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APAR status

  • Closed as program error.

Error description

  • Change Team finds this storage shortage is due to the build up
    of ITRQP control blocks (which are created for a page chain)
    however that chain changes while a client channel scans it
    to check for messages on the queue.
    When that browsing process completes the current browse/get
    operation should free the ITRQPs however this operation never
    completes because the pagechain continues to change while it is
    being searched. This results in the accumulation of ITRQPs on
    the free chain which eventually leads to storage exhaustion
    (which can be represented by the abend in ADDITIONAL SYMPTOMS)
    The root of the problem is the length of the page chain (which
    contains thousands of empty pages which are being scanned each
    time, and the time taken to scan through these increases the
    chance of the pagechain changing while that scan takes place).
    The scavenger should be removing any empty pages from the page
    chain and, for an indexed queue, this is done when an
    application informs the scavenger that a page may be empty.
    Dump review shows this process is occurring, but every so often
    a page is missed, leading to an accumulation over time of empty
    pages on the page chain. The problem occurs when the last
    remaining message is got from a page by an application that
    uses MQGMO_MARK_SKIP_BACKOUT and the application backs out.
    During backout processing the IPSE to inform the scavenger that
    the page may be empty is deleted (by design) however due to
    mark skip backout processing the message is never returned to
    the queue to be go again. This means no new IPSE will be
    created for that page (ie. the scavenger is never informed that
    the page is empty and does not deallocate it)
    Under normal circumstances this is not noticeable, however the
    partial scavenger would also scan the pagechain removing empty
    pages, and so would locate and process these missed pages (In
    this case that does not occur due to a valid message in the
    head page. The partial scavenger only processes empty pages at
    the head of the pagechain)
    Additional symptoms/keywords:
    5C6 00E2000B ABEND5C6 ABENDS5C6 S5C6 S05C6
    SP229 KEY7 leak

Local fix

Problem summary

  • ****************************************************************
    * USERS AFFECTED:                                              *
    * All users of IBM MQ for z/OS Version 9 Release 0             *
    * Modification 0.                                              *
    * PROBLEM DESCRIPTION:                                         *
    * Abend 5C6-00E2000B and abnormal queue manager termination    *
    * due to a short on storage (SOS) condition due to a build up  *
    * of ITRQP control blocks.                                     *
    *                                                              *
    * In some cases, empty pages are not freed, leading to         *
    * unexpected pageset expansion or MQRC 2192                    *
    * RECOMMENDATION:                                              *
    * none                                                         *
    While an application browsing or getting messages from a queue
    searches for the next available message, ITRQP control blocks
    are created by other tasks performing operations that could
    require the application to restart its search. If the
    application determines that it needs to restart the search (for
    example, due to an ITRQP indicating that an uncommitted message
    is now available earlier on the queue, or due to the page chain
    being changed by the scavenger), it moves any unneeded ITRQPs to
    a freechain to be freed later, when the browse/get operation
    In rare cases, the browse/get operation can be required to
    restart the search multiple times, causing the number of ITRQPs
    on the freechain to grow. As the number of ITRQPs on the
    freechain increases, the time to process them increases, which
    increases the likelihood of needing to restart the search again,
    compounding the problem and leading to the get/browse operation
    never completing, and consequently never freeing the ITRQPs on
    the freechain, and eventually resulting in storage exhaustion.
    In the reported case the browse/get the search of the pagechain
    was taking a long time due to a large number of empty pages on
    the pagechain - in the time taken to scan through the empty
    pages the chain would be changed by the scavenger, requiring the
    search to restart, and so triggering the ITRQP build up. The
    buildup of empty pages was due to mark skip backout
    (MQGMO_MARK_SKIP_BACKOUT) processing, in this case from the CICS
    bridge, failing to inform the scavenger when skipping the
    backout of the last message on a page.

Problem conclusion

  • CSQIATRQ is changed to free unneeded ITRQPs immediately rather
    than accumulating them on the freechain, and consequently
    preventing the buildup of ITRQPs and resulting storage shortage.
    CSQIDLM1 is changed to inform the scavenger when skipping the
    backout of a message on an indexed queue, ensuring the page is
    deallocated if it is now empty and preventing the build up of
    empty pages on the pagechain.

Temporary fix


APAR Information

  • APAR number


  • Reported component name

    MQ Z/OS V9

  • Reported component ID


  • Reported release


  • Status


  • PE




  • Special Attention

    NoSpecatt / Xsystem

  • Submitted date


  • Closed date


  • Last modified date


  • APAR is sysrouted FROM one or more of the following:


  • APAR is sysrouted TO one or more of the following:




Fix information

  • Fixed component name

    MQ Z/OS V9

  • Fixed component ID


Applicable component levels

  • R000 PSY UI41040

       UP16/10/14 P F610

Fix is available

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Document Information

Modified date:
19 May 2017