mmap的性能

　　mmap操作提供了一种机制，让用户程序直接访问设备内存，这种机制，相比较在用户空间和内核空间互相拷贝数据，效率更高。在要求高性能的应用中比较常用。mmap映射内存必须是页面大小的整数倍，面向流的设备不能进行mmap，mmap的实现和硬件有关。（百度百科）

      下面的实例来自网络，简单修改了一下，比较了传统的OPEN/WRITE与mmap的性能，网上有很多原理分析，就不赘述了。 

 1 #include<unistd.h> 
 2 #include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<sys/types.h>
#include<sys/stat.h>
#include<sys/time.h>
#include<fcntl.h>
#include<sys/mman.h>

#define MAX 1000

int main()
{
    int i=0;
    int count=0, fd=0;
    struct timeval tv1, tv2;
    int *array = (int *)calloc( MAX,sizeof(int));
    int *pmmap =NULL;
    /*create*/

    fd = open( "mmap_test", O_CREAT );
    write( fd, (void *)array, sizeof(int)*MAX );
    close (fd);
    /*mmap*/
    gettimeofday( &tv1, NULL );//获得微秒级时间
    fd = open( "mmap_test", O_RDWR );
    pmmap = mmap( NULL, sizeof(int)*MAX, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0 );
    munmap( pmmap, sizeof(int)*MAX );31     close( fd );
    gettimeofday( &tv2, NULL );
    printf( "Time of mmap: %dms
", tv2.tv_usec-tv1.tv_usec );
    
    /*read*/
    gettimeofday( &tv1, NULL );
    fd = open( "mmap_test", O_RDWR);
    if( sizeof(int)*MAX != read( fd, (void *)array, sizeof(int)*MAX ) )
    {
        printf( "Reading data failed...
" );
        return -1;
    }

    if( sizeof(int)*MAX != write( fd, (void *)array, sizeof(int)*MAX ) )
    {
        printf( "Writing data failed...
" );
        return -1;
    }
    free( array );
    close( fd );
    gettimeofday( &tv2, NULL );
    printf( "Time of read/write: %dms
", tv2.tv_usec-tv1.tv_usec );

    
    return 0;
}