1、# 创建两个优先级不同的swap文件，swapfile2优先级较低
dd if=/dev/zero of=./swapfile bs=1024 count=1048576
dd if=/dev/zero of=./swapfile2 bs=1024 count=1048576
mkswap swapfile 
mkswap swapfile2
swapon swapfile
swapon swapfile2

2、编译如下测试代码进行测试

#include <sys/mman.h>
#include <stdio.h>
#include <memory.h>
#define MEMORY_LEN 500*1024*1024
int main()
{
        char *addr;
        int map_flags;
        int madv_flags;
        map_flags = MAP_PRIVATE | MAP_ANONYMOUS;
        madv_flags = MADV_PAGEOUT;
        addr = (char *)mmap(NULL, MEMORY_LEN, PROT_READ | PROT_WRITE, map_flags, -1, 0);
        if (addr == MAP_FAILED) {
            perror("map mem");
            addr = NULL;
            return -1;
        }
        memset(addr, 'a', MEMORY_LEN);
        getchar();//此时设置测试进程的preferred_swap
        madvise(addr, MEMORY_LEN, madv_flags);
        getchar();//此时验证swap使用情况
        munmap(addr, MEMORY_LEN);
        return 0;
}

3、测试步骤如下：
[root@localhost home]# swapon -s
Filename                                Type            Size            Used            Priority
/dev/dm-1                               partition       4153340         1708            -2
/home/swapfile                          file            1048572         0               -3
/home/swapfile2                         file            1048572         0               -4
[root@localhost home]# ./a.out
^Z
[1]+  Stopped                 ./a.out
[root@localhost home]# echo `pidof a.out`
1930248
[root@localhost home]# echo /home/swapfile2 > /proc/1930248/preferred_swap
[root@localhost home]# cat /proc/1930248/preferred_swap
/home/swapfile2
[root@localhost home]# fg
./a.out

^Z
[1]+  Stopped                 ./a.out
[root@localhost home]# swapon -s
Filename                                Type            Size            Used            Priority
/dev/dm-1                               partition       4153340         1964            -2
/home/swapfile                          file            1048572         0               -3
/home/swapfile2                         file            1048572         512000          -4
[root@localhost home]#