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Tune Key Variables Affecting Performance


This variable controls whether a spawned child process will be started in the same address space as the login shell. The OMVS command, which is used to start an OpenEdition shell from TSO, also has a SHAREAS/NOSHAREAS option.

Some functions will not work in a SHAREAS=YES environment:

Functions which change the security environment
For example, newgrp cannot be issued when _BPX_SHAREAS=YES, because all processes running in the address space would have their security environment altered. Similarly, SETUID or SETGID programs must run in a separate address space. APF considerations may prevent the use of SHAREAS=YES.
Functions that require exclusive use of address space resources
For example, if a process requires a large amount of memory, it may not be able to execute in the same address space as the original TSO session. Also, processes running in the same address space share the same data set allocations so they are responsible for coordinating their use.

Some functions will behave differently in a SHAREAS=YES environment:

CPU time accumulation
The CPU time limit for an address space applies to the total CPU consumed by all processes running in that address space. However, OMVS services that affect the CPU time limit only examine the CPU time consumed by the current process.
Virtual memory protection
A process may be able to overwrite memory in use by another process running in the same address space. It may be desirable to isolate an application from other processes while it is being debugged.


This variable controls whether a shell script will be run directly by the spawn callable service. Setting _BPX_SPAWN_SCRIPT=YES improves performance, but may produce undesirable behaviour under certain circumstances - for instance, when spawn is being invoked from an application that is not intended to support shell script invocation.

These functions cannot be used with the tcsh shell.
Big Iron