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IT33488: Queries might hang in UOW-Exec waiting on SQLO_LT_SQLB_HASH_BUCKET_GROUP_HEADER__groupLatch in sqlbFindPageInBPOrSim function

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APAR status

  • Closed as program error.

Error description

  • Under rare timing conditions, with high concurrency, queries
    might get stuck in UOW-Executing state with the following
    function on the top of the stack of the coordinator agent
    (db2agent EDU)
    
    SQLO_SLATCH_CAS64::getConflict
    sqlbFindPageInBPOrSim
    sqlbfix
    
    
    At the same time, multiple prefetcehrs (db2pfchr EDUs) will be
    show waiting in:
    
    SQLO_SLATCH_CAS64::getConflict
    sqlbFindPageInBPForPrefetch
    sqlbGetBuffer
    sqlbProcessRange
    sqlbPFPrefetcherEntryPoint
    
    and one db2agent with sqlbRemovePageFromSimulationArea function
    on the stack:
    
    SQLO_SLATCH_CAS64::getConflict
    sqlbRemovePageFromSimulationArea
    sqlbFindPageInBPOrSim
    sqlbfix
    
    db2pd -latches will report multiple waiters on the same
    SQLO_LT_SQLB_HASH_BUCKET_GROUP_HEADER__groupLatch address, with
    the latch holder EDU ID changing across iterations.
    
    MON_GET_LATCH query, e.g.:
    
    select substr(latch_name,1,40) latch_name,
        substr(memory_address,1,20) memory_address,
        edu_id,
        substr(edu_name,1,20) edu_name,
        application_handle,
        member,
        latch_status,
        latch_wait_time
    from table ( mon_get_latch( null, -2 ) ) where latch_status =
    'W' order by latch_wait_time
    
    might show at least one EDU_ID waiting on that latch address for
    significant amount of time.
    
    Problem is specific to databases where size of bufferpool(s) is
    controlled by STMM (SIZE AUTOMATIC) and it is more likely to be
    exposed on systems with large number of CPUs, very large
    bufferpool and under a heavy prefetch activity.
    

Local fix

  • Disable self tuning for bufferpools:
    
    ALTER BUFFERPOOL <BP_NAME> SIZE <NUMBER_OF_PAGES>
    

Problem summary

  • ****************************************************************
    * USERS AFFECTED:                                              *
    * ALL                                                          *
    ****************************************************************
    * PROBLEM DESCRIPTION:                                         *
    * See Error Description                                        *
    ****************************************************************
    * RECOMMENDATION:                                              *
    * Update to Db2 version 11.1 Mod 4 Fix Pack 6 or newer.        *
    ****************************************************************
    

Problem conclusion

  • First fixed in Db2 version 11.1 Mod 4 Fix Pack 6
    

Temporary fix

  • Disable self tuning for bufferpools:
    
    ALTER BUFFERPOOL <BP_NAME> SIZE <NUMBER_OF_PAGES>
    

Comments

APAR Information

  • APAR number

    IT33488

  • Reported component name

    DB2 FOR LUW

  • Reported component ID

    DB2FORLUW

  • Reported release

    B50

  • Status

    CLOSED PER

  • PE

    NoPE

  • HIPER

    NoHIPER

  • Special Attention

    NoSpecatt / Xsystem

  • Submitted date

    2020-07-09

  • Closed date

    2020-11-26

  • Last modified date

    2020-11-26

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

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

    IT33546

Fix information

  • Fixed component name

    DB2 FOR LUW

  • Fixed component ID

    DB2FORLUW

Applicable component levels

[{"Line of Business":{"code":"LOB10","label":"Data and AI"},"Business Unit":{"code":"BU053","label":"Cloud & Data Platform"},"Product":{"code":"SSEPGG","label":"Db2 for Linux, UNIX and Windows"},"Platform":[{"code":"PF025","label":"Platform Independent"}],"Version":"11.5"}]

Document Information

Modified date:
27 November 2020