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an EREP report. The MDRs can also be seen online using the NetView
Hardware Monitor (or an equivalent) product. These records contain
information on why the resource became INOP. They are very useful for
problem determination.
• Are all users that are in session working and only those that are causing a
session to be established hung?
In this circumstance, the problem could be a VTAM problem. Remember
that most of the send and receive data processing is done in the user′s
address space. All session establishment is done in VTAM′s address space.
Check the following areas:
− VTAM commands
Do VTAM commands work? If they do, VTAM is working. If they do not,
VTAM may be in a wait (unlikely) or loop (more likely) or perhaps is
unable to get a share of the CPU.
− CPU usage
Does any particular address space have very high CPU utilization? If so,
monitor this, as the address space could be in a loop.
− VTAM paging
Is VTAM doing a large amount of paging? If so, what looks like a loop in
VTAM could be VTAM running a very long chain of control blocks.
• Are all users hung?
If so, this is could be a more fundamental problem with the operating
system. Check whether any MVS commands are working.
In any hang situation, the results (or lack of results) from a variety of displays
can give a clear picture of the scope of the problem. The more information that
is available, the easier the problem diagnosis is, and, usually, the faster the
resolution will be. It is a discouraging and time-consuming task to try to find a
problem in a VTAM dump when all you know is that there is a hang.
If the hang only occurs on a session when one particular request is made, often
a VTAM buffer trace can be used to see the last PIUs flowing on the session.
These PIUs can often hold the key to the problem.
Loops
When a loop is suspected, a variety of actions can be taken:
• CPU usage
Usually a loop can be readily found by displaying CPU usage. Of course, it
is necessary to have some idea of “normal” CPU usage for the suspected
address space, for comparison purposes. In general, any address space
using an unusually high amount of CPU should be suspect.
• Paging rates
The paging rates can be monitored to check for any abnormally high rates.
Of course, “normal” usuage is again needed for comparison purposes.
• Loop recording
The 3090 and 308X CPUs have a hardware facility to record up to 490 PSWs.
This facility is activated from the hardware console and can be used to trace
a loop. The output is dumped when an SVC or stand-alone dump is taken.
Chapter 6. Problem Determination 217