IBM Support

PM88804: CHANGE DISCARDDATA TO BE TRIGGERED BY MONITOR STATUS RATHER THAN ALWAYS BEING DONE IN CERTAIN CASES.

A fix is available

Subscribe

You can track all active APARs for this component.

 

APAR status

  • Closed as program error.

Error description

  • DISCARDDATA of REAL STORAGE frames results in RSM serialization
    to manage the frame status and page faults for those frames
    which were unbacked with KEEPREAL(NO).  The combination of these
    on an LPAR where paging typically does not occur, results in
    significant cpu overhead.
    

Local fix

Problem summary

  • ****************************************************************
    * USERS AFFECTED: All DB2 10 users.                            *
    ****************************************************************
    * PROBLEM DESCRIPTION: Frequent thread terminations without    *
    *                      re-use may result in significant CPU    *
    *                      consumption as 64bit thread pool        *
    *                      storage is freed and REAL frames are    *
    *                      unbacked.                               *
    ****************************************************************
    * RECOMMENDATION:                                              *
    ****************************************************************
    When threads terminate, their pool storage may either be
    completely freed if the thread is not cached for reuse, or a
    portion may be freed.  In either case, storage which is only
    virtually freed by DB2 since it is submanaged, will have its
    REAL frames unbacked in many cases.  This unbacking requires RSM
    latch serialization and also results in a pagefault when those
    pages are again used.  This whole process can show significant
    CPU usage depending on the number of threads and their
    allocation/deallocation frequency.  Similarly, thread
    contraction which may occur at specific commit frequencies, also
    may result in pool storage being virtually freed with the REAL
    frames unbacked.  REALSTORAGE_MANAGEMENT which has default of
    AUTO, should not result in the discard of REAL frames unless the
    LPAR comes under stress and begins paging or nears the
    REALSTORAGE_MAX value if specified.
    

Problem conclusion

  • This apar changes the way 64 bit Above-the-bar (ATB) real
    storage frames are managed by DB2 based on the DB2 zparms
    REALSTORAGE_MANAGEMENT,REALSTORAGE_MAX and general RSM LPAR
    storage monitor fields.  DB2 ATB storage management is changed
    to no longer issue an IARV64 DISCARDDATA request to free real
    storage frames unless the DB2 storage monitor has detected that
    DISCARD MODE should be turned ON. When DISCARD MODE is ON the
    member will return to the same behavior it had prior to PM88804
    and issue an IARV64 DISCARDDATA request when freeing storage.
    
    Not freeing the real storage frames can improve system
    performance due to a significant reduction in first reference
    page faults, RSM lock contention, and uncaptured CPU time.
    
    *** NOTE ***
    PM88804 may result in higher REAL FRAME counts being reported by
    DB2 monitor reports and RMF, especially in environments that
    never page to aux.  For those customers who experience frequent
    or periodic paging, the counts may not be significantly higher
    but will likely still be higher than before PM88804.  PM88804
    reverses some of the affect of PK25427.
    The major reporting fields for 64 bit storage are:
    qw0225ShrStg_Real - Shared storage most heavily used for threads
                        will likely show the most increase.
    qw0225ComStg_Real - High Common
    qw0225PriStg_Real - High address space private
    

Temporary fix

Comments

APAR Information

  • APAR number

    PM88804

  • Reported component name

    DB2 OS/390 & Z/

  • Reported component ID

    5740XYR00

  • Reported release

    A10

  • Status

    CLOSED PER

  • PE

    NoPE

  • HIPER

    YesHIPER

  • Special Attention

    NoSpecatt

  • Submitted date

    2013-05-10

  • Closed date

    2013-06-24

  • Last modified date

    2013-07-02

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

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

    UK95350

Modules/Macros

  • DSNSCON2 DSNSVSFM DSNSVSPC DSNSVSVP
    

Fix information

  • Fixed component name

    DB2 OS/390 & Z/

  • Fixed component ID

    5740XYR00

Applicable component levels

  • RA10 PSY UK95350

       UP13/06/29 P F306

Fix is available

  • Select the PTF appropriate for your component level. You will be required to sign in. Distribution on physical media is not available in all countries.

[{"Business Unit":{"code":"BU059","label":"IBM Software w\/o TPS"},"Product":{"code":"SSEPEK","label":"Db2 for z\/OS"},"Component":"","ARM Category":[],"Platform":[{"code":"PF025","label":"Platform Independent"}],"Version":"10.1","Edition":"","Line of Business":{"code":"LOB10","label":"Data and AI"}},{"Business Unit":{"code":"BU054","label":"Systems w\/TPS"},"Product":{"code":"SG19M","label":"APARs - z\/OS environment"},"Component":"","ARM Category":[],"Platform":[{"code":"PF025","label":"Platform Independent"}],"Version":"10.1","Edition":"","Line of Business":{"code":"","label":""}}]

Document Information

Modified date:
02 July 2013