Common Tuning Opportunities

I got the tee shirt

Andrew Jepeal@compuware com

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Common Tuning Opportunities

• COBOL• File I/O• CICS• DB2

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COBOL - Table initialization

• Do we have to initialize?• Keeping a count of rows• Rewriting data anyway

• COBOL Initialize is fast

• Table to Table copy is fastest• 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

• 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• In:

– Arithmetic

– Counters

– Compares

• Forces a conversion of working storage items, sometime multiple conversions

• Get the compile listing PMAP (pseudo assembler) and see exactly what’s going on.

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File I/O - Buffering

• QSAM• We’re finding buffering isn’t helping anymore. The DASD and

Tape hardware are compensating for bad buffering & blksize(ymmv)

• VSAM • Sequential – data buffers (1/2*CIs per CA +1)• Direct

• 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

• Repetitively opening and closing files

• 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• 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

• Reading a whole table with singleton selects• 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

• 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

• See a lot of large tables without DB2 compression.

• It’s not your father’s compression

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DB2 – Useless SQL

• 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

• PDT Modules – CA Detector