通过free -m 查看到 buff/cache的值比较大，导致可使用的内存有120M左右了

通过下面的命令，清除缓存

echo 1 > /proc/sys/vm/drop_caches
echo 2 > /proc/sys/vm/drop_caches
echo 3 > /proc/sys/vm/drop_caches

whereis a2ensite
a2ensite: /usr/sbin/a2ensite /usr/share/man/man8/a2ensite.8.gz
ls -l /usr/sbin |grep a2
lrwxrwxrwx 1 root root        7 Jul 15  2016 a2disconf -> a2enmod
lrwxrwxrwx 1 root root        7 Jul 15  2016 a2dismod -> a2enmod
lrwxrwxrwx 1 root root        7 Jul 15  2016 a2dissite -> a2enmod
lrwxrwxrwx 1 root root        7 Jul 15  2016 a2enconf -> a2enmod
-rwxr-xr-x 1 root root    15424 Apr  6  2016 a2enmod
lrwxrwxrwx 1 root root        7 Jul 15  2016 a2ensite -> a2enmod
-rwxr-xr-x 1 root root     9870 Jul 15  2016 a2query