Contention for CF paths

When a CF request obtains a subchannel, in most cases, it will proceed down the path to the CF and complete the processing with no further delays. However, if this is a PR/SM environment with multiple z/OS images sharing coupling facility links, the request can encounter a "path busy" condition.

A path busy condition occurs when a coupling facility operation is attempted on a subchannel and at that moment physical resources ("link buffers") that are associated with any of the currently online CHPIDs for the target coupling facility are not available. Without an available link buffer to use, the system cannot start the request. The request is rejected synchronously with a path busy indication that z/OS handles by re-driving the request until a link buffer is available; at that time, the request is successfully accepted for delivery to the coupling facility.

Path busy conditions generally occur only when coupling CHPIDs are shared across multiple z/OS LPARs and when overcommitment of the number of active subchannels exists across the sharing LPARs relative to the number of link buffers. Each coupling CHPID provides a fixed number of physical link buffers that can be used to process coupling facility requests. Each sharing z/OS image is assigned a number of subchannels equal to the number of link buffers; therefore, each sharing z/OS image is capable of using the entire link buffer capacity that is associated with the shared CHPID. However, if multiple sharing z/OS images try to initiate a large enough number of coupling facility requests, all of the available link buffers in the shared CHPID become saturated before the aggregated subchannels across all of the sharing LPARs are saturated. This excess demand causes path busy conditions to occur. In effect, you can consider path busy conditions as the contention caused by too many subchannels chasing after too few link buffers.

Path busy conditions are handled by z/OS synchronously re-driving them. The amount of time spent doing such re-drive processing is included in the reported service time for the coupling facility request. Additionally, z/OS counts and reports the total number of coupling facility requests that experienced path busy conditions before being successfully initiated. Therefore, there are two ways to observe the occurrence and effects of path busy conditions:

By determining the percentage of requests that experienced one or more path busy conditions (count of requests that experienced one or more path busy conditions divided by the total number of coupling facility requests)
By observing an elongation of the service time associated with coupling facility requests, representing the time spent re-driving those requests.

Note that many different sysplex configuration and workload-related factors can affect the occurrence of path busy conditions, such as the following:

Insufficient total link capacity (that is, too few shared CHPIDs, given the coupling facility workload)
"Over-shared" link capacity (that is, CHPIDs shared across too many z/OS LPARs, given the coupling facility workload)
Intensive coupling facility workloads that drive too many concurrent coupling facility requests (enough to saturate all available link buffers) across all sharing LPARs during steady-state operation.
Workloads that normally do not drive enough concurrent coupling facility requests to saturate the available link buffers except during occasional "bursts" when the link buffers can become saturated.
Workloads that contain many long-running coupling facility operations, which occupy link buffers for relatively long periods of time.
Various configuration factors that cause increased coupling facility service times, such as extended link distance (for example, in GDPS environments), use of system-managed duplexing, high coupling facility utilization, shared-processor coupling facility images, and so forth. These conditions all cause coupling facility operations to occupy link buffers for relatively long periods of time.

The system reports the total number of requests that experienced one or more "Path Busy" conditions on the RMF CF Subchannel Activity report in the column labeled PTH BUSY.