Limiting the expansion of subpool 229 using SNA pacing

Subpool 229 can 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. The PACING parameter controls the flow of traffic from the network control program (NCP) to the terminal and does not affect the processor activity as such. The VPACING parameter 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 z/OS® Communications Server application program by another SNA logical unit without requiring that an acknowledgment (a pacing response) is returned. The host sends data path information units (PIUs) according to the definition of the VPACING parameter. 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 so 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, might cause a significant increase in host activity.

Normally, the VPACING parameter is implemented when a shortage of NCP buffers requires controlling the volume of flow between the host and the NCP. You can lessen the effect on the processor by increasing the VPACING parameter to a value that the NCP can 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 the PACING and VPACING parameters causes both response time degradation and increased processor activity, and increased network traffic.

Specify the PACING and VPACING parameters for all terminals to prevent a “runaway” transaction from flooding the SNA network with messages and requiring large amounts of buffer storage. If a transaction loops while issuing SEND commands to a terminal, IOBUF (CSA storage) and NCP buffers can fill up causing slowdowns and CSA shortage conditions.

Specify the PACING and VPACING parameters high enough so that normal data traffic can flow without being regulated, but excessive amounts of data are prevented from entering the network and impairing the normal flow of data.

For secondary to primary pacing, you must code in the following way:
  • SSNDPAC=nonzero value in the LOGMODE entry pointed to by the secondary application program
  • VPACING=nonzero value on the APPL definition for the secondary application.

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. Ensure that the device supports this form of pacing as specified in the component description manual for that device.