The default minimum window size might be too small for a VTAM® channel-to-channel virtual route. VTAM increases the window size only when the virtual route pacing response is not returned fast enough to prevent one window worth of PIUs from being queued up on the virtual route (a HELD condition). A channel-to-channel route is so fast that the virtual route pacing responses are turned around fast enough to prevent HELD conditions from occurring very often. Because of this, the current virtual route window size tends not to increase, but to stay very close to the minimum VR window size.

The smaller the VR window size, the more virtual route pacing responses flow over the channel. This can cause higher CPU utilization than necessary.

Setting a higher minimum window size reduces the number of virtual route pacing responses and helps CPU utilization and throughput. For channel-to-channel one-hop routes, the default minimum VR window size is 1. The recommended value is 15. The recommended maximum virtual route pacing window size value is 50 for this environment.

If VTAM is expanding its IOBUF buffer pool to service a virtual route, the lack of VTAM buffers is considered major congestion for the route. Therefore, VTAM decreases the virtual route window size to the minimum for all VRs that it services. You should monitor this VTAM buffer pool to ensure that there is not excessive expansion and contraction if you are using dynamic buffering.

You can also use VTAM tuning statistics to analyze a virtual route impact on the channel-to-channel connection. A virtual route that uses transmission priority two and traverses the channel-to-channel connection causes VTAM to immediately schedule the data transfer operation. A virtual route pacing response is also high-priority traffic, which has the same effect. The PRI tuning statistic in VTAM indicates the number of times that a VTAM channel program is started to transfer this high priority data (TP2 or VR pacing response) to the other VTAM host. If this number is high and the channel-to-channel connection is not used extensively for TP2 traffic, the minimum virtual route pacing window size is probably too small. Also, the higher this number is in relation to the sum of TIMERS, QDPTH, and BUFCAP, the less outbound coattailing occurs.

For more information about pacing, see Session-level pacing.