try opening the file with O_DIRECT in the flags.
O_DIRECT allows reads and writes to avoid the linux fs cache

举例;

#include <stdio.h>
#include <assert.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

unsigned long long get_sys_time();
int main(int argc,char*argv[])
{
        int i;
        int nRet=0;
        if(argc!=3)
        {
        printf("Usage:pro file size\n");
                return 0;
        }
        unsigned long long llBeg =0;
        unsigned long long llEnd =0;
        int fp;
        int nLen = atoi(argv[2]);

        char *pBuf =  (char*)calloc(1,nLen);
        llBeg = get_sys_time();
        fp = open(argv[1],O_RDONLY,O_DIRECT);
        assert(fp>0);
        memset(pBuf,0,nLen);
        nRet = read(fp,pBuf,nLen);
        close(fp);
        llEnd = get_sys_time();
free(pBuf);
printf("readed=%d\n",nRet);
printf("elapsed time:=%d\n",(llEnd-llBeg)/1000);
return 0;

}