virtual memory exhausted: Cannot allocate memory

[root@Byrd byrd]# free -m
             total       used       free     shared    buffers     cached
Mem:           512        108        403          0          0         28
-/+ buffers/cache:         79        432
Swap:            0          0          0

[root@Byrd ~]# mkdir /opt/images/

[root@Byrd ~]# rm -rf /opt/images/swap

[root@Byrd ~]# dd if=/dev/zero of=/opt/images/swap bs=1024 count=2048000
2048000+0 records in
2048000+0 records out
2097152000 bytes (2.1 GB) copied, 82.7509 s, 25.3 MB/s

[root@Byrd ~]# mkswap /opt/images/swap
mkswap: /opt/images/swap: warning: don't erase bootbits sectors
        on whole disk. Use -f to force.
Setting up swapspace version 1, size = 2047996 KiB
no label, UUID=59daeabb-d0c5-46b6-bf52-465e6b05eb0b

[root@hz mnt]# swapon /opt/images/swap

[root@hz mnt]# free -m
             total       used       free     shared    buffers     cached
Mem:           488        481          7          0          6        417
-/+ buffers/cache:         57        431
Swap:          999          0        999

内存太小，增加内存可以解决。
使用完毕后可以关掉swap：

[root@hz mnt]# swapoff swap  
[root@hz mnt]# rm -f /opt/images/swap 

关机重启需要重新运行命令