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 pages to handle workload demand, it can use free 1 MB large pages to back 4 KB page requests, enabling the system to react dynamically to changing system storage frame requirements.

If additional 1 MB large pages are required, the system can recombine 4 KB frames that had been taken from the 1 MB large page area and place them back into the LFAREA as 1 MB pages. However, frequent use of this decomposition and recombination function can indicate a system configuration and tuning issue—the large-page allocation (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 large page frame area (LFAREA) or adjust the workload to reduce the demand for 4 KB frames.

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 pages could be demoted to 256 4 KB pages (2 GB pages are never demoted to 4 KB pages). Because a CPU cost exists when the system converts 1 MB pages to 4 KB pages and vice-versa, select an LFAREA value that can accommodate your 2 GB and 1 MB page application requirements and your workload requirements for 4 KB frames.
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 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.
While 1 MB and 2 GB large pages can provide enhanced performance for your system, they also consume real storage. Ensure that your LFAREA specification reserves the optimal amount of real storage in balance with other system considerations to maximize overall system performance.

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.