13 Copyright © 2006, Oracle. All rights reserved. Tuning PGA and Temporary Space

13Copyright © 2006, Oracle. All rights reserved.

Tuning PGA and Temporary Space

Copyright © 2006, Oracle. All rights reserved.13-2

Objectives

After completing this lesson, you should be able to do the following:

• Diagnose PGA memory issues

• Size the PGA memory

• Diagnose temporary space issues

• Specify temporary tablespace parameters for efficient operation


SQL Memory Usage

• Memory-intensive SQL operators:– Sort-based (sort, group-by, rollup, window, ...)– Hash-join– Bitmap operators (merge and inversion)

• Concept of work area:– Memory allocated by a memory-intensive operator

to process its input data

• Performance impact of memory:– Optimal: Input data fits into the work area (cache).– One-pass: Perform one extra pass over input data.– Multi-pass: Perform several extra passes over input

data.


Performance Impact

Response Time

Memory

..

1-pass optimalM-pass

Sort

Response Time

..Hash-Join

M1-pass Moptimal

M1-pass

Moptimal

1-pass optimalM-passMemory


Automatic PGA Memory

• Dynamically adapts the SQL memory allocation based on:– PGA memory available

– SQL operator needs

– System workload

• Improves manageability:– No need to set *_AREA_SIZE parameters

– DBA sets a memory target: PGA_AGGREGATE_TARGET

• Improves performance:– PGA memory is really returned to the OS.

– Memory is allocated to the operation to maximize throughput.

– Maximize overall memory utilization by dynamically adapting memory with workload variation.

– Operation adapts its memory usage during the execution.


SQL Memory Manager

...

HJ

GB

HJ

WP1

Lo

cal SM

M

HJ

GB

WP5

Server-1

Server-n

SGA

Globalmemorybound Global SMM (CKPT)

AggregatePGA

auto-target

Memory BoundComputation

every 3sWP2

Lo

cal SM

M

WP3

WP4

V$SQL_WORKAREA_ACTIVE

V$SQL_WORKAREA

PGA MemoryUsage

Statistics

PGA_AGGREGATE_TARGET

V$PROCESSV$PROCESS_MEMORY

V$PGASTAT

WP5

WP2

WP1

V$PGASTAT

Auto-TargetComputation

WP3

WP4


Configuring Automatic PGA Memory

• PGA_AGGREGATE_TARGET:– Specifies the target aggregate amount of PGA

memory available to the instance– Can be dynamically modified at the instance level– Examples: 100,000 KB; 2,500 MB; 50 GB– Default value: 10 MB or 20% of the size of the SGA,

whichever is greater

• WORKAREA_SIZE_POLICY:– Optional– Can be dynamically modified at the instance or

session level– Allows you to fall back to static SQL memory

management for a particular session


Setting PGA_AGGREGATE_TARGET Initially

• Leave 20% of the available memory to other applications.

• Leave 80% of memory to the Oracle instance.

• For OLTP:

• For DSS:

PGA_AGGREGATE_TARGET=(total_mem*80%)*20%

PGA_AGGREGATE_TARGET=(total_mem*80%)*50%


Monitoring SQL Memory Usage

V$SQL_WORKAREA_ACTIVE

V$SQL_WORKAREAV$PGASTAT

V$SQL_PLANV$SQL

V$SQL_WORKAREA_HISTOGRAMS

V$PROCESS_MEMORY

V$SYSSTATV$TEMPSEG_USAGE


Monitoring SQL Memory Usage: Examples

SELECT sql_text, sum(onepass_executions) onepass_cnt, sum(multipasses_executions) mpass_cntFROM V$SQL s, V$SQL_WORKAREA wa WHERE s.address = wa.addressGROUP BY sql_textHAVING sum(onepass_executions+multipasses_executions)>0;

SELECT TO_NUMBER(DECODE(sid, 65535, NULL, sid)) sid, operation_type OPERATION, TRUNC(expected_size/1024) ESIZE, TRUNC(actual_mem_used/1024) MEM, TRUNC(max_mem_used/1024) MAXMEM, number_passes PASS, TRUNC(tempseg_size/1024) TSIZEFROM V$SQL_WORKAREA_ACTIVEORDER BY 1,2;

1

2


Tuning SQL Memory Usage

• Determine the best PGA_AGGREGATE_TARGET value by using:– V$PGA_TARGET_ADVICE– V$PGA_TARGET_ADVICE_HISTOGRAM

• Monitor AWR reports/Statspack reports for:– direct path read temp– direct path write temp


PGA Target Advice Statistics

• V$PGA_TARGET_ADVICE predicts how cache hit percentages shown in V$PGASTAT evolve.

• STATISTICS_LEVEL must be set to at least TYPICAL.

V$PGA_TARGET_ADVICE

13%

25%

50%

75%

100%

120%

140%

160%

180%

200%

300%

400%

PGA_TARGET_FACTORESTD_PGA_CACHE_HIT_PERCENTAGE

CurrentvalueO

ve

r a

llo

ca

tio

nzo

ne

Goodvalue

ESTD_OVERALLOC_COUNT

0102030405060708090100


PGA Target Advice Histograms

• V$PGA_TARGET_ADVICE_HISTOGRAM predicts how histograms shown in V$SQL_WORKAREA_HISTOGRAM evolve.

• STATISTICS_LEVEL must be set to at least TYPICAL.


Automatic PGA and Enterprise Manager


Automatic PGA and AWR Reports


Temporary Tablespace Management: Overview

• Temporary data generated by a database include:– Bitmap merges– Hash-join– Bitmap index creation– Sort– Temporary LOBs– Global temporary tables

• Data persists for the duration of a transaction or session.

• High concurrency of the space management operation is critical.

• Media and instance recovery is not required.


Temporary Tablespace: Best Practice

Using locally managed temporary tablespace:

• Allows high-concurrency space management– At steady state, all space metadata is cached in

SGA.– Operations are serialized by the SGA latch.

• Allows faster writes to temp files. Redo generated on temporary blocks is not written to disk.

• Makes READ ONLY databases possible


Configuring Temporary Tablespace

• Locally managed temporary tablespaces are uniform-extent tablespaces.

• 1 MB to 10 MB extent size:– For DSS, OLAP applications involving huge work

areas– Large temporary LOBs are predominant.

• 64 KB or multiple less than 1 MB:– Small global temporary tables are predominant.– OLTP

• Use V$TEMPSEG_USAGE to monitor space usage and workload distribution.


Temporary Tablespace Group: Overview

• Groups multiple temporary tablespaces

• Characteristics:– At least one temporary tablespace– Same namespace as tablespaces– Created implicitly on first assignment– No explicit deletion

Default tablespace

EXAMPLE …

Default temporary tablespace group TEMP

Tablespace

TEMP1

Tablespace

TEMPn


Temporary Tablespace Group: Benefits

Enables a user to use multiple temporary tablespaces:

• Same user in multiple sessions

• One particular parallel operation

Temporary tablespace group TEMP

Tablespace

TEMP1

Tablespace

TEMP2

Tablespace

TEMP3

SerialParallelHR

HR


Creating Temporary Tablespace Groups


Maintaining Temporary Tablespace Groups


Data Dictionary Changes

SELECT group_name, tablespace_nameFROM DBA_TABLESPACE_GROUPS;


Monitoring Temporary Tablespace

• Use V$TEMPSEG_USAGE to monitor space usage and workload distribution:

•

• Use V$SORT_SEGMENT to determine space usage percentage:

SELECT session_num, username, segtype, blocks, tablespaceFROM V$TEMPSEG_USAGE;

SELECT (s.tot_used_blocks/f.total_blocks)*100 as pctusedFROM (SELECT SUM(used_blocks) tot_used_blocks FROM V$SORT_SEGMENT WHERE tablespace_name='TEMP') s, (SELECT SUM(blocks) total_blocks FROM DBA_TEMP_FILES WHERE tablespace_name='TEMP') f;


Practice Overview: Tune PGA Memory

This practice covers the following topics:

• Enable Automatic PGA Memory

• Tune PGA_AGGREGATE_TARGET• Tune Temporary Tablespace Performance


Summary

In this lesson, you should have learned how to:

• Diagnose PGA memory issues

• Size the PGA memory

• Diagnose temporary space issues

• Specify temporary tablespace parameters for efficient operation

Documents

13 Copyright © 2006, Oracle. All rights reserved. Tuning PGA and Temporary Space