IBM Support

PH03409: SEEMINGLY ERRATIC THREAD POOL GROWTH DURING LOW OR NO-LOAD SITUATIONS AFTER UPGRADING TO 18.0.0.1

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APAR status

  • Closed as program error.

Error description

  • After installing fix pack 18.0.0.1, some users occasionally
    
    observed the Liberty default executor thread pool growing
    
    erratically to large sizes for short periods of time, even
    
    though the Liberty server was processing little or no work.
    As
    
    a result, Liberty could abend if the Operating
    
    System/environment's per-process thread limits or it's
    native
    
    memory is exhausted while attempting to create these new
    
    threads.
    

Local fix

  • Setting Liberty's maxThreads property will prevent the thread
    pool from growing beyond that size regardless of workload, even
    if the algorithm would otherwise have decided to grow.  This
    can also help ensure that Liberty does not attempt to use more
    threads than the environment pre-process limits would allow.
    For z/OS Connect:  As a circumvention to this problem
    check OMVS MAXTHREADS and update server.xml to set
    <executor name="Default Executor" maxThreads="nnn"/>
    where maxThreads is approximately 30 below the nnn value that
    was specified.
    

Problem summary

  • ****************************************************************
    * USERS AFFECTED:  All users of IBM WebSphere Application      *
    *                  Server Liberty                              *
    ****************************************************************
    * PROBLEM DESCRIPTION: Occasional/random excess threadpool     *
    *                      growth in low/no-load situations        *
    ****************************************************************
    * RECOMMENDATION:                                              *
    ****************************************************************
    Changes made in 18001 to enable the threadpool controller to
    better handle high-latency workloads had the unexpected side
    effect of occasionally causing the poolsize to grow to excessive
    size.
    

Problem conclusion

  • The threadpool controller mostly decides to grow/shrink the pool
    based on historical throughput data at different pool sizes.
    This core mechanism is augmented by some general principles, for
    example: If the CPU usage is high, the controller becomes
    reluctant to add more threads, because more threads is unlikely
    to improve throughput in a high-cpu environment.
    The problem reported in this APAR is now addressed by a similar
    'rule-of-thumb': If the throughput is low (measured as ratio of
    tasks completed by the threadpool per controller cycle to number
    of threads in the pool) and no tasks are waiting in queue, the
    controller will be reluctant to grow the threadpool. This
    provides a general downward pressure on the poolsize in no/low-
    load situations, which greatly reduces the opportunity for the
    type of excess pool growth reported in this APAR.
    
    The fix for this APAR is currently targeted for inclusion in fix
    pack 18.0.0.4.  Please refer to the Recommended Updates page for
    delivery information:
    http://www.ibm.com/support/docview.wss?rs=180&uid=swg27004980
    

Temporary fix

  • This problem can be worked around by setting the maxThreads
    value in the executor clause of the server.xml to a value
    suitable for the particular environment.
    
                 <executor maxThreads="N" />
    

Comments

APAR Information

  • APAR number

    PH03409

  • Reported component name

    LIBERTY PROF -

  • Reported component ID

    5655W6514

  • Reported release

    CD0

  • Status

    CLOSED PER

  • PE

    NoPE

  • HIPER

    NoHIPER

  • Special Attention

    NoSpecatt / Xsystem

  • Submitted date

    2018-09-28

  • Closed date

    2018-12-11

  • Last modified date

    2018-12-11

  • 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

    LIBERTY PROF -

  • Fixed component ID

    5655W6514

Applicable component levels

  • RCD0 PSY

       UP



Document information

More support for: WebSphere Application Server for z/OS

Software version: CD0

Reference #: PH03409

Modified date: 11 December 2018