How STATSPACK Was Used to Solve Common Performance Issues Brian Hitchcock OCP 8, 8i, 9i DBA Sun Microsystems [email protected] [email protected] Dedicated

How STATSPACK Was Used to Solve Common Performance Issues

Brian HitchcockOCP 8, 8i, 9i DBA

Sun Microsystems

[email protected]

[email protected]

Dedicated to Pramitha Chowrira, the Goddess of the Rockies,Mike Waldron, the Student who became the Master,Sheryl Driscoll, who leads purely for the glory andAnn Bischoff, who took me in trade for a developer...

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Brian Hitchcock November 13, 2003 Page 1

mailto:[email protected]

mailto:[email protected]

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What STATSPACK Is

Set of SQL and PL/SQL Collects performance data from v$ tables Stores collected data in separate tables Each collection of data is a ‘snapshot’ Reports deltas in data between snapshots Supports ad-hoc SQL queries of the snapshot

data

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STATSPACK Details

Works for 8.1.7 onwards Gathers data for a single instance Snapshot levels

– Determine how much data is collected– Defaults are fine

Snapshot interval of 15 minutes suggested Long report periods miss transient events

– Reports over an instance restart are not valid

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STATSPACK -- Good

Free (very cool!) Gathers a wide range of data

– You don’t know what you’re looking for at first– Root cause isn’t usually obvious

Standard process to collect performance data– Gathers the same data on all instances– Easy to share with vendors, support groups

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STATSPACK -- Not Perfect

Gathers a wide range of data– Ocean of data– Any information?– Easy to get lost

Does not tell you what the problem is– Shows what is happening in the instance– You need to figure out if this is a problem or not

Does not tell you the solution Does not tell you that you are done tuning...

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How to Interpret Output?

Requires experience with your system– No single way to analyze output– Must have history of your system– Look for possible problem areas– Trial and error to change problem behavior– Only you can tell if you have a performance

problem

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STATSPACK Report Sections– Instance Summary, Efficiency– Top 5 Wait Events, Wait Events– SQL Ordered by Gets, Reads, Executions– Instance Activity Stats– Tablespace IO Stats Ordered by IOs, Tblspc-file– Buffer Pool Statistics– Rollback Segment Stats, Storage– Latch Activity, Sleep, Miss Sources– Dictionary Cache Stats– Library Cache Activity– SGA Memory Summary, Breakdown– init.ora Parameters

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Documentation of output No comprehensive documentation

– Oracle 8i Reference Appendix A -- Wait Events defined Appendix B -- Enqueue Names defined Appendix C -- Statistics Descriptions

– Database Performance Guide and Ref 9.0.1 Chapters 21-23, Supplied packages, how to use

– $ORACLE_HOME/rdbms/admin/spdoc.txt– ORACLE High-Performance Tuning with STATSPACK, Donald K.

Burleson, Oracle Press ISBN 0-07-213378-3

No explanation of – What output means for your system

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Configuration Used

Oracle 8.1.7.2 Snapshots every 15 minutes

– snapshots taken continuously

Default STATSPACK snapshot ‘level’ Application loads and analyzes web site click

stream data– Lots of data– More data all the time– We don’t know what vendor code looks like

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Actual Use

4 Performance issues in 2002– Case 1) Reports Running Slow

STATSPACK output didn’t show the problem

– Case 2) Vendor Demo Slow STATSPACK output didn’t show the problem

– Case 3) Data Load Slow STATSPACK output led to 18x speedup (1800%)

– Case 4) Data Load Time Varies STATSPACK output led to the root cause

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Case 1) Reports Running Slow Vendor code allows users to setup reports

– Vendor code generates SQL for report

Long run interferes with next day’s data load STATSPACK captures SQL

– Generate explain plan(s)

Report SQL doesn’t generate where clause properly to use partition pruning

Vendor refuses to change their code We simply removed the reports

– Performance issue ‘resolved’

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Case 2) Vendor Demo Slow

Due to issues like Case 1)– New vendor sets up demo, data load slow– Data load runs twice as fast at vendor– Statspack output doesn’t show anything obvious– Compare configuration of vendor and our dbs– Vendor has only one redo log file per group– We had two redo log files per group– We drop one file per group, performance issue

resolved

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Case 3) Data Load Slow First time loading new type of web log data No baseline to compare with

– Classical performance tuning doesn’t always apply to the real world

Data load so slow no time for daily reporting Must run faster or the data won’t be loaded We don’t know if this load will run faster Do we have a ‘performance’ issue?

– Yes, data load must run faster to be useful– No, perhaps this is as fast as it can be...

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Case 3) Data Load Slow

SQL -- Highest Gets per Exec

SQL ordered by Gets for DB: BHDATA04 Instance: BHDATA04 Snaps: 4426 -4427-> End Buffer Gets Threshold: 10000-> Note that resources reported for PL/SQL includes the resources used by all SQL statements called within the PL/SQL code. As individual SQL statements are also reported, it is possible and valid for the summed total % to exceed 100

Buffer Gets Executions Gets per Exec % Total Hash Value--------------- ------------ -------------- ------- ------------ 634,304 68 9,328.0 97.5 3948948039SELECT t526.keyvalueid FROM bh_lqueryvalue t526 WHERE t526.queryinfoid = :ph0 ORDER BY t526.keyvalueid ASC

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Bad SQL? SQL shouldn’t cost much

– Select looking for one row– Table has two indexes– Explain plan shows ‘index full scan’– Should show ‘index range scan’– Explain plan with hint to force one index– Verify cost of each index– Optimizer is choosing wrong index!

Drop the costly index!– Indexes added by vendor ‘to be safe’...

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Explain Plan Force Index1

SQL> truncate table plan_table;

Table truncated.

SQL> explain plan set Statement_Id = 'TEST' forSELECT /*+ INDEX(t526 X_LQRYVL_QUERYIDKYVL) */t526.keyvalueid FROM bh_lqueryvalue t526WHERE t526.queryinfoid = 100ORDER BY t526.keyvalueid ASC; 2 3 4 5

Explained.

SQL> @$ORACLE_HOME/rdbms/admin/utlxpls

Plan Table--------------------------------------------------------------------------------| Operation | Name | Rows | Bytes| Cost | Pstart| Pstop |--------------------------------------------------------------------------------| SELECT STATEMENT | | 3 | 24 | 1 | | || INDEX RANGE SCAN |X_LQRYVL_ | 3 | 24 | 3 | | |--------------------------------------------------------------------------------

Cost 4

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Explain Plan Force Index2

SQL> explain plan set Statement_Id = 'TEST' forSELECT /*+ INDEX(t526 X_LQYVAL_KYVLQRYID) */t526.keyvalueid FROM bh_lqueryvalue t526WHERE t526.queryinfoid = 100ORDER BY t526.keyvalueid ASC; 2 3 4 5

Explained.

SQL> @$ORACLE_HOME/rdbms/admin/utlxpls

Plan Table--------------------------------------------------------------------------------| Operation | Name | Rows | Bytes| Cost | Pstart| Pstop |--------------------------------------------------------------------------------| SELECT STATEMENT | | 3 | 24 | 2334 | | || INDEX FULL SCAN |X_LQYVAL_ | 3 | 24 | 9333 | | |--------------------------------------------------------------------------------

SQL>

Cost 11,667

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Solution

After dropping costly index– data load time was 18 hours, became 1 hour– 18:1 improvement (1800%)

Why did optimizer choose wrong index?– No idea, Oracle requested running 18 hour data load

to gather instance data– Business users said “NO!”– Indexes created by vendor, no need for both indexes

Know when to quit tuning!

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What About Wait Events?

Popular DBAs– Wait Events are all that matters

I want (desperately) to be popular too…– Return to Case 3)– Examine Top 5 Wait Events section– Try to understand what is causing the wait time

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Case 3) Wait Events

Top 5 Wait Events

Top 5 Wait Events~~~~~~~~~~~~~~~~~ Wait % TotalEvent Waits Time (cs) Wt Time-------------------------------------------- ------------ ------------ -------PX Deq: Execution Msg 335 1,156 45.30latch free 1,740 522 20.45control file parallel write 88 329 12.89db file sequential read 253 323 12.66log file parallel write 2 42 1.65 -------------------------------------------------------------

PX -- parallel query issues?– Contact Oracle Tech Support

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Ask the Experts

Oracle Tech Support– Many wait events in STATSPACK output should

be ignored (a bug perhaps? Or an RFE?)– Requests the full STATSPACK report– Tells me that the latch free wait event must be

addressed– set session_cached_cursors = 100– Performance improvement will be ‘significant’– Data load now takes 19 hours (about 10% worse)

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What Happened? How could the experts miss the bad SQL? Wait events are important

– If the total wait time is the largest problem

In this case– Bad SQL dominated the overall run time

Wait event analysis– Many events should be ignored– Need to determine how much of total run time is due to

wait events only

Go back and fix the SQL issue

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Total CPU Time

Total CPU time is 27475 10s of milliseconds– 10 milliseconds is 1 centisecond (cs) = 0.01 sec– 27457 --> 27475 cs = 274.75 seconds– Report interval was 4.60 minutes (276 seconds)– confused? How many cs left until Happy Hour?

From Instance Activity StatsInstance Activity Stats for DB: BHDATA04 Instance: BHDATA04 Snaps: 4426 -4427

Statistic Total per Second per Trans--------------------------------- ---------------- ------------ ------------CPU used by this session 27,441 99.4 27,441.0CPU used when call started 27,475 99.6 27,475.0………

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Total Wait Time Look at all Wait Events

Wait Events for DB: BHDATA04 Instance: BHDATA04 Snaps: 4426 -4427-> cs - centisecond - 100th of a second-> ms - millisecond - 1000th of a second-> ordered by wait time desc, waits desc (idle events last) Avg Total Wait wait WaitsEvent Waits Timeouts Time (cs) (ms) /txn---------------------------- ------------ ---------- ----------- ------ ------PX Deq: Execution Msg 335 0 1,156 35 335.0latch free 1,740 1,046 522 3 ######control file parallel write 88 0 329 37 88.0db file sequential read 253 0 323 13 253.0log file parallel write 2 0 42 210 2.0enqueue 61 0 40 7 61.0refresh controlfile command 602 0 37 1 602.0PX Deq: Msg Fragment 64 0 29 5 64.0PX Deq: Parse Reply 59 0 26 4 59.0control file sequential read 2,673 0 16 0 ######log file sync 1 0 15 150 1.0PX Deq: Signal ACK 2 2 11 55 2.0PX Deq: Join ACK 57 0 3 1 57.0PX Deq: Execute Reply 5 0 3 6 5.0SQL*Net more data to client 37 0 0 0 37.0file open 6 0 0 0 6.0db file parallel write 4 0 0 0 4.0PX Idle Wait 4,316 4,287 877,818 2034 ######SQL*Net message from client 2,213 0 27,615 125 ######SQL*Net message to client 2,214 0 1 0 ###### -------------------------------------------------------------

-----------> Total Wait Time 907997 cs

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Real Total Wait Time

Remove idle events– MetaLink Note:191103.1 PQ Wait Events– STATSPACK report should filter out idle events– Database Performance Guide and Ref 9.0.1

Explains more about this ‘feature’

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Real Total Wait Time Idle Events

Wait Events for DB: BHDATA04 Instance: BHDATA04 Snaps: 4426 -4427-> cs - centisecond - 100th of a second-> ms - millisecond - 1000th of a second-> ordered by wait time desc, waits desc (idle events last) Avg Total Wait wait WaitsEvent Waits Timeouts Time (cs) (ms) /txn---------------------------- ------------ ---------- ----------- ------ ------PX Deq: Execution Msg 335 0 1,156 35 335.0 <----- removelatch free 1,740 1,046 522 3 ######control file parallel write 88 0 329 37 88.0db file sequential read 253 0 323 13 253.0log file parallel write 2 0 42 210 2.0enqueue 61 0 40 7 61.0refresh controlfile command 602 0 37 1 602.0PX Deq: Msg Fragment 64 0 29 5 64.0PX Deq: Parse Reply 59 0 26 4 59.0control file sequential read 2,673 0 16 0 ######log file sync 1 0 15 150 1.0PX Deq: Signal ACK 2 2 11 55 2.0 <----- removePX Deq: Join ACK 57 0 3 1 57.0PX Deq: Execute Reply 5 0 3 6 5.0SQL*Net more data to client 37 0 0 0 37.0file open 6 0 0 0 6.0db file parallel write 4 0 0 0 4.0PX Idle Wait 4,316 4,287 877,818 2034 ###### <----- removeSQL*Net message from client 2,213 0 27,615 125 ###### <----- removeSQL*Net message to client 2,214 0 1 0 ###### -------------------------------------------------------------

----------> Total Wait Time 1397 cs

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Total Response Time

Total CPU Time + Total Wait Time– 27475 cs + 1397 cs = 28872 cs

Total Wait Time– 1397/28872 = 0.05– 5% of Total Response Time

Wait Time was never an issue! If you don’t remove the idle events

– 907997/(27475+907997) = 97%

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Bad SQL Rules Slow data load time

– Time due to Bad SQL– Time due All Others

Including Wait Events

Bad SQL

All Others

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Case 4) Data Load Time Varies

Data Load Time– Normal 6.5 hours, Long 16 hours– Varies randomly, no pattern

Generate STATSPACK report– Normal, Long– Compare reports– Look for differences between reports

Tablespace IO Stats Section– Normal -- 11 tablespaces accessed– Long -- 74 tablespaces accessed

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Problem and Solution

Vendor data load shouldn’t touch all tables What process would access all tables? Production db supported by another group

– We aren’t allowed to connect as ‘oracle’– Can’t see what they might be running (cron?)

Turns out– Production DBAs decided we needed full exports– We weren’t notified– Stop the exports, performance issue goes away!

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What About Cache Hit Rates? Back to the subject of experts

– Remember when it was cool to discuss hit rates?

For Case 4)– Compute buffer cache hit ratio from tables– Tables larger than physical memory– Can’t have all pages in memory at once– Buffer cache hit ratio won’t be 100%

Even if we had 100%– Bad SQL (index) was the real problem– Buffer cache hit ratio wasn’t relevant

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Select Buffer Cache Hit Ratio

00.20.40.60.8

11.2

Snapshot Id

Hit

Rati

o

Case 4)

Data Load without Exports running

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Total Wait Time? For Case 4), 15 minute report interval

– Wait Time is 28% of total time

Top 5 Wait Events~~~~~~~~~~~~~~~~~ Wait % TotalEvent Waits Time (cs) Wt Time--------------------------------------- ------------ ------------ -------PX Deq: Table Q Normal 495,566 1,017,115 30.12 <-- removeslave wait 25,580 744,985 22.06PX Deq Credit: send blkd 406,262 678,556 20.10 <-- removePX Deq: Execution Msg 33,009 438,309 12.98 <-- removelatch free 42,575 134,840 3.99 ------------------------------------------------------------- 879825 cs

total time = 2237915 + 879825 = 3117740 Wait time is 879825/3117740 = 28%

Instance Activity Stats for DB: BHDATA01 Instance: BHDATA01 Snaps: 13817 -1381

Statistic Total per Second per Trans--------------------------------- ---------------- ------------ ------------CPU used by this session 1,422,921 1,556.8 154.4CPU used when call started 2,237,915 2,448.5 242.8 <-----

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Review For the 4 issues we had, STATSPACK output

– Was useful for all 4 Provided standard set of data for all involved

– Fixed 2 issues Provided the data that led to the root cause Verified that the fix was working

Performance improvements were substantial– Tuning process much faster with STATSPACK– Same process worked for all 4 issues– Decided not to look for further improvements

Wait Time analysis might be useful...

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oraperf.com Analyzer

Website oraperf.com– Submit STATSPACK report– Analyzer reviews report– Generates detailed analysis

CPU time Wait time

– Gives specific advice– Not perfect, but it is fast and free!

Has same issues with idle wait events as STATSPACK report

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oraperf.com Who or what is oraperf.com?

– From the website...

Oraperf.com is run by Anjo Kolk. Anjo has worked for over 16 years at Oracle

(1985-2001). While at Oracle he worked in different countries and different departments.

Many people generate utlbstat/utlestats and statspack reports, but don't know how to interpret the data. People that do look at these are reports also mostly looking at the wrong information and end up making the wrong tuning decisions. That is why the reports are analyzed based on the YAPP method. The YAPP method will show what component of the total response time should be tuned first.

YAPP-Method -- Yet Another Performance Profiling Method

http://oraperf.com/download/yapp_anjo_kolk.pdf

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oraperf.com -- Case 3)

Upload report from slow data load Analyzer shows

– Response time 91.63% CPU Time 8.37% Wait Time

– Advice? Reduce the number of buffer gets or executions

Wait time– Matters only as a % of total response time

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Upload report from fast data load Analyzer shows

– Response time 5.16% CPU Time 94.84% Wait Time

– Advice? Tune PX Deq: Execution Msg event

But this is an idle event...

Non-idle wait time is only about 25% total time

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Conclusion– oraperf.com analyzer provides another tool for

performance tuning– Well worth using if only to compute

Response Time CPU Time Wait Time

Check for idle wait events...

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No Excuses

Install STATSPACK Generate two snapshots Generate standard report Upload to oraperf.com Review advice Fast, free performance analysis!

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Installing STATSPACK

Create separate tablespace Create PERFSTAT user Execute SQL script to create tables Setup job to execute snapshots Setup process to purge data over time Set timed_statistics = TRUE

– Not required, but needed to get wait time data

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Installing STATSPACK

As user ‘SYS’

create tablespace perfstat datafile'/xxx/xxx/perfstat_01.dbf' size 500M;

cd $ORACLE_HOME/rdbms/adminsqlplus [email protected]

Enter value for default_tablespace: perfstatEnter value for temporary_tablespace: temp

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Generate Standard Report

Report SQL supplied by Oracle

sqlplus perfstat/perfstat@<SID>execute statspack.snapexecute statspack.snap

sqlplus perfstat/perfstat@<SID>@$ORACLE_HOME/rdbms/admin/spreport.sqlEnter value for begin_snap: 1Enter value for end_snap: 2Enter value for report_name: testing

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Select STATSPACK Data

Query the tables directlyselect to_char(snap_time,'yyyy-mm-dd HH24') mydate,new.name buffer_pool_name, (((new.consistent_gets-old.consistent_gets)+(new.db_block_gets-old.db_block_gets))-(new.physical_reads-old.physical_reads))/((new.consistent_gets-old.consistent_gets)+(new.db_block_gets-old.db_block_gets)) bhrfromperfstat.stats$buffer_pool_statistics old,perfstat.stats$buffer_pool_statistics new,perfstat.stats$snapshot snwherenew.snap_id > 13125and new.snap_id < 13149andnew.name = old.nameandnew.snap_id = sn.snap_idandold.snap_id = sn.snap_id-1;

Based on SQL fromORACLE High-Performance Tuning with STATSPACKDonald K. BurlesonOracle Press ISBN 0-07-213378-3

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Buffer Cache Hit Ratio Case 4)

Output of SQL on previous slideyr. mo dy Hr BUFFER_POOL_NAME BHR------------- -------------------- -----2002-09-17 02 DEFAULT 1.002002-09-17 02 DEFAULT 1.002002-09-17 02 DEFAULT .942002-09-17 02 DEFAULT .872002-09-17 03 DEFAULT .902002-09-17 03 DEFAULT .932002-09-17 03 DEFAULT .972002-09-17 03 DEFAULT .822002-09-17 04 DEFAULT .782002-09-17 04 DEFAULT .782002-09-17 04 DEFAULT .762002-09-17 04 DEFAULT .802002-09-17 05 DEFAULT .692002-09-17 05 DEFAULT .732002-09-17 05 DEFAULT .762002-09-17 05 DEFAULT .692002-09-17 06 DEFAULT .812002-09-17 06 DEFAULT 1.002002-09-17 06 DEFAULT 1.002002-09-17 06 DEFAULT 1.002002-09-17 07 DEFAULT 1.002002-09-17 07 DEFAULT 1.002002-09-17 07 DEFAULT 1.00

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Space Used

Snapshot size varies with– Number of tablespaces– Number of SQL statements captured

Db1 21 tablespaces --> 0.15 Mb/snapshot Db2 376 tablespaces --> 0.37 Mb/snapshot Assuming a snapshot every 15 minutes

– 96 snapshots per day– Db1 --> 14.4 Mb/day– Db2 --> 35.6 Mb/day

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Removing Snapshot Data

Oracle supplied SQL– SQL removes snapshot data for a range of

snapshot id numbers

Example sqlplus perfstat/perfstat@<SID>@$ORACLE_HOME/rdbms/admin/sppurge...... (listing of all existing snapshots)...Specify the Lo Snap Id and Hi Snap Id range to purge~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Enter value for losnapid: 4001Using 4001 for lower bound.

Enter value for hisnapid: 5000Using 5000 for upper bound.

Deleting snapshots 4001 - 5000.

commit;Note: large deletes may fill rollback segments

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Summary

STATSPACK– Free, easy to install, easy to run– Output can be very useful or confusing– Real-world use has resulted in big performance

gains– Useful for all instances– Standard way to gather performance data

Documents

How STATSPACK Was Used to Solve Common Performance Issues Brian Hitchcock OCP 8, 8i, 9i DBA Sun Microsystems [email protected] [email protected] Dedicated