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

Shared class cache
This section contains special information about the shared class cache usage on
z/OS.
Overview
The shared class cache is used to share WebSphere Application Server and
user classes between multiple JVMs.
JVMs that use the shared class cache start up more quickly, and have lower
storage requirements than JVMs that do not. The overall cost of class loading is
also reduced when JVMs use the shared class cache. When a new JVM that
shares the class cache is initialized, it uses the preloaded classes instead of
reading them from the file system. A JVM that shares the class cache still owns
all the working data (objects and variables) for the applications that run in it. This
helps to maintain the isolation between the Java applications being processed in
the system.
The first JVM, after an IPL or after the cache has been destroyed, will take
between 0–5% longer, to fill the cache. The startup time of subsequent JVMs will
decrease by 10–40%, depending on the number of classes being loaded.
The z/OS implementation links pages in the private area of the address space
that uses the cache to the frames of the original location of the cache. Because
shared memory is used, BPXPRMxx parmlib settings affect the cache
performance.
Important settings
Consider these factors when using shared class cache in your environment:
► Cache size limits
The maximum theoretical cache size is 2 GB. The size of cache you can
specify is limited by the amount of physical memory and swap space available
to the system. The cache for sharing classes is allocated using the System V
IPC Shared memory mechanism. Because the virtual address space of a
process is shared between the shared classes cache and the Java heap, if
you increase the maximum size of the Java heap you might reduce the size of
the shared classes cache you can create.
► BPXPRMxx settings for shared memory
The following settings affect the amount of shared memory pages available to
the JVM.
– MAXSHAREPAGES
– IPCSHMSPAGES
– IPCSHMMPAGES
– IPCSHMMSEGS
470 WebSphere Application Server V7.0: Concepts, Planning, and Design