Significance of WCHAN in ps output

From the man ps ouput on linux --
       nwchan      WCHAN     address of the kernel function where the process  
                                             is sleeping (use wchan if you want the kernel
                                             function name). Running tasks will display a dash
                                             ('-') in this column.

The following text from the troubleshooting link is worth reading carefully --
http://www.tldp.org/HOWTO/SCSI-2.4-HOWTO/trouble.html

Text Follows --

When it looks like something has partially locked up the system, the ps command can be useful for finding out what may be causing the problem. The following options may be useful for identifying what part of the kernel may be causing the problem. This information could be forwarded to the maintainers.

ps -eo cmd,wchan ps -eo fname,tty,pid,stat,pcpu,wchan ps -eo pid,stat,pcpu,nwchan,wchan=WIDE-WCHAN-COLUMN -o args

The most interesting option for finding the location of the "hang" is "wchan". If this is a kernel address then ps will use /proc/ksyms to find the nearest symbolic location. The "nwchan" option outputs the numerical address of the "hang". If the system is not responding to keystrokes, then <Alt+ScrollLock> in text mode should output a stack trace while <Ctrl+ScrollLock> should output a list of all processes. If the log is still working, the output will be sent there as well as appearing on the console.
If the kernel has been built with the CONFIG_MAGIC_SYSRQ, then in text mode <Alt+SysRq+H> will list available commands. Of these <Alt+SysRq+S> is useful for doing an emergency sync while <Alt+SysRq+U> will remount file systems in read only mode. After that <Alt+SysRq+B> to reboot the machine might be your next move.