mysql 自动关闭 启动后一段时间又会关闭

下面是日志，谁能帮忙分析一下是什么原因，第一次相差半个月，现在频率越来越快


100422 23:27:27 [Note] MySQL: ready for connections.
Version: '5.0.18-nt'  socket: ''  port: 3306  MySQL Community Edition (GPL)
100427 11:42:50 [Note] MySQL: Normal shutdown

100427 11:42:53 [Warning] MySQL: Forcing close of thread 99065  user: 'root'

100427 11:42:53 [Warning] MySQL: Forcing close of thread 99063  user: 'root'

100427 11:42:53 [Warning] MySQL: Forcing close of thread 99057  user: 'root'

100427 11:42:53 [Warning] MySQL: Forcing close of thread 99055  user: 'root'

100427 11:42:53 [Warning] MySQL: Forcing close of thread 99054  user: 'root'

100427 11:42:53 [Warning] MySQL: Forcing close of thread 99053  user: 'root'

100427 11:42:53 [Warning] MySQL: Forcing close of thread 99048  user: 'root'

100427 11:42:53 [Warning] MySQL: Forcing close of thread 99046  user: 'root'

100427 11:42:53 [Warning] MySQL: Forcing close of thread 99045  user: 'root'

100427 11:42:53 [Note] MySQL: Shutdown complete

100427 11:44:07 [Note] MySQL: ready for connections.
Version: '5.0.18-nt'  socket: ''  port: 3306  MySQL Community Edition (GPL)
100428 17:09:23 [Note] MySQL: ready for connections.
Version: '5.0.18-nt'  socket: ''  port: 3306  MySQL Community Edition (GPL)

每次出现自动关闭都有这样的记录是什么意思
100427 11:44:07 [Note] MySQL: ready for connections.
Version: '5.0.18-nt'  socket: ''  port: 3306  MySQL Community Edition (GPL)
100428 17:09:23 [Note] MySQL: ready for connections.
Version: '5.0.18-nt'  socket: ''  port: 3306  MySQL Community Edition (GPL)

上面的是正常启动的记录。