Is It a LOOP?

Symptoms of a loop include the continual repetition of messages or the continual repetition of a set of instructions. Repeated messages appear in the log file or the console of a virtual machine or in a report file. Review the log file, the console, or the report file.

To determine if a virtual machine is looping, log on to the virtual machine in question and enter:

     BEGIN

to resume processing.

To capture all console output, enter:

     #CP SPOOL CONSOLE START *

Then enter:

     #CP TRACE INST RUN

to display the instructions being processed.

As soon as the command is accepted by CP, the instructions are displayed as they are processed. If nothing is displayed, then the virtual machine is in a wait state and should be handled as such.

If instructions are displayed, then the application is doing work. Check the addresses associated with each instruction closely. Repeating addresses indicate that a loop exists. The repetition of addresses may be noticed immediately or it may take some time to observe it. Be patient. If no repetition of addresses is noticed after a reasonable length of time, then no loop exists.

Once you have determined that instructions are being repeated, you must dump the virtual machine.

Enter:

     #CP TRACE END ALL

to stop the trace.

Enter:

     #CP TRACE INST NORUN

to place the virtual machine in single-instruction execution mode.

When CP accepts the command, an instruction is displayed and the virtual machine is placed in CP READ. Once this occurs, enter:

     VMDUMP 0:END FORMAT DFSMS * loop in smsmaster

Note: Include a “dumpid” following the asterisk (*). This is a 1 to 100 character string describing the reason for the dump. The resulting dump is placed in the virtual machine’s reader.

The console file should also be saved. Enter:

     SPOOL CONSOLE CLOSE

Both files should be saved as CMS files and be available when reporting a problem to the IBM Support Center. Use the DUMPLOAD command to load the reader spool file into a CMS file. To do this, enter:

     DUMPLOAD SPOOL NOPRINT

This could result in a large file and may require a large file space. For additional information about DUMPLOAD, see z/VM: CP Commands and Utilities Reference.