Chapter 3. z/OS 125
Diagnosis (CTC paths)
Due to the nature of XCF channel programs, queued requests are added to the CCW chain to
increase efficiency; however, this distorts the “AVG Q LNGTH” on the RMF XCF ACTIVITY
report (XCF PATH STATISTICS section) and therefore this is
not a good indicator.
Instead, the DISPLAY XCF,PI,DEV=ALL command, which was updated by XCF APAR
OW38138 to display the “MXFER TIME” for each signal path as an indicator of path response
time. The “MXFER TIME” is the mean transfer time in microseconds for up to the last 64
signals received within the last minute. A value of less than 2 milliseconds (or 2000
microseconds) is an indicator of sufficient CTC capacity for the current workload.
As the DISPLAY XCF command is a snapshot of a small period of time, it may be worthwhile
to generate an RMF Channel Activity report for an extended period (24 hours, 7 days, etc.) to
determine regular periods of peak activity during which the DISPLAY XCF command results
may be assessed for capacity and performance under load conditions.
Resolution (Coupling Facility paths and CTC paths)
This is generally resolved through capacity changes:
Add new signaling paths to the transport group to increase capacity via the SETXCF
START command, or redirect existing under utilized paths via the SETXCF MODIFY
Review the RMF XCF ACTIVITY report (XCF PATH STATISTICS section) for the
connected system for non-zero values for “BUFFER UNAVAIL” for inbound traffic from
connected systems. If this is non-zero it may indicate that the outbound path “BUSY”
count is due to insufficient inbound message buffer capacity on the connected system
causing delayed message delivery.
Review the RMF XCF ACTIVITY report (XCF USAGE BY MEMBER section) to determine
group message counts. Where a transport class supports multiple group workloads it may
be worthwhile routing a high message sending group through a dedicated transport class
with dedicated signaling resources.
Too many paths for a transport class
Where there are multiple outbound signaling paths assigned to a transport class leading to
the same system, it is possible that the transport class is overconfigured and valuable
signaling resources that could be used by other transport classes to the same system may be
Examine the RMF XCF ACTIVITY report (XCF PATH STATISTICS section) for disparity
between the “REQ OUT” counts for multiple signaling paths through the same transport class
to the same system. XCF attempts to use the signaling path that is providing the best
performance and regularly choosing the same path is an indication of excess capacity.
Relocate an under utilized signaling path to another transport group via the SETXCF
MODIFY command with due consideration to redundancy.
3.10.5 XCF message buffer length performance problems
Message buffer space is formatted into buffers of a set length as determined by the
CLASSLEN value for the related CLASSDEF entry as follows: