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IT07070: STMM MAXIMUM TARGETS MAY BE TEMPORARILY TOO HIGH, SOME SWAPPING MAY OCCUR ON LINUX SYSTEMS

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APAR status

  • Closed as program error.

Error description

  • This problem affects DB2/Linux systems using STMM to tune the
    overall database memory usage based on system memory
    availability, i.e.
    - INSTANCE_MEMORY is not set to a fixed value
    - DATABASE_MEMORY is set to AUTOMATIC
    - SELF_TUNING_MEM is set to YES
    
    If the Linux VMM is under pressure to reclaim memory, some DB2
    shared memory pages may become unmapped, i.e. removed from the
    db2sysc process page table.  If the memory is not subsequently
    swapped out, DB2's self tuning memory manager (STMM) will assume
    this memory is part of the file cache, and available for other
    usage.  This may cause STMM maximum memory targets to be too
    high.
    
    This should be a temporary condition, but may trigger additional
    swapping on Linux systems
    - STMM memory targets are conservative, which generally avoid
    the need for reclaiming shared memory.
    - If the unmapped shared memory is swapped out, it is accounted
    for and memory targets reduced
    - Even if maximum targets are too high, there must be a
    performance benefit to commit additional memory to DB2 to
    increase memory usage.  For this to occur, generally memory is
    in high demand and frequently accessed, in which case any
    unmapped memory is likely to be re-referenced within a short
    time period, increasing the "Mapped" counter to be again in line
    with the Shmem counter.
    
    The overshoot in STMM's maximum memory target can be calculated
    by subtracting the "Mapped" field from the "Shmem" field in the
    /proc/meminfo output.  For example :
    Mapped:         91855876 kB
    Shmem:          119439256 kB
    
    In this case there is approximately 119439256-91855876 =
    27583380 or 27GB of unmapped shared memory, causing STMM's
    maximum memory target to be 27GB too high.  It does not mean
    that STMM will increase the configuration by this amount, but it
    is a possibility, and STMM may not correctly detect a
    constrained memory situation on the system.
    

Local fix

  • Specify a fixed INSTANCE_MEMORY or DATABASE_MEMORY setting that
    limits DB2's memory usage to reasonable values (this can be done
    based on checking historical database_memory tunings in the
    db2diag.log or STMM log)
    

Problem summary

  • ****************************************************************
    * USERS AFFECTED:                                              *
    * Systems using STMM to tune Database_Memory on Linux AND      *
    * Instance_Memory is set to AUTOMATIC                          *
    ****************************************************************
    * PROBLEM DESCRIPTION:                                         *
    * See Error Description                                        *
    ****************************************************************
    * RECOMMENDATION:                                              *
    * upgrade to DB2 Version 9.7 Fix Pack 11                       *
    ****************************************************************
    

Problem conclusion

  • Problem first fixed in DB2 Version 9.7 Fix Pack 11
    

Temporary fix

  • see Local Fix
    

Comments

APAR Information

  • APAR number

    IT07070

  • Reported component name

    DB2 FOR LUW

  • Reported component ID

    DB2FORLUW

  • Reported release

    970

  • Status

    CLOSED PER

  • PE

    NoPE

  • HIPER

    NoHIPER

  • Special Attention

    NoSpecatt

  • Submitted date

    2015-02-10

  • Closed date

    2015-10-23

  • Last modified date

    2015-10-23

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

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

    IT07074 IT07075

Modules/Macros

  • sqo
    

Fix information

  • Fixed component name

    DB2 FOR LUW

  • Fixed component ID

    DB2FORLUW

Applicable component levels

  • R970 PSN

       UP



Document information

More support for: DB2 for Linux, UNIX and Windows

Software version: 9.7

Reference #: IT07070

Modified date: 23 October 2015