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DB-Time-based Oracle Performance Tuning: Theory and Practice Graham Wood, Uri Shaft, John BeresniewiczOracle Corporation RMOUG

Feb 2008

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Agenda

• Brief History of Oracle Performance Tuning Methods• DB Time: What is it?• DB Time: Data Sources• DB Time Method

Oracle Tuning Methods

• Prehistory• Debug code

• Dark Ages• Counters/Ratios

• BSTAT/ESTAT

• SQL*Trace

• Renaissance• Increased instrumentation inc. Wait Events

• Move from counters to timers

• STATSPACK

Oracle Tuning Methods

• More Recent Methods – Time-based methods• YAPP

• Instance tuning - instance statistics• Non intrusive• Always on

• Method R• Session tuning – sql trace based

• Tightly scoped• Must be highly selective

• Modern advances• DB Time Tuning

• Instrumentation improvements• ASH, AWR,ADDM

Why Do We Care About Time? Performance Is Always About Time

• Human time is critical to the enterprise

• System time includes human and IT resource time to accomplish business goals

• System performance affects business goals• “Time is money.”

• Performance improvement usually means doing things faster

• Method: find where system time is spent – reduce it!

The DB Time Method

• Uses combination of cumulative and sampled DB Time• ‘Always on’ data only

• Combines best of current methods• Low intrusion• Detailed data

• No scope necessary for collections• No requirement to reproduce problem• Works for concurrency problems such as locking

The DB Time Method

• Supports multiple scopes for diagnosis• Database• Instance• Session• Client id• Module/Action• SQL ID

• More inclusive, less intrusive

Database time (DB Time)

• Time spent in database calls by foreground sessions

• Includes CPU time, IO time and wait time

• Excludes idle wait time

• The lingua franca for Oracle performance analysis

Database time is total time spent by user processes either actively working or actively

waiting in a database call.

Single session with Database Black Box server

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TIME = time spent in database

A Single Session

Active Session = Session currently spending time in a database call

Database Time (DB Time) = Total time session spent in all database calls

Average Activity of the Session (% Activity) = The ratio of time active to total wall clock time

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TIME = time spent in database

Fundamental concepts

Active sessions

• Foreground sessions in a database call• Backgrounds are also interesting

• Either on CPU, waiting for IO, or waiting (not idle)

• V$ACTIVE_SESSION_HISTORY is a collection of timed regular samples of active session attributes

Active sessions are foreground sessions contributing to DB time in any given moment.

= time spent in database

Multiple Sessions

TIME

User 1

User 2

User 3

User n

t

At time t we have 2 active sessions

DB Time = Sum of DB Time Over All Sessions

Avg. Active Sessions = Sum of Avg. Activity Over All Sessions

= time spent in database

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TIME

Avg. Active Sessions =Wall Clock (Elapsed) Time

Database Time

The Basic Relationship

DB time

System load and DB time

• More users• => More calls

• => DB time increases

• Larger transactions• => Longer calls

• => DB time increases

DB time increases as system load increases.

System performance and DB time

• IO performance degrades • => IO time increases

• => DB time increases

• Application performance degrades• => Wait time increases

• => DB time increases

DB time increases as system performance degrades.

System performance and DB time

Where to find DB time?

• V$SYS_TIME_MODEL• STAT_NAME = ‘DB time’• Accumulated value over entire instance

• V$WAITCLASSMETRIC_HISTORY• AVERAGE_WAITER_COUNT

• It is precisely Average Active Sessions

• V$SYSMETRIC_HISTORY • “Database Time Per Second”, “CPU Usage Per Sec” • Units are Centi-seconds per second• Value is 100 x Average Active Sessions

Where to find DB time?

• V$SQL• ELAPSED_TIME• Also wait class times

• V$ACTIVE_SESSION_HISTORY• Sample per second• Count = time

timet0 t1

Estimating DB time with ASH

Active sessions

t = 1 sec

ASH sample count is value of active sessions function at sample times

DB Time

DB time is area under curve

Integral approximation using ASH

second) 1 (where

1*

*)(lim

)( time DB

1

0

10

t1

t0

t

ASHsamples

ttionsActiveSess

dttionsActiveSess

tsampletime

tsampletime

n

kk

t

EM Top Activity page

• ASH-estimated DB time by wait class• Aggregated over 15 second intervals

Sampled vs. cumulative DB time

Where is DB time used?

• ADDM

• AWR and AWR compare periods reports

• EM Performance page and drill downs

• ASH report

• Server-generated Alerts

EM Performance page

• Cumulative DB time by wait class• v$waitclassmetric_history and v$sysmetric_history• 1 minute intervals

DB Time Tuning

• DB Time can be aggregated at multiple levels:• Database / instance• Service / module / action• Session / user / client id• SQL id / rowsource

• Performance improvement for Oracle database means doing the same work in less DB Time

Performance Problem Resolution 101

• Discover the problem:• User phone call or other complaint • Metric threshold alert or system monitoring

• Scope the problem:• How widespread is it?• How severe is it?• In other words: Who or what is wasting DB Time and how

much is being wasted?

• Diagnose the problem

• Scope the solution• How much of the pain can be relieved?

The DB Time Method

1. Scope

2. Set Goal

3. Investigate DB time distribution

4. Identify the largest potential for improvement

5. Modify system

6. Evaluate against Goal

The DB Time Method

• Scope• What is the problem?

• Business Requirements• Resource capacity• Resource contention

• System wide or individual Business Function

The DB Time Method

• Set Goal• Quantitative • Establishes the STOP TUNING criteria• Should be business driven for applications (X per day)

The DB Time Method

• Investigate DB time distribution• Identify major contributors to DB time at the selected scope• System scope

• V$SYS_TIME_MODEL• V$ACTIVE_SESSION_HISTORY• V$SQL• Identify high load service, sessions and SQL• Identify resource constraints or contention

The DB Time Method

• Investigate DB time distribution• Session scope

• V$SESS_TIME_MODEL• V$ACTIVE_SESSION_HISTORY• Identify if database is the problem• Identify high load SQL• Identify application efficiency issues• Identify resource constraints or contention

The DB Time Method

• Identify the largest potential for improvement• What can be changed that will produce the greatest

reduction in ‘scoped’ DB time?• Parameters• System• Application• SQL• Design

• Modify system

The DB Time Method

• Evaluate against Goal • Did our changes to the system achieve our goal?• If not return to step 3 and repeat • If we have reached our goal STOP

Summary

DB Time is the fundamental performance metric

The DB Time Method uses many different sources of DB time within the database to allow many different scopes of performance tuning

Time based diagnosis removes ‘value judgments’ from performance analysis

New In 11g – Enhancements for RAC

• ADDM has “Database” analysis mode• New AWR “Database” report• EM Performance screens for RAC enhanced to

support new server capabilities.