OA34024: INSUFFICIENT DOCUMENTATION ON HOW TO SELECT A LFAREA SIZE

APAR status

• Closed as documentation error.

Error description

• THERE IS INSUFFICIENT DOCUMENTATION TO PROVIDE GUIDANCE ON HOW
  TO SIZE THE LFAREA.
  

Local fix

Problem summary

• ****************************************************************
  * USERS AFFECTED: Users of HBB7750 and up wishing to exploit   *
  *                 large (1 MB) pages                           *
  *                                                              *
  ****************************************************************
  * RECOMMENDATION:                                              *
  ****************************************************************
  When Large (1 MB) Pages were introduced, there was insufficient
  documentation describing how to select a proper LFAREA size.
  

Problem conclusion

• The following text will be included in the MVS Initialization
  and Tuning Reference under the LFAREA parameter section for
  IEASYSxx (system parameter list):
  
  Large (or 1 MB) pages are backed by 256 contiguous 4-KB real
  storage frames and are allocated only from the virtual area
  reserved for large-page-frame allocations (as specified by
  the LFAREA keyword). However, if the system becomes
  constrained for lack of sufficient 4-KB pages to handle
  workload demands, free large pages can be used to back 4-KB
  page requests, thereby enabling the system to react
  dynamically on demand for system storage frames.
  
  Alternatively, if more large pages are needed, the frames
  reserved for backing large pages can be recombined for use
  as large pages again. Note that a high use of this
  decomposition-and-recombine function can be indicative of a
  system configuration and tuning issue; the large-page
  allocation (LFAREA) may be too large or the demands for
  4-KB frames may be higher than anticipated. Using an
  understanding of the unique system and its workload
  requirements, either the size of the large-page-frame area
  (LFAREA) can be decreased or adjustments can be made to the
  workload to reduce the demands for 4-KB frames.
  
  The IEASYSxx LFAREA parameter specifies the amount of real
  storage to be made available for 1 MB pages.  The system
  requires an IPL in order to change the LFAREA value.
  Therefore, the following considerations apply:
  
  - If the value specified for LFAREA is too small, there
    may be no available 1 MB pages for applications that
    could benefit from 1 MB page utilization.
  
  - If the value specified for LFAREA is too large such
    that the system does not have enough 4 KB pages to
    satisfy workload needs, this may result in the
    conversion of 1 MB pages into 4 KB pages.  Since
    there is a CPU cost for the system to convert 1 MB
    pages to 4 KB pages and vice-versa, it is recommended
    that another LFAREA value is selected that will
    accommodate both your 1 MB page application needs and
    leave enough 4K frames available to accomodate your
    4K frame workload needs.
  
  - Determine the total number of 1 MB pages that
    applications on the system plan to use.  This should
    be the minimum LFAREA value.
  
  - Use the DISPLAY VIRTSTOR,LFAREA system command
    (available with APAR OA31116) to list the high water
    marks for the number of 1 MB pages used on behalf of
    1 MB requests as well as on behalf of 4 KB requests.
    These high water marks can be used to determine if the
    value specified for LFAREA is too small or too large.
    See message IAR019I for further details.
  
    - A high water mark for the number of 1 MB pages
      used on behalf of 1 MB requests that is equal to
      the LFAREA value may imply that the installation
      specified too small a LFAREA value.  Installations
      should re-evaluate the number of 1 MB pages that
      applications on the system actually need.
  
    - A high water mark for the number of 1 MB pages
      used on behalf of 1 MB requests that is less than
      the LFAREA value implies that the LFAREA value is
      sufficiently large to handle installation needs.
      Note: Installations should consider reducing the
      LFAREA value if the high water mark for the number
      of 1 MB pages used on behalf of 4 KB requests is
      greater than zero.
  
  
  Examples:
  
  For DB2 users, DB2 requests 1 MB pages for buffer pools that
  are defined with the PGFIX(YES) option.  Therefore,
  determining an optimum LFAREA value can be performed by
  calculating the sum of the DB2 buffer pool sizes that are
  defined with the PGFIX(YES) option.  The following link is
  a good starting point for understanding DB2 buffer pools:
  
  http://publib.boulder.ibm.com/infocenter/dzichelp/v2r2/topic/
  com.ibm.db2z10.doc.perf/db2z_tunedbbufferpools.htm
  
  Note: The material presented in the above link can also be
  found in the following publication:
  - DB2 10 for z/OS Managing Performance (SC19-2978-00)
  
  For JAVA users, The Java Virtual Machine (JVM) has an address
  space, just like any process, from which it can allocate
  chunks of memory for various purposes. The most important
  (and definitely the most familiar to Java programmers)
  of these chunks is the Java heap. The Java heap is used for
  allocating the Java objects, and you can monitor its usage
  through verbosegc traces. The size of a Java heap can be
  explicitly controlled using -Xms and -Xmx command line
  arguments.  Therefore, determining an optimum LFAREA value
  can be performed by calculating the sum of sizes of all the
  JAVA heaps defined on your system (in megabyte units).
  

Temporary fix

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APAR Information

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