The z/OS MEMLIMIT parameter limits the amount of 64-bit (above-the-bar) storage that the CICS address space can use. This storage includes the CICS dynamic storage areas above the bar (collectively called the GDSA) and MVS storage in the CICS region outside the GDSA.
A CICS region requires at least 4GB of 64-bit storage. You can’t start a CICS region with a MEMLIMIT value lower than 4GB. If you attempt to do so, CICS issues a message and terminates.
Note: CICS doesn’t try to obtain the MEMLIMIT amount of storage when initializing; 64-bit storage is obtained as required.
CICS Storage Manager domain was greatly enhanced to manage 64-bit storage and provide additional statistical information about 64-bit storage consumption. In addition, for 31-bit storage, the minimum and default EDSALIM values have changed to 48MB to ensure there’s sufficient storage for CICS initialization.
Exploiting 64-Bit Storage
CICS temporary storage is one of the major exploiters of 64-bit storage in CICS TS 4.2. TS main temporary storage queues can now use 64-bit storage. CICS provides new facilities so you can check the storage use of main temporary storage queues and limit that storage use. Auxiliary temporary storage queues and shared temporary storage queues continue to use 31-bit storage.
Main temporary storage is in 64-bit storage rather than 31-bit (above-the-line) storage, depending on the version of z/OS and whether the CICS region operates with transaction isolation. If your CICS applications use large amounts of main temporary storage, the move to 64-bit storage can increase the available storage elsewhere in your CICS regions. An additional new capability is provided to clean up unwanted queues after a specified time interval.
CICS trace domain exploits 64-bit storage (depending on the version of z/OS being used and whether the CICS region operates with transaction isolation) by allocating the CICS internal trace table above the bar. This provides virtual storage constraint relief for 31-bit storage and allows for much larger trace tables to aid problem determination. Trace control blocks and transaction dump trace tables also move above the bar, as do message tables used by the message domain.
For CICS Java applications, all Java Virtual Machines (JVMs) now run in AMODE 64 instead of AMODE 31, increasing the capacity for running more JVMs in a CICS region. JVM servers and pooled JVMs use 64-bit storage, significantly reducing the storage constraints in a CICS region for running Java applications. You can therefore reduce the number of CICS regions that run Java to simplify system management and reduce infrastructure costs. You can also use System z Application Assist Processors (zAAPs) to run eligible Java workloads.
The scalability enhancements provided by CICS Transaction Server for z/OS for V4.2 provide, via the OTE enhancements, the ability for more workload to exploit the power of the mainframe. The 64-bit enhancements provide the ability to scale vertically and do more work in a single CICS region, establishing a foundation for even greater capacity in the future.
- Scalability enhancements in CICS TS 4.2: http://publib.boulder.ibm.com/infocenter/cicsts/v4r2/topic/com.ibm.cics.ts.whatsnew.doc/themes/theme5.html
- Threadsafety learning path: http://publib.boulder.ibm.com/infocenter/cicsts/v4r2/topic/com.ibm.cics.ts.doc/lpaths/threadsafe/overview.html
- “Threadsafe Considerations for CICS” Redbooks publication: www.redbooks.ibm.com/abstracts/sg246351.html.