Maximizing coattailing

To maximize coattailing, you need to transfer more data either inbound to the host or outbound without generating an attention interrupt. If more messages are transferred in or out of the host than the number of READ or WRITE channel programs issued, coattailing is taking place. The effect is to transfer more data in a single channel I/O operation. Coattailing can provide greater throughput between a host and an SNA controller; however, response time can increase.

The amount of coattailing that you can achieve is directly related to the message traffic in your network. However, you can influence the amount of data that is available for a data transfer operation.

Coattailing allows more PIUs to be queued for transmission before a channel program is issued. This entails a trade-off between response time and host/controller cycles. Specifying a large value for coattailing delay usually results in a larger number of PIUs being accumulated before a channel program is issued. However, PIUs will experience a longer wait time, and consequently, the overall response time might increase. The benefits of specifying a coattailing delay are dependent on the volume, distribution, and predominant direction (if any) of traffic flow.

The coattailing delay is implemented based on PIU arrivals. The arrival of a PIU causes VTAM® to compute the elapsed time since the beginning of the delay interval and determine if the coattailing delay has been exceeded. This has implications for the benefits of coattailing:

At low traffic volumes, PIU arrivals might be infrequent. This could cause a PIU to stay in the queue until the next PIU arrives or the backup three-second timer expires. Setting a nonzero coattailing delay could delay transmission of a PIU up to three seconds. The coattailing delay should be set to 0 for low traffic volumes.
At high traffic volumes, sufficient numbers of PIUs are queued while a previous channel program is running. This results in "automatic" coattailing. Specifying a coattailing value greater than 0 might not result in greater throughput or reduced host processing cycles.
Coattailing can be beneficial at high processor utilization with moderate traffic volumes. In this case, tuning the coattailing delay parameters might result in reducing the impact on host processor cycles and provide improved throughput.
For predominantly unidirectional data traffic, specifying a coattailing delay for traffic in the nonpredominant direction might result in larger overall delays and reduced throughput. For example, file transfers with application level acknowledgments for segments of files transferred might see lower throughputs with a nonzero coattailing value because the acknowledgment PIUs can be delayed for up to three seconds if no other PIU triggers the coattailing timer.

In general, transaction processing applications using 3270 data streams are characterized by bidirectional traffic with relatively small messages. In this case, a nonzero value of coattailing delay might be beneficial at some traffic volumes. Newer client-server applications are usually characterized by fewer and larger messages. These applications will see minimal benefit with nonzero coattailing delays.