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MustGather: High CPU causing hang or loop running on z/OS



MustGather for problems with the WebSphere Application Sever for z/OS experiencing High CPU. This technote explains how to obtain a dump of an address space in a WebSphere Application Server for z/OS environment. This should only be done at the request of IBM™ Support personnel.

Resolving The Problem

If you have not contacted Support, click MustGather: Read first for WebSphere Application Server for z/OS

If it is necessary to obtain a dump of an address space in WebSphere Application Server for z/OS environment when you are experiencing a hang or loop situation:

  1. Understand the System Resource Considerations described below.

  2. Verify that your DUMP dataset is large enough for the dump you are taking.

  3. Recreate the problem (the reason why you need to take the dump).

  4. If this is a possible LOOP or HIGH CPU condition, consider turning on BRANCH tracing.

  5. Issue the DUMP command, using the example in this technote.
  6. Turn off BRANCH tracing if previously enabled.

System resource considerations

CPU utilization
This depends on the size and number of address spaces you are dumping. It is hard to quantify the impact. Think in terms of the size of your typical dumps and how long they take. This should give you a feel for the length of time required to take this dump. You can use SDSF to approximate the size of the dump.

During DUMP processing a copy of all the pages in an address space is made in virtual storage. The amount of virtual storage required depends on the size of the address spaces you are dumping. If your dump is large, consider the following:
    MAXSPACE setting
    This setting limits the amount of virtual storage all dumps currently being processed can occupy. If this value is exceeded, the result is a Partial Dump, which is virtually useless. The default setting is 500 meg, which is small for a WebSphere Application Server for z/OS environment. Issue the following MVS command to display your existing MAXSPACE setting:

    D D,ST

    Then issue MVS command, CHNGDUMP, to increase this setting, if necessary. This command is described in MVS System Commands in the Knowledge Center.

    For example, to increase the MAXSPACE setting to 2 gig, issue the following command:


    PAGE space setting

    These dumps are usually large, anywhere from 500 meg to 1 gig, on an average. They can be significantly larger if you are dumping several address spaces. Most of this storage ends up in your PAGE datasets. MAXSPACE is there to protect your system from running out of PAGE datasets. If you raise the MAXSPACE, be sure you have the PAGE datasets to support the same value.

    Dump dataset size
    If your dataset is not large enough, the result is a partial dump. Look for message IEA611I.

DUMP command
In the WebSphere Application Server for z/OS environment, the OMVS address space and its DataSpaces is usually dumped. Following is an example of such a dump:

DUMP COMM=(Descriptive name for this Webserver dump)
R rn,jobname=(OMVS,server_name),CONT
R rn,dspname=('OMVS'.*),END                

For a listing of technotes, downloads and educational materials specific to High CPU, search the WebSphere Application Server for z/OS support site.

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Document Information

Modified date:
15 June 2018