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Dedicated Memory

In the demand paging model, real storage is assigned to the application upon demand, whether that is at the time of an IARV64 GETSTOR or page fault. Some applications perform better when they do not have to compete with other applications for memory.

Dedicated Memory provides a way to assign memory directly to the application at job-step start, while ensuring that the memory remains with the job step until it ends. Even if the system is in a frame shortage, Dedicated Memory is not paged. Logically, Dedicated Memory is defined at the top of Central Storage to take advantage of systems configured with large amounts (>4 Terabytes) of memory that would otherwise be managed as part of the 2G LFAREA.

The installation defines the amount of Dedicated Memory at system initialization time in IARPRMxx and memory is then assigned to applications based on SMFLIMxx specifications. SMFLIMxx has a broad set of filters to determine which applications are to be assigned Dedicated Memory including Sysname, Pgmname, and Jobname. At job-step start, the specified amount of memory in 2G units of real storage is assigned to the address space where the job step is about to run. Once the memory is assigned, Dedicated Memory is locally managed within the address space, minimizing system overhead. Dedicated Memory is restricted to High Virtual private, but can be used for all frame types: 4K, 1M fixed, 1M Pageable and 2G. While the job step is running, the system passively reforms 1M frames, but not 2G frames.

Before the IPL, the System Administrator must decide how much Dedicated Memory to define on the system. DUMPSRV, zCX instances, and other applications that place high demands on memory are recommended applications. The OMVS address space is not eligible for a Dedicated Memory Assignment. Historical real memory usage for High Virtual Private provides a good estimate of the amount of Dedicated Memory to assign to each application. The following SMF 30 fields provide this information:
  • SMF30HVR – High water mark of the number of real storage frames that are used to back 64-bit private storage.
  • SMF30HVA – High water mark of the amount of auxiliary storage that is used to back 64-bit private storage.
  • SMF30_InUseAs2GHWM – High water mark of the number of 2G frames in use by the job step.
  • SMF30_Num2GFailed – Number of 2G frames that could not be obtained because none were available at the time of the IARV64 GETSTOR request.

((SMF30HVR+SMF30HVA)*4K+SMF30_InUseAs2GHWM*2G) is a rough upper bound on memory usage for a particular job step and can be used to estimate the amount of Dedicated Memory to assign. Since Dedicated Memory is only used for High Virtual storage, the amount of Dedicated Memory that is assigned to a job step should not exceed its Memlimit.

About 2G out of every 124G of Dedicated Memory is used by the system to manage Dedicated Memory and therefore is ineligible to be assigned to a job step.

Dedicated Memory can be configured offline and moved to a different LPAR. For more information about storage reconfiguration, see z/OS MVS System Commands.

Figure 1. Dedicated Memory Usage Diagram
Diagram boxing off Dedicated Memory assigned to the job step, Dedicated Memory, and all of central storage
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