2. DB-Time-based Oracle Performance Tuning: Theory and Practice
RMOUG
Feb 2008
Graham Wood, Uri Shaft, John Beresniewicz
Oracle Corporation

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The following is intended to outline our general product direction. It is intended for information purposes only, and may not be incorporated into any contract. It is not a commitment to deliver any material, code, or functionality, and should not be relied upon in making purchasing decisions. The development, release, and timing of any features or functionality described for Oracle’s products remains at the sole discretion of Oracle.

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

5. Oracle Tuning Methods Prehistory Dark Ages Renaissance Debug code
Counters/Ratios
BSTAT/ESTAT
SQL*Trace
Renaissance
Increased instrumentation inc. Wait Events
Move from counters to timers
STATSPACK

6. 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

7. 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!

8. 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

9. The DB Time Method Supports multiple scopes for diagnosis
Database
Instance
Session
Client id
Module/Action
SQL ID
More inclusive, less intrusive

10. 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.

11. A Single Session Single session with Database Black Box server TIME
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TIME
= time spent in database

12. Fundamental concepts
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

14. 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.

15. Multiple Sessions DB Time = Sum of DB Time Over All Sessions
Avg. Active Sessions = Sum of Avg. Activity Over All Sessions t
At time t we have 2 active sessions
User 1
User 2
User 3
User n
TIME
= time spent in database

16. Wall Clock (Elapsed) Time
The Basic Relationship
Database Time
Avg. Active Sessions =
Wall Clock (Elapsed) Time
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= time spent in database
TIME

17. DB time

18. Breaking down DB Time (example)
Sessions do different database things at different times
User 1
User 2
User 3
User n
TIME
= time spent in database

19. Breaking down DB Time (example)
Maybe I should investigate other wait time?
CPU
I/O
Other Waits
User 1
User 2
User 3
User n
TIME

20. Database time (DB Time)
Database time vs Wall clock time
Database time vs Response time

21. DB time increases as system load increases.
System load and DB time
More users
=> More calls
=> DB time increases
Larger transactions
=> Longer calls
DB time increases as system load increases.

22. System performance and DB time
IO performance degrades
=> IO time increases
=> DB time increases
Application performance degrades
=> Wait time increases
DB time increases as system performance degrades.

23. System performance and DB time

24. Where to find DB time? V$SYS_TIME_MODEL V$WAITCLASSMETRIC_HISTORY
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

25. Where to find DB time? V$SQL V$ACTIVE_SESSION_HISTORY ELAPSED_TIME
Also wait class times
V$ACTIVE_SESSION_HISTORY
Sample per second
Count = time

26. Active Session History (ASH)
Persisted samples of active session information
Sessions contributing to DB time at time of sampling
One-second sampling interval is a great default
Allows simplified AAS computations
DB time and Average active sessions can be computed by aggregating ASH samples

27. Estimating DB time with ASH
ASH sample count is value of active sessions function at sample times
Active sessions
time t0 t1
DB Time
DB time is area under curve
t = 1 sec

28. Integral approximation using ASH

29. EM Top Activity page ASH-estimated DB time by wait class
Aggregated over 15 second intervals

30. Sampled vs. cumulative DB time

31. Where is DB time used? ADDM AWR and AWR compare periods reports
EM Performance page and drill downs
ASH report
Server-generated Alerts

33. Average active sessions
Average active sessions is the rate of change of DB time over time.
Time-normalized DB time
Full-time equivalent sessions
Not whole sessions
How many full-time virtual sessions to do the work?
Comparable
Across systems
Across time periods

34. What are the units? Time / time = unitless?
DB time accumulates in micro- or centi-seconds
Time-normalized metrics are per second of elapsed
Centi-seconds (foreground time) per second (elapsed)
Centi-users per second
User seconds per elapsed second (normalize time units)
Active session seconds per second
Active sessions

35. EM Performance page Cumulative DB time by wait class
v$waitclassmetric_history and v$sysmetric_history
1 minute intervals

36. % activity = DB time *100 / elapsed time Used for individual sessions
Percent Activity
% activity = DB time *100 / elapsed time
Used for individual sessions

37. Percent Activity

38. 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

39. 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?

40. The DB Time Method Scope Set Goal Investigate DB time distribution
Identify the largest potential for improvement
Modify system
Evaluate against Goal

41. The DB Time Method Scope What is the problem? Business Requirements
Resource capacity
Resource contention
System wide or individual Business Function

42. The DB Time Method Set Goal Quantitative
Establishes the STOP TUNING criteria
Should be business driven for applications (X per day)

43. 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

44. 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

45. 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

46. The DB Time Method Need to have examples here
Instance level and SQL level maybe?

47. 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

52. 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

53. 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.