mmap()的效率咋就这么低呢?
设计到大文件的编程,不可能一次将文件内容全部读到内存,就用了mmap()函数,将文件映射到内存,网上很多人说mmap速度要快一些,可我的测试结果是mmap的读取速度比正常的read慢的多,下面是我的测试代码:
//--------------------------------------------------------------
//mmap.c
int main(int argc, char **argv)
{
int fd = -1;
struct stat f_stat;
unsigned char *start = NULL;
unsigned char *buf = NULL;
if (argc != 2)
return -1;
if ((fd = open(argv[1], O_RDONLY)) == -1)
return -1;
if (fstat(fd, &f_stat))
return -1;
if ((start = mmap(start, f_stat.st_size, PROT_READ, MAP_PRIVATE, fd, 0)) == NULL)
return -1;
time_t time1,time2;
int len = 1024 * 1024 * 10;
buf = (unsigned char *)malloc(len);
int i;
time1 = time(NULL);
for (i = 0; i < f_stat.st_size/len; i++)
{
memcpy(buf, start + i * len, len);
}
time2 = time(NULL);
free(buf);
printf("consume time:%d\n", time2 - time1);
if (munmap(start, f_stat.st_size))
return -1;
close(fd);
}
//------------------------------------------------------------------
//fread.c
int main(int argc, char **argv)
{
int fd = -1;
struct stat f_stat;
unsigned char *start = NULL;
unsigned char *buf = NULL;
if (argc != 2)
return -1;
if ((fd = open(argv[1], O_RDONLY)) == -1)
return -1;
if (fstat(fd, &f_stat))
return -1;
time_t time1,time2;
int len = 1024 * 1024 * 10;
buf = (unsigned char *)malloc(len);
int i;
time1 = time(NULL);
for (i = 0; i < f_stat.st_size/len; i++)
{
read(fd, buf, len);
}
time2 = time(NULL);
free(buf);
printf("consume time:%d\n", time2 - time1);
close(fd);
}
//-------------------------------------------------------------
读取的文件大小为740MB,mmap用时137秒,而正常的文件读写仅用20秒,敬请高手解答!!!