(const std::string& strFilepath)
{
    unsigned long crc = 0;
    FILE*   fp = NULL;

    if (fopen_s(&fp, strFilepath.c_str(), "rb") == 0 && fp != NULL)
    { 
     crc ^= 0xFFFFFFFF;

     char c[ 655360 ];
     int size;
     while ( size = fread(&c, 1, sizeof(c), fp) )
     {
      ++repeat;
      std::cout << repeat << std::endl;
      for ( int i = 0; i < size; ++i )
      {
       crc = (crc >> 8) ^ crc32_table[(crc ^ c[i]) & 0xFF];
      }
     }

     crc ^= 0xFFFFFFFF;

     fclose(fp);
    }

    return crc;
}

#include <mmsystem.h>

int _tmain(int argc, _TCHAR* argv[])
{
    DWORD t0, t1;
    unsigned long crc;

    t0 = timeGetTime();
    crc = GetCRC(argv[1]);
    t1 = timeGetTime();
    printf( "CRC : %X, time : %f sec\n", crc, (t1-t0)/1000.0f );

    return 0;
}

I'm getting CRC value using above code. My problem is that its difference is too big. If I test 1GB file, first time, it took about 90 seconds and second time, it took about 3 seconds. I think there may be something like cache. At first time, hard-disk-read-LED was ON and at second time, the LED was not ON. After some seconds later, it took about 90 seconds again.

How this can be happen such a wide range of difference?

I'm using Windows XP Professional with Service Pack 3 and Core2(2.6GHz) with 2GB of RAM. Before I test, I terminated most of my application except some file explorer and Putty(terminal client). CPU usage was almost zero and PF usage was about 1GB.

Answer 1

That's because the OS (and maybe even the drive) caches the file. Upon first access it's read into memory, upon further accesses it's served from memory. You should never include the first access to a file into measurement, except when you want to measure disk access.

BTW, this also means that your program spends the overwhelming amount of its time waiting for IO. So there's hardly any reason to further optimize it - except if you find a way to optimize IO.