Common Tuning
Opportunities
I got the tee shirt
Andrew Jepeal@compuware com
Common Tuning Opportunities
• COBOL
• File I/O
• CICS
• DB2
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COBOL - Table initialization
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Do we have to initialize?
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Keeping a count of rows
Rewriting data anyway
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COBOL Initialize is fast
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Table to Table copy is fastest
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05 COBOL-TABLE-INIT
````````````````````````````````
100-MAIN
INITIALIZE COBOL-TABLE-INIT
````````````````````````````````````````````
MOVE COBOL-TABLE-INIT TO COBOL-TABLE-ORIGINAL
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COBOL - Inspect verb
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It’s expensive
It’s more expensive when using COBOL
reserved words (low-values, spaces)
• Use a literal instead ‘ ‘
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COBOL – Mismatched Data Types
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In:
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Arithmetic
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Counters
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Compares
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Forces a conversion of working storage items,
sometime multiple conversions
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Get the compile listing PMAP (pseudo assembler)
and see exactly what’s going on.
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File I/O - Buffering
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QSAM
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We’re finding buffering isn’t helping anymore. The DASD and Tape
hardware are compensating for bad buffering & blksize(ymmv)
VSAM
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Sequential – data buffers (1/2*CIs per CA +1)
Direct
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Index buffers (Total CIs – Sequence set CIs +1)
Batch LSR (JCL or SMS) – advanced buffering, caches the
whole index and LRR data CIs.
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File I/O – OPEN/CLOSE
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Repetitively opening and closing files
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SVC 19 & 20 (may also include SVCs 18,26,48,56,99,120, 130)
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File I/O – Inserts
High Wait Time and High CA splits
• Is the input file sorted correctly?
• Is the VSAM file in the right order?
• It’s better to insert at the end of file.
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File I/O – Read or update the same data
Look at the number of EXCPs compared to
number of records.
• Number of Logical Operations (read, update) vs.
number of records
• We’ve seen very unusual numbers
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Hundreds of thousands of reads to VSAM files that
contain a few thousand records
Batch LSR is a good band aid but fixing the logic is
the real solution.
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CICS – Memory Management
Module with Getmain / Freemain Descriptors
• SVC 120
• Use the CPU attribution report to determine what
modules / transactions are causing the modules
to be invoked
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CICS – Auxiliary Trace
High CPU to Trace Domain modules(DFHTRPX)
• We turn them on to debug a problem, then never
turn them off
• They can be dynamically turned on/off (CETR)
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CICS – Monitors
High CPU time to modules associated with CICS
monitors
• Monitors are not bad
• Sometimes turning certain options on, is
• Contact the Vendor to discuss
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DB2 – Access Path
• Table Space scan generally bad
• # matching index columns vs predicates
• Stage 1 vs Stage 2 predicates
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DB2 - Reading Data Repetitively
Same as the File I/O, some programs read the
same data over and over (reference data), rather
than reading it once and storing locally.
• Will probably get a DB2 buffer hit so the SQL runs
fast but still incur the CPU overhead of going to
DB2
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DB2 – Improper Cursor Usage
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Reading a whole table with singleton selects
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Probably a place that a table space scan would be
better
The number or executions on the select matches the
number of rows in the table
Using a cursor when only one row is returned
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The number of Opens = the number of Fetches = the
number of Closes
Incurring the overhead of setting up a cursor when all
or most of the time, only one row is returned
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DB2 – Lack of Joins
Especially in shops that migrated from IMS to DB2
• Two or more separate SQL statement that are run
the exact same number of times
• Look for common columns
• Using the joins can significantly reduce the
number of calls to DB2
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DB2 – Full Runstats
Still see a lot of objects that do not have current
Runstats, especially tables that have been set up
for distributed application
• DB2 has gotten better in choosing the correct
access path if you give it the right information to
make the decision
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DB2 – Compression
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See a lot of large tables without DB2
compression.
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It’s not your father’s compression
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DB2 – Useless SQL
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Mostly in DB2 DDF
Thousands of executions in a 15 min period
Distributed app trying to keep a thread or
connection open ?
SELECT COUNT(*) FROM SYSIBM.SYSTABLES
SELECT CURRENT DATE FROM
SYSIBM.SYSDUMMY1
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DB2 – Monitors
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PDT Modules – CA Detector
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