关于ipp在linux下通过cpp程序测试

Question

您好我写了一个简单的代码来读取大型（10GB）二进制文件到内存并测量kvm的访客virtual machine中的运行时间。

大文件由fallocate -l 10GB test.bin

创建

四个结果

案例1.第一次或[shutdown->boot]之后：17s

案例2.第二次：4s

案例3.清除缓存缓冲区后[通过运行sync && echo 3 > /proc/sys/vm/drop_caches或rm file and recreate file]：6s

案例4.重启后：13s

我认为案例1,3,4应该得到相同的结果，因为缓冲区缓存已被清除，但为什么它们现在得到不同的结果？

这是代码段

int test()
{
    int ret = 0;
    streampos size;
    char* buffer;

    //define file pointer pointed to a large_file
    ifstream large_file("test.bin", ios::in|ios::binary|ios::ate);

    if(large_file.is_open()){

        size = large_file.tellg();//get the size of large_file
        buffer = new char[size];//allocate memory
        large_file.seekg(0, ios::beg);//set get position to the begin of the file

        start = wtime();
        large_file.read(buffer, size);
        large_file.close();
        end = wtime();
        printf("File size = %ldGB Time = %.5f\n", size/1024/1024/1024, end-start);

        delete buffer;
        return ret;
    }
    else printf("failed to open the file\n");

    ret = 1;

    return ret;
}