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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