Subpool 229 may be expanded if batch type terminals send data faster than a CICS® transaction can process that data. The use of secondary to primary pacing, sometimes called inbound pacing, limits the amount of data queued in subpool 229 for any given batch terminal.
PACING controls the flow of traffic from the network control program (NCP) to the terminal and does not affect the processor activity as such. VPACING on the other hand controls the flow of traffic between the host and the NCP.
The VPACING parameter of the CICS APPL statement determines how many messages can be sent in a session to the VTAM® application program by another VTAM logical unit without requiring that an acknowledgment (called a "pacing response") be returned. The host sends data path information units (PIUs) according to the definition of VPACING. The first PIU in a group carries a pacing indicator in the RH. When this PIU is processed by the NCP, the NCP sends a response to the host with the same pacing indicator set to request a new pacing group. This means that, for every x PIUs to a terminal and every y PIUs to a printer, the pacing response traffic must flow from the NCP to the host which, based on the volume of traffic, could cause a significant increase in host activity.
Normally, VPACING is implemented when a shortage of NCP buffers requires controlling the volume of flow between the host and the NCP. You may be able to lessen the effect on the processor by increasing the VPACING value to what the NCP can actually tolerate.
The PACING parameter is required for most printers, to match the buffer capacity with the speed of printing the received data. Terminals do not normally require pacing unless there is a requirement to limit huge amounts of data to one LU, as is the case with some graphics applications. Use of pacing to terminals causes response time degradation. The combination of PACING and VPACING causes both response time degradation and increased processor activity, and increased network traffic.
PACING and VPACING should be specified for all terminals to prevent a "runaway" transaction from flooding the VTAM network with messages and requiring large amounts of buffer storage. If a transaction loops while issuing SENDs to a terminal, IOBUF (CSA storage) and NCP buffers may fill up causing slowdowns and CSA shortage conditions.
PACING and VPACING should always be specified high enough so that normal data traffic may flow without being regulated, but excessive amounts of data are prevented from entering the network and impairing normal data flow.
For secondary to primary pacing, you must code:
The value used is coded on the VPACING parameter. If either of these values are zero, no pacing occurs.
Specify VPACING on the APPL statement defining the CICS region, and any nonzero value for the SSNDPAC parameter on the LU statement defining the batch device. You should ensure that the device supports this form of pacing by referring to the component description manual for that device.
For further information on the selection criteria for values for the PACING and VPACING parameters, see the ACF/VTAM Version 2 Planning and Installation Reference manual.