linux 用户空间watchdog

在用户空间，还有一个叫做 watchdog 的守护进程，它可以定期对系统进行检测，包括：

* Is the process table full?
* Is there enough free memory?
* Are some files accessible?
* Have some files changed within a given interval?
* Is the average work load too high?
* Has a file table overflow occurred?
* Is a process still running? The process is specified by a pid file.
* Do some IP addresses answer to ping?
* Do network interfaces receive traffic?
* Is the temperature too high? (Temperature data not always available.)
* Execute a user defined command to do arbitrary tests.

如果某项检测失败，则可能导致一次　soft reboot （模拟一次　shutdown　命令的执行），它还可以通过 /dev/watchdog 来触发内核 watchdog 的运行。

内核级”watchdog”与用户空间的”watchdog”的主要区别是，内核态的”watchdog”抗干扰能力强，运行稳定。