The shared library region is a z/OS® feature that enables address spaces to share dynamic link library (DLL) files. This feature enables your CICS® regions to share the DLLs that are needed for JVMs, rather than each region having to load them individually. This can greatly reduce the amount of real storage used by MVS™, and the time it takes for the regions to load the files.
The storage that is reserved for the shared library region is allocated in each CICS region when the first JVM is started in the region. (This might be the master JVM that initializes the shared class cache.) The amount of storage that is allocated is controlled by the SHRLIBRGNSIZE parameter in z/OS, which is in the BPXPRMxx member of SYS1.PARMLIB. The minimum is 16MB, and the z/OS default is 64MB. You can tune the amount of storage that is allocated for the shared library region by investigating how much space you actually need, bearing in mind that other applications besides CICS might be using the shared library region, and adjusting the SHRLIBRGNSIZE parameter accordingly.
If you want to reduce the amount of storage that is allocated for the shared library region, first check that you do not have wasted space in your shared library region. Bring up your normal workload on the z/OS system, then issue the command D OMVS,L to display the library statistics. If there is unused space in the shared library region, you can reduce the setting for SHRLIBRGNSIZE to remove this space. If CICS is the only user of the shared library region, you can reduce the SHRLIBRGNSIZE to the minimum of 16MB, because the DLLs needed for the JVM only use around 10MB of the region.
If you find that all the space in the shared library region is being used, but you still want to reduce this storage allocation in your CICS regions, there are three possible courses of action that you can consider:
If you choose to remove the extended attribute +l from any of your files, when you replace those files with new versions (for example, during a software upgrade), remember to check that the new versions of the files do not have this attribute.
You can find more information about shared libraries in z/OS UNIX on the z/OS UNIX System Services Web site at http://www.ibm.com/servers/eserver/zseries/zos/unix/perform/sharelib.html.