HTML5 / CSS3无类框架(HTML5/CSS3 classless framework)

 差异似乎是由于使用数组而不是std :: vector，我仍然很难相信。 我的两组代码如下所示，以供比较。 第一个可以从节目1以约2500MB / s的速率进行摄取。 第二个可以以3100MB / s的速率进行摄取。 
 
 计划1（2500 MB /秒） 
 
int main(int argc, char **argv)
{
    int fd = open("/tmp/fifo2", O_RDONLY);

    std::vector<char> buf(8*1024*1024);

    while(1)
    {
       read(fd, &buf[0], 8*1024*1024);
    }
}
 
 计划2（3100 MB /秒） 
 
int main(int argc, char **argv)
{

    int fd = open("/tmp/fifo2", O_RDONLY);

    char buf[8*1024*1024];

    while(1)
    {
       read(fd, &buf[0], 8*1024*1024);
    }
}
 
 两者都使用gcc版本4.4.6编译-O3。 如果任何人都可以解释这个原因，我会非常感兴趣（因为我理解std :: vector基本上是一个数组的包装）。 
 
 编辑 ：我刚刚测试了下面的程序3，它可以使用ifstream并以3000 MB / s运行。 因此，使用ifstream而不是'read（）'会导致性能下降很小。 比使用std :: vector的命中少得多。 
 
 计划3（3000 MB / s） 
 
int main(int argc, char **argv)
{
    ifstream file("/tmp/fifo2", ios::in | ios::binary);

    char buf[8*1024*1024];

    while(1)
    {
       file.read(&buf[0], 32*1024);
    }
}
 
 编辑2： 
 
 我修改了程序2的代码，以便在堆栈中使用malloc'd内存而不是内存，并且性能下降以匹配向量性能。 谢谢，ipc，为我键入了这个。 

The difference appears to be due to using an array instead of std::vector, which I still have a hard time believing. My two sets of code are shown below for comparison. The first can ingest from Program 1 at a rate of about 2500 MB/s. The second can ingest at a rate of 3100 MB/s.
 
Program 1 (2500 MB/s)
 
int main(int argc, char **argv)
{
    int fd = open("/tmp/fifo2", O_RDONLY);

    std::vector<char> buf(8*1024*1024);

    while(1)
    {
       read(fd, &buf[0], 8*1024*1024);
    }
}
 
Program 2 (3100 MB/s)
 
int main(int argc, char **argv)
{

    int fd = open("/tmp/fifo2", O_RDONLY);

    char buf[8*1024*1024];

    while(1)
    {
       read(fd, &buf[0], 8*1024*1024);
    }
}
 
Both are compiled with -O3 using gcc version 4.4.6. If anyone can explain the reason for this I'd be very interested (since I understand std::vector to basically be a wrapper around an array).
 
Edit: I just tested Program 3, below, that can uses ifstream and runs at 3000 MB/s. So it appears that using ifstream instead of 'read()' incurs a very slight performance degradation. Much less than the hit taken from using std::vector.
 
Program 3 (3000 MB/s)
 
int main(int argc, char **argv)
{
    ifstream file("/tmp/fifo2", ios::in | ios::binary);

    char buf[8*1024*1024];

    while(1)
    {
       file.read(&buf[0], 32*1024);
    }
}
 
Edit 2:
 
I modded Program 2's code to use malloc'd memory instead of memory on the stack and the performance dropped to match the vector performance. Thanks, ipc, for keying me onto this.