What is best method to debug freezes/hangs?

[Per olof Ljungmark]

yar wrote:
> My question is how best to go about debugging sporadic freezes and hangs.
> Just looking for general methods.    On a couple of our IDS systems there
> are sporadic hangs or freezes that occur every couple weeks or so, that are
> recoverable by either system reboots or taking out and reinserting the Gig
> fiber ethernet sniffing cables, but I  never can find anything in
> /var/logs/messages to tell what is going on. I'm assuming that may have to
> compile in kernel debugger support, but from what I read briefly, doesn't
> that require an actual crash with the resultant reboot and a debugger
> readable file  left in /var/crash?  That doesn't happen, there is no
> automatic reboot just a freeze/hang (box falls off network and at the
> console unable to type until one of the two above mentioned measures are
> take. 

In my case with the above behaviour a debug kernel and ctrl+alt+esc 
would work and that is probably true most of the times.

http://www.freebsd.org/doc/en_US.ISO8859-1/books/developers-handbook/kerneldebug-deadlocks.html

HTH,
--per