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 
frames
to handle workload
demand, it can use free 1 MB large frames to
back 4 KB non-preferred page requests, enabling
the system to react dynamically to changing system storage frame requirements.
If 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. 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.
When 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.
- 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 large frames could be used as
4 KB frames (2 GB pages are never used as 4
KB pages). However, 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.The 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.
- 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.
- 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.