PM86952: CONTRACTION FOR MULTI-BLOCK SEGMENTS FOR 64BIT DB2 STORAGE POOLS

Obtain the fix for this APAR.

APAR status

• Closed as new function.

Error description

• When the original design was done for storage pools consisting
  of 64bit Above-the-bar ( ATB ) storage, it was felt that it
  would not be necessary to contract these pools.  Having more
  thread storage shifted to these pools from 31bit usage, has
  proven that there are some environments which result in very
  large amounts of storage to accumulate in these multi-block
  segments.  When this storage is later freed, much of this
  storage will remain backed in REAL because it is not contracted.
  Also, there have been cases where POOL FULL RSN00E2000C occur
  which may be relieved by contraction, even though there are
  tuning or application changes which may reduce the actual
  storage usage.
  

Local fix

Problem summary

• ****************************************************************
  * USERS AFFECTED: All DB2 V10 users                            *
  ****************************************************************
  * PROBLEM DESCRIPTION: (1)                                     *
  *                      Thread 64bit Shared Variable storage    *
  *                      slowly increases over time as observed  *
  *                      in IFCID225 record, field QW0225SV.     *
  *                      (2)                                     *
  *                      ABEND0C4 DSNSVBK when the thread is     *
  *                      canceled while performing contraction   *
  *                      and DSNSCON2 FRR returns to DSNSVBK     *
  *                      with a zero recovery stack pointer.     *
  *                      Since contraction is likely during      *
  *                      a MUST COMPLETE process, DB2 may        *
  *                      terminate.                              *
  *                      (3)                                     *
  *                      DSNSVSVB ABEND04E RC00E2000C when the   *
  *                      thread encounters Pool Full.            *
  ****************************************************************
  * RECOMMENDATION: Apply apar                                   *
  ****************************************************************
  (1)
  The original design implemented for DB2 storage pool
  contraction, did not include those size slots which contain
  multiple fixed blocks per expansion segment. In analyzing
  various dumps from customers and internal testing, it was
  observed that pools sometimes had significant FREE storage tied
  up in these slots.  By contracting these slots, REAL FRAME usage
  may be reduced since the corresponding virtual storage would be
  available for reuse by other threads, thus not having to back
  additional frames.  Reducing the virtual storage within a given
  pool could potentially prevent requests from being abend04E
  with RC00E2000C, pool full condition.
  (2)
  When a thread attempts to get storage from a pool and finds it
  is full, it will synchronously call contraction for that pool.
  If it is canceled while in the contraction module, that FRR
  recovers but does not handle the recovery stack correctly.
  (3)
  When threads are attempting to get 64bit storage from a system
  level pool and it is full, they will all try to perform
  contraction on that pool but only the one succeeds.  The
  remaining threads are not allowed to wait.  Instead, they
  return and retry their storage request but the pool is
  still full and they abend04E RC00E2000C.
  

Problem conclusion

Temporary fix

• *********
  * HIPER *
  *********
  

Comments

• (1)
  This apar extends the contraction function to include all slot
  sizes, those with multiple blocks per segment and those with one
  block per expansion segment.
  (2)
  Contraction will not retry canceled threads.
  (3)
  Contraction will be serialized and multiple threads contracting
  the same pool, will be allowed to wait while that occurs.
  

APAR Information

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

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

  UK97692

Modules/Macros

• DSNLTMSC DSNSCON2 DSNSINTM DSNSSTUP DSNSVSFB DSNSVSFM DSNSVSGM
  DSNSVSPP DSNSVSVB DSNSVSVP DSNTXSTB
  

Fix information

• Fixed component name

  DB2 OS/390 & Z/

• Fixed component ID

  5740XYR00

Applicable component levels

• RA10 PSY UK97692

     UP13/10/04 P F310 ½

Fix is available

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