%META:TOPICINFO{author="ChrisBartram" date="1170379776" format="1.1" version="1.2"}%
%META:TOPICPARENT{name="Hp3000Tips"}%
---+ Will turning off the system console of an HP3000 lock up the system? And other HP3000 myths

Today, the system doesn't care if the console is switched on or not. However, if the console is left hung via a control-s, console message
buffers will eventually fill up and any process that tries to send a console message will be suspended until the message can be delivered, at
least for a minute or so.

  --[[BruceToback]]

There are 63 what I call genmsg buffers. Every console message requires a genmsg buffer and a TELL or WARN requires
a buffer per destination terminal.  When the system detects that all of the buffers are in use, it starts a 30 sec. timer.  If no buffers free up after
that time, the system issues a hard-preemptive I/O to the console in an attempt to free it up.  Then, the system waits another 30 sec.  If the
console is still hung, then we abort every I/O that's using a genmsg buffer.

So, if the console can never complete I/Os, for whatever reason, then there is a 63 message/one minute timeout/abort 63 I/Os cycle that continually repeats itself.  A system with a lot of console/TELL/WARN I/O traffic will seriously bog down as the result of a hung console.

  --[[MikePaivinen]]

After that, the system gives up and starts tossing all its undelivered messages into the system log file. So if you _have_ managed to hang up your
console (turning it off won't do it by itself), you'll need to look in your log files. I'm not sure what happens if you don't have console logging
turned on -- the system won't crash, but it's not clear what, if anything, will happen to undeliverable messages.

  --[[BruceToback]]

Actually, the system doesn't do anything special with the aborted I/Os. If console logging is enabled, then the aborted console messages will be found in the console log files.   If console logging is NOT enabled, then these messages are lost.  If the aborted message was targeted at a non-console terminal, then this message is always lost.

  --[[MikePaivinen]]

-- Main.ChrisBartram - 17 May 2006