CRC计算性能

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;
}

我使用上面的代码获取 CRC 值。我的问题是它的差异太大了。如果我测试 1GB 文件，第一次大约需要90 秒，第二次大约需要3 秒。我认为可能有缓存之类的东西。第一次，硬盘读取 LED 亮起，第二次，LED 不亮起。几秒钟后，又过了大约90秒。

怎么会出现如此大范围的差异呢？

我使用的是带有 Service Pack 3 的 Windows XP Professional 和带有 2GB RAM 的 Core2(2.6GHz)。在测试之前，我终止了大部分应用程序，除了一些文件浏览器和 Putty（终端客户端）。 CPU 使用率几乎为零，PF 使用率约为 1GB。

Answer 1

这是因为操作系统（甚至可能是驱动器）缓存该文件。第一次访问时，它被读入内存，进一步访问时，它会从内存中提供。永远不应该将对文件的第一次访问纳入测量中，除非您想要测量磁盘访问。

顺便说一句，这也意味着您的程序花费了大量时间等待 IO。所以几乎没有任何理由进一步优化它——除非你找到了优化 IO 的方法。