WATCHDOG(4) BSD Kernel Interfaces Manual WATCHDOG(4)
watchdog -- hardware timers/counters for quick crash recovery
Hardware watchdog timers are devices that reboot the machine when it
hangs. The kernel continually resets the watchdog clock on a regular
basis. Thus, if the kernel halts, the clock will time out and reset the
machine. Watchdog timers may be configured to be reset from userland to
cause a reboot if process scheduling fails; see watchdogd(8) for further
A number of hardware watchdogs are supported, and all are configured
using sysctl(8) under the kern.watchdog name:
kern.watchdog.auto Automatically reset ('tickle') the watchdog
timer but disable it at system shutdown time.
kern.watchdog.period The timeout in seconds. Setting it to zero
disables the watchdog timer.
In situations where the machine provides vital services which are not
handled completely in kernel space, e.g. mail exchange, it may be desir-
able to reboot the machine if process scheduling fails. This is done by
setting kern.watchdog.auto to zero and running a process which repeatedly
sets kern.watchdog.period to the desired timeout value. Then, if process
scheduling fails, the process resetting the timer will not be run, lead-
ing to the machine being rebooted. Note that the kernel will not auto-
matically disable an enabled watchdog at system shutdown time when
kern.watchdog.auto is set to zero.
Watchdog timers should be used in high-availability environments where
getting machines up and running quickly after a crash is more important
than determining the cause of the crash. A watchdog timer enables a
crashed machine to autonomously attempt to recover quickly after a system
Note that this also means that it is unwise to combine watchdog timers
with ddb(4) since the latter may prevent the former from resetting the
watchdog timeout before it expires. This means that the machine will be
rebooted before any debugging can be done. In other words: For mission
critical machines, disable ddb(4) by adding ``ddb.panic=0'' to
sysctl.conf(5) since this will give the chance to perform a crash dump
and reboot. Simply setting the watchdog will lose the debug trace of
what went wrong.
ddb(4), sysctl.conf(5), config(8), sysctl(8), watchdogd(8)
For systems with multiple watchdog timers available, only a single one
can be used at a time. There is currently no way of selecting which
device is used; the first discovered by the kernel is selected.
BSD May 21, 2009 BSD