When a new thread connects to mysqld, mysqld will spawn a new thread to
handle the request. This thread will first check if the hostname is in
the hostname cache. If not the thread will call gethostbyaddr_r() and
gethostbyname_r() to resolve the hostname.
If the operating system doesn't support the above thread-safe calls, the
thread will lock a mutex and call gethostbyaddr() and gethostbyname()
instead. Note that in this case no other thread can resolve other
hostnames that is not in the hostname cache until the first thread is
ready.
You can disable DNS host lookup by starting mysqld with
--skip-name-resolve. In this case you can however only use IP names in
the MySQL privilege tables.
If you have a very slow DNS and many hosts, you can get more performance
by either disabling DNS lookop with --skip-name-resolve or by
increasing the HOST_CACHE_SIZE define (default: 128) and recompile
mysqld.
You can disable the hostname cache with --skip-host-cache. You can clear
the hostname cache with FLUSH HOSTS or mysqladmin flush-hosts.
If you don't want to allow connections over TCP/IP, you can do this by starting mysqld with --skip-networking.
大致翻譯如下：
當一個新連接連接mysql服務器時，mysql服務器會對此次連接的合法性進行判定，具體通過查詢mysql.user表實現。mysql的權限設置將user和host（客戶端的地址）聯繫起來，只有當兩者都符合條件時才能進行下一步認證。
當客戶端連接的時候，客戶端的地址假如不在mysql.host表中時，mysql服務器會調用gethostbyaddr和gethostbyname名字進行解析（同步方法），或者gethostbyaddr_r和gethostbyname_r（異步）來解析客戶端地址，這樣會導致效率下降。
因此建議安裝完畢之後從my.cnf中刪除skip-name-resolve，然後在調用grant命令時全部寫成ip地址。