WebSphere Application Server V7.0: Concepts ... - IBM Redbooks

Note: The JVM uses these memory pages for the shared classes cache. If
you request large cache sizes, you might have to increase the amount of
shared memory pages available.
The shared page size for a z/OS Unix System Service is fixed at 4 KB for
31-bit and 1 MB for 64-bit. Shared classes try to create a 16 MB cache by
default on both 31- and 64-bit platforms. Therefore, set IPCSHMMPAGES
greater than 4096 on a 31-bit system.
If you set a cache size using -Xscmx, the VM will round up the value to the
nearest megabyte. You must take this into account when setting
IPCSHMMPAGES on your system.
For further information about performance implications and use of these
parameters, refer to IBM publications z/OS MVS Initialization and Tuning
Reference, SA22-7592, and zOS Unix System Services Planning Guide,
GA22-7800.
Persistence for shared class cache
WebSphere Application Server for z/OS V7.0 uses the IBM Java Standard
Edition (SE) 6. This JVM implementation offers the shared class cache that
allows multiple JVM’s to access the same classes, both application and system
classes, without loading them multiple times into the memory.
While the IBM implementation for distributed platforms (Windows, Linux, AIX)
offers the option to write the content to a file system so that it can survive an
operating system restart, z/OS only supports the non-persistent cache.
Compressed references
The use of compressed references improves the performance of many
applications because objects are smaller, resulting in less frequent garbage
collection and improved memory cache use. Certain applications might not
benefit from compressed references. Test the performance of your application
with and without the option to determine if it is appropriate.
When using compressed references, the following structures are allocated in the
lower area of the address space:
► Classes
► Threads
► Monitors
Chapter 14. WebSphere Application Server for z/OS 471