User address space for x86 servers
The user address space for x86 servers is generated with a size of 2 GB and cannot be modified.
User address space for /390 servers
For /390 servers the total address space consists of the user address space (including the space requirement for shared products; see the SHRSIZE parameter) and the system address space. For the size of the total address space on /390 servers there is a choice of the values 1 or 2 GB.
On S servers the size of the user address space automatically determines the size of the system address space as the difference to the next higher value of the total address space. It is no longer necessary to specify the SYSSIZE parameter in the parameter service during system startup.
By default, the user address space for /390 servers is defined with a size of 1,808 MB. This size can be changed using the SYSPRC.BS2000-EXEC.<version> procedure (see the “System Installation” manual [32 (Related publications)]).
With configurations which require a larger system address space, the user address space must be reduced correspondingly (e.g. to 1,792 MB when SYSSIZE=256 MB, with all intermediate values being possible from 240 through 512 MB at intervals of 16 MB).
One of the reasons the subdivision of the total address space was introduced was to reduce the main memory requirement for the address space management (segment table). Depending on the size of the total address space (1 or 2 GB), the following main memory per task is occupied for the segment tables:
4 KB for an address space of 1,024 MB
8 KB for an address space of 2,048 MB
With a small number of tasks, selection of the largest possible address space (2 GB) is not critical. With a large number of tasks, you must check whether the extra main memory required by some large applications is justified.
Page-table management requires 3 KB main memory per megabyte of occupied address space and task. If, for example, 10 tasks completely occupy the maximum available user address space of 1,808 MB, the following main memory (resident class-3 system address space) is required for managing the address space alone (segment table and page table)
10 * 8 KB = 80 KB for segment table
10 * 1.808 MB * 3 KB/MB = 54.240 KB or page table
Sum 54.320 KB
With large virtual address spaces, you must make sure that the paging areas are large enough. The user-specific size (/ADD-USER, /MODIFY-USER-ATTRIBUTES) must be defined carefully. If many large address spaces are to be used, a correspondingly large sized-main memory is required.