The RAPOOL system initialization parameter specifies the number of concurrent receive-any requests that CICS® is to process from VTAM®. RAPOOL determines how many receive-any buffers there are at any time and, therefore, if VTAM has a lot of input simultaneously, it enables VTAM to put all the messages directly into CICS buffers rather than possibly having to store them itself elsewhere. The first operand (value1) is for non-HPO systems, the second operand (value2) is for HPO systems.
The HPO value for the non-HPO operand is derived according to the formula shown in the CICS System Definition Guide. The second operand (value2) for HPO systems is used with minimal adjustment by the formula.
Initially, task input from a terminal or session is received by the VTAM access method and is passed to CICS if CICS has a receive-any request outstanding.
For each receive-any request, a VTAM request parameter list (RPL), a receive-any control element (RACE), and a receive-any input area (RAIA)--the value specified by RAMAX (see Setting the size of receive-any input areas (RAMAX)) are set aside. The total area set aside for VTAM receive-any operations is:
If HPO=YES, both RACE and RPL are above the 16MB line.
See topic Setting the size of receive-any input areas (RAMAX) for RAIA considerations.
In general, input messages up to the value specified in RAPOOL are all processed in one dispatch of the terminal control task. Because the processing of a receive-any request is a short operation, at times more messages than the RAPOOL value may be processed in one dispatch of terminal control. This happens when a receive-any request completes before the terminal control program has finished processing and there are additional messages from VTAM.
VTAM receive-any processing is for the first terminal message in a transaction, so RAPOOL has no effect on further inputs for conversational tasks. Those additional inputs are processed with VTAM receive-specific requests.
The pool is used only for the first input to start a task; it is not used for output or conversational input. VTAM posts the event control block (ECB) associated with the receive any input area. CICS then moves the data to the terminal I/O area (TIOA) ready for task processing. The RAIA is then available for reuse.
Use the RAPOOL operand in networks that use the VTAM access method for terminals.
If the RAPOOL value is set too low, this can result in terminal messages not being processed in the earliest dispatch of the terminal control program, thereby inducing transaction delays during high-activity periods. For example, if you use the default and five terminal entries want to start up tasks, three tasks may be delayed for at least the time required to complete the VTAM receive-any request and copy the data and RPL. In general, no more than 5 to 10% of all receive-any processing should be at the RAPOOL ceiling, with none being at the RAPOOL ceiling if there is sufficient storage.
If the RAPOOL value is set too high, this can use excessive virtual storage, but does not affect real storage because the storage is not page-fixed and is therefore paged out.
Whether RAPOOL is significant or not depends on the environment of the CICS system: whether, for example, HPO is being used.
In some cases, it may sometimes be more economical for VTAM to store the occasional peak of messages in its own areas rather than for CICS itself to have a large number of RAIAs, many of which are unused most of the time.
Furthermore, there are situations where CICS reissues a receive-any as soon as it finds one satisfied. It thereby uses the same element over and over again in order to bring in any extra messages that are in VTAM.
CICS maintains a VTAM RECEIVE ANY for n of the RPLs, where n is either the RAPOOL value, or the MXT value minus the number of currently active tasks, whichever is the smaller. See the the CICS System Definition Guide for more information about these SIT parameters.
A general recommendation is to code RAPOOL with the number of fixed request parameter lists (RPLs) that you require. When it is not at MXT, CICS maintains a receive-any request for each of these RPLs. The number of RPLs that you require depends on the expected activity of the system, the average transaction lifetime, and the MXT specified.
The RAPOOL value you set depends on the number of sessions, the number of terminals, and the ICVTSD value (see topic Adjusting the terminal scan delay (ICVTSD)) in the system initialization table (SIT). Initially, for non-HPO systems, you should set RAPOOL to 1.5 times your peak local 2 transaction rate per second plus the autoinstall rate. This can then be adjusted by analyzing the CICS VTAM statistics and by resetting the value to the maximum RPLs reached.
For HPO systems, a small value (<= 5) is usually sufficient if specified through the value2 in the RAPOOL system initialization parameter. Thus, RAPOOL=20, for example, is specified either RAPOOL=(20) or RAPOOL=(20,5) to achieve the same effect.
RAPOOL is a system initialization parameter.
The CICS VTAM statistics contain values for the maximum number of RPLs posted on any one dispatch of the terminal control program, and the number of times the RPL maximum was reached. This maximum value may be greater than the RAPOOL value if the terminal control program is able to reuse an RPL during one dispatch. See Interpreting VTAM statistics for more information.