Sample CPExpert "rule" description
extracted from the
MVS Component User Manual


Rule MVS605:    XCF inbound message buffer space may be too small


Finding:    CPExpert has determined that a large percent of the cross system coupling
            facility (XCF) inbound messages were rejected because of constraints on
            the amount of inbound message buffer space.


Impact:     This finding can have a MEDIUM IMPACT or HIGH IMPACT on the
            signalling performance of the sysplex.


Logic flow: This a basic finding.  There are no predecessor rules.


Discussion: The XCF component of MVS/ESA allows authorized programs on one MVS
            system in a sysplex to communicate with programs on the same system or
            on other systems.  A typical example of this communication is between
            CICS regions; CICS regions often communicate with other CICS regions
            in the same system or with CICS regions on other systems in the sysplex.

            Please refer to the discussion associated with Rule MVS601 for additional
            information about XCF buffers.

            Inbound message buffers are used to receive messages from another
            system.  These buffers are allocated, as needed, to support the message
            traffic load.  Message buffer space for inbound messages is separated by
            signalling path.

            Message buffer space for inbound traffic is assigned by the MAXMSG
            parameter on the PATHIN statement for each inbound signalling path.  If
            no MAXMSG parameter is specified, the value on the MAXMSG parameter
            of the COUPLE statement is used as a default buffer space specification.

            Message buffers associated with an inbound signalling path do not receive
            messages over any other inbound signalling path.  If the inbound message
            buffer space required to support messages on a particular inbound
            signalling path is exhausted, MVS will reject additional messages until
            message buffer space becomes available in for the inbound signalling path.

            SMF Type 74 (Subtype 2) provides statistics about the number of inbound
            messages received, where the messages are sent, how many messages
            were rejected because there was insufficient message buffer space, and
            how much input message buffer space was allocated.

            CPExpert analyzes this information to determine whether sufficient
            message buffer space has been defined.  CPExpert computes the total
            inbound message traffic.  CPExpert concludes that the inbound message
            buffer space is too small when more than the value specified for the
            PCTREJ guidance variable of the inbound messages were rejected
            because of no buffer space.  The default specification for the PCTREJ
            guidance variable is %LET PCTREJ = 0.1; indicating that Rule MVS605
            will fire when more than one-tenth of a percent of the inbound traffic is
            rejected for insufficient buffer space.

            CPExpert fires Rule MVS605 to alert you that a significant percent of
            inbound messages have been rejected because of insufficient buffer space.

            The following example illustrates the output from Rule MVS605:





RULE MVS605: THE XCF INBOUND MESSAGE BUFFER SPACE MAY BE TOO SMALL

   The inbound message buffer space may be too small.  CPExpert noticed
   that XCF input requests were rejected because of constraints on the
   amount of input message buffer space.  An asterisk beside the buffer
   space means that the buffer space DECREASED during the reported
   measurement interval, from the preceding measurement interval.  You
   should consider increasing the amount of input message buffer space.
   This finding applies to the following measurement intervals:

                         RECEIVED   TOTAL    REJECTED      PCT     BUFFER
   MEASUREMENT INTERVAL    FROM   REQUESTS   REQUESTS   REJECTED    SPACE
   13:00-13:30,26MAR1996   J80      9,242       462        5.0       500K ***




Suggestion: The available inbound buffer space for an inbound path can be too small
            because (1) the amount initially specified on the PATHIN statement was too
            low, (2) a system operator could have decreased the amount of inbound
            message buffer space for one or more paths, or (3) one or more paths have
            been deleted or have failed.

            If Rule MVS605 is produced, CPExpert suggests that you consider the
            following alternatives:

            -  You should evaluate the amount of message space specified on the
               MAXMSG parameter of the PATHIN statement.  You should consider
               increasing the inbound message buffer space.

            -  You should assess whether a system operator changed the amount of
               inbound message buffer space assigned to an inbound path.  CPExpert
               will notify you (by placing '***' beside the buffer space value) if the
               amount of allocated message buffer space assigned to the inbound path
               decreased from the previous RMF measurement interval.

            -  If the system operator did make a change resulting in less message
               buffer space for an inbound path, you should verify that there was a
               sound rationale for the action.

            -  You should assess whether there has been a decrease in the number of
               inbound paths.  A system operator could have issued the SETXCF STOP
               command to delete a signalling path, or a path could have failed.

            -  When Rule MVS605 is produced, CPExpert often will produce Rule
               MVS620 to identify the outbound/inbound path combination that is
               experiencing problems.

               -  It is possible that Rule MVS605 would be produced but CPExpert
                  cannot identify an outbound/inbound path combination causing
                  problems.  This situation could occur when there is a general
                  problem with the inbound buffer space over all paths, but no path
                  combination causes the problem.

               -  It is possible that Rule MVS605 would not be produced, but CPExpert
                  could produce Rule MVS620.  This situation could occur when there
                  is not a general problem with the inbound buffer space for all paths,
                  but a particular outbound/inbound path combination is experiencing
                  problems.

             -  Rule MVS605 is based on the PCTREJ guidance variable, which
                guides the assessment of rejects of outbound messages (analyzing
                SMF Type 74, Subtype 2, System Data).  Rule MVS620 is based on
                comparing the outbound path BUSY with the inbound path BUFFER
                UNAVAILABLE condition (analyzing SMF Type 74, Subtype 2, Path
                Data).

             -  Since different data are analyzed by different logic paths, it is not
                always possible for CPExpert to produce both Rule MVS605 and Rule
                MVS620.

             - If Rule MVS605 occurs frequently and there is no action you wish take,
               you should change the guidance to CPExpert by altering the PCTREJ
               guidance variable in USOURCE(MVSGUIDE).


Reference:  MVS/ESA: Setting Up a Sysplex (GC28-1449)
              Section 5: Planning Signalling Services in a Sysplex

            MVS/ESA: Initialization and Tuning Reference (GC28-1452)
              COUPLExx (Cross-System Coupling Facility Parameters)

            OS/390: Setting Up a Sysplex (GC28-1779)
              Section 5: Planning Signalling Services in a Sysplex

            OS/390: Initialization and Tuning Reference (GC28-1752)
              COUPLExx (Cross-System Coupling Facility Parameters)

            "Parallel Sysplex Performance: tuning tips and techniques,"
            Kelley, Joan (IBM, Poughkeepsie, NY), SHARE 86, February 1996.

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Last updated by Don Deese on 11/10/97. WLM Component