Using extended system queue area (ESQA)

The extended system queue area (ESQA) is a major element of z/OS® virtual memory above the 16MB line. This storage area contains tables and queues relating to the entire system, and duplicates above the 16MB line the system queue area (SQA). Kernel services use ESQA in support of several functions. You can use formulas to predict some of the ESQA usage, but others can only be estimated. The maximum amount of ESQA consumed by z/OS UNIX shared memory functions can be limited by the value specified in MAXSHAREPAGES.

The following functions consume ESQA:

Signaling uses SRBs to notify the target of a signal. Signaling frequency is typically not very high and the SRBs are short-lived. For most installations, additional ESQA does not need to be allocated in order to support signaling. If you run applications that use signals frequently, increase your ESQA allocation.
Using asynchronous socket services causes SRBs to be allocated. Allocate an additional 100KB of ESQA if there is heavy use of asynchronous socket services.
The following functions use an MVS™ service called IARVSERV:
- ptrace(), debugger support
- shmat(), shared memory attach
- mmap(), memory map files
- fork() when fork is using Copy on Write (COW) mode
For each real page of shared storage affected by IARVSERV, RSM allocates a 32-byte anchor block in ESQA. For each virtual page connected to a shared real page, RSM allocates a 32-byte control block in ESQA.