16.20 - Anticipated-number-of-concurrent-sessions - Teradata Tools and Utilities

Teradata® Call-Level Interface Version 2 Reference for Mainframe-Attached Systems

prodname
Teradata Tools and Utilities
vrm_release
16.20
created_date
September 2019
category
Programming Reference
featnum
B035-2417-108K

Anticipated-number-of-concurrent-sessions is a four-byte field that refers to the maximum number of sessions expected to be concurrently established.

In this language... The variable name for Anticipated-number-of-concurrent sessions is...
COBOL DBCAREA-MAX-NUM-SESS
PL/I MAX_NUM_SESS
C, C++ max_num_sess
IBM Assembler DBCISMAX
Routine Action
DBCHINI writes
DBCHCL reads (CON; RSUP; IRQ; IWPF; CMD)
DBCHWAT  
Maximum Number of Sessions is used by... To...
applications write

The value of zero for Anticipated-number-of-concurrent-sessions indicates that the default value from the HSHSPB is used for the Anticipated-number-of-concurrent-sessions. The application may change the value of Anticipated-number-of-concurrent-sessions by supplying the preferred value in the DBCAREA. The minimum value is 1, and the maximum value is 999.

Anticipated-number-of-concurrent-sessions is used for two purposes. When CLIv2 must wait for one or more sessions to complete, it must build a list of system-dependent control blocks on which it will wait. If CLIv2 obtains a list of a particular size and then on a subsequent call needs a larger list, it must free the existing list and obtain a larger one. You can avoid this overhead by obtaining an appropriately sized list the first time. CLIv2 rounds the specified value up to the next multiple of 16 and obtains a list of this size.

When a request or response is processed, CLIv2 must lookup the CLIv2 internal information for that session. The larger the number of simultaneous sessions, the less efficient such session lookup can be. CLIv2 uses the value set for the Anticipated-number-of-concurrent-sessions when the first session is connected to optimize the internal lookup algorithm. So setting an accurate value initially can reduce the CPU used by CLIv2 on all subsequent calls.