LFAREA parameter

The IEASYSxx LFAREA parameter specifies the amount of real storage to be made available for 1 MB and 2 GB large pages. All 1 MB and 2 GB pages are backed by contiguous 4 KB real storage frames, and are allocated from real storage as specified by the LFAREA parameter. If the system becomes constrained by a lack of sufficient 4 KB Start of changeframesEnd of change to handle workload demand, it can use free 1 MB large Start of changeframesEnd of change to back 4 KB Start of changenon-preferredEnd of change page requests, enabling the system to react dynamically to changing system storage frame requirements.

Start of changeIf 1 MB large frames are required but they are currently in use as 4 KB frames, the system will attempt to reform them. However, frequent 1 MB frame decompositions and reforms can indicate a system configuration and tuning issue—the LFAREA might be too large, or the demand for 4 KB frames might be higher than expected.End of change To resolve this issue, you can either decrease the size of the LFAREA or adjust the workload to reduce the demand for 4 KB frames.

Start of changeWhen the system’s supply of pageable 1 MB frames is depleted, available 1 MB frames from the LFAREA will be used to back pageable 1 MB pages. If subsequent demand for fixed 1 MB frames occurs, pageable 1 MB pages that are backed by such frames will relinquish the frames, resulting in either paging or demotion to pageable 4 KB frames.End of change

Because the IEASYSxx LFAREA parameter requires an IPL in order to change the LFAREA value, the following considerations apply:
  • If the value specified for LFAREA is too small, available 1 MB and 2 GB pages might not exist for applications that could benefit from large page usage.
  • If the value specified for LFAREA is too large, such that the system does not have enough 4 KB pages to satisfy workload requirements, 1 MB Start of changelarge frames could be used as 4 KB framesEnd of change (2 GB pages are never Start of changeused asEnd of change 4 KB pages). Start of changeHowever, when such frames are used as 4 KB frames, they are not treated as preferred storage (cannot back SQA or long term fixed storage in a non-swappable address space). Additionally, there is a CPU cost associated with the conversion.End of change

    Start of changeThe specification of INCLUDE1MAFC in the LFAREA parameter will cause the system to take the available 1 MB large frames into account when making paging decisions. IBM recommends that you specify this parameter unless you want the system to preserve the use of 1 MB large frames for 1 MB large fixed pages at the expense of greater paging.End of change

  • The LFAREA request can be specified with target and minimum values for 2 GB and 1 MB pages. If the amount of online real storage at IPL is not sufficient to reserve both areas at their target values, the system will attempt to reserve the target value for 2 GB pages by reducing the 1 MB request, provided that the 1 MB request can be satisfied at or above its minimum value. If reducing the 1 MB request to its minimum is still not enough, the system will reduce the 2 GB request toward its minimum value. If the request cannot be satisfied at the specified 2 GB and 1 MB minimum values, the system will prompt the operator to re-specify the LFAREA request when PROMPT was specified, either explicitly or by default, or will reserve zero 1 MB and 2 GB pages when NOPROMPT was specified.
  • Determine the total number of 1 MB and 2 GB pages that your applications require, and consider using this number as a starting point for your LFAREA target values.
  • Use output from the DISPLAY VIRTSTOR,LFAREA system command as an estimate for the maximum number of 1 MB pages used on behalf of both 1 MB and 4 KB requests. Use this estimate to determine if your LFAREA value is too small or too large; refer to message IAR019I for additional details.
Refer to the following documentation for additional details on LFAREA:
  • For specific details on specifying the IEASYSxx LFAREA parameter, refer to z/OS MVS Initialization and Tuning Reference.
  • For information on calculating the LFAREA value based on DB2® requests, refer to IBM® DB2 10 for z/OS® Managing Performance.
  • For information on calculating the LFAREA value based on JAVA heaps, refer to IBM SDK for z/OS, Java™ Technology Edition.