z/OS Subchannel Tuning

z/OS supports a function called subchannel tuning to dynamically manage and tune the number of in-use subchannels across z/OS LPARs that are sharing coupling CHPIDs. z/OS attempts to reduce the overcommitment of the number of subchannels relative to the underlying shared physical link buffers that can cause path busy conditions. To perform subchannel tuning, each z/OS image monitors its own path busy percentage with respect to each coupling facility over short monitoring time intervals. At the end of each of these intervals:

If the observed path busy percentage is "high," z/OS reduces the number of in-use subchannels in the partition by a small amount while keeping a minimum number of subchannels available for use at all times.
If the observed path busy percentage is "low," z/OS adds back some of the subchannels that were previously removed from use.
Otherwise, z/OS takes no action to change the number of subchannels currently in use.

z/OS makes these changes to gradually tune the number of subchannels in use within each sharing partition and attempt to achieve an acceptable level of path busy as seen by that partition. By reducing the number of subchannels available for use, z/OS can convert the "spin" to re-drive path busy conditions into a software subchannel queueing model (as requests queue up to wait for the scarcer subchannels to become available).

You can see the dynamic effects of subchannel tuning in RMF reports or reports of similar monitoring products. At the end of each monitoring interval the RMF or equivalent report shows the number of configured subchannels and the current "snapshot" of the number of subchannels that are actually in-use. The number of in-use subchannels can fluctuate from interval to interval as a result of subchannel tuning actions. Because the time granularity of z/OS subchannel tuning is very fine (measured in seconds) compared to RMF monitoring intervals (typically 30 or 60 minutes), subchannel tuning actions might have occurred many times during the RMF interval. The reported path busy rates, subchannel queueing rates and other measurements reflect the overall net effects given whatever subchannel tuning occurs during the interval.