introduction to parallel execution 4119

8/8/2019 Introduction to Parallel Execution 4119

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Tuning & Tracing Parallel Execution

(An Introduction)

Doug Burns([email protected])


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Introduction Introduction

Parallel Architecture

Configuration

Dictionary Views

Tracing and Wait Events

Conclusion


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Parallel Query Option introduced in 7.1 Now called Parallel Execution

Parallel Execution splits a single large task intomultiple smaller tasks which are handled byseparate processes running concurrently. Full Table Scans

Partition Scans Sorts

Index Creation

And others

Introduction


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A little history

So why did so few sites implement PQO?

Introduction

- Lack of understanding

- Leads to horrible early experiences

- Community's resistance to change

- Not useful in all environments

- Needs time and effort applied to the initial design!

Isnt Oracles Instance architecture parallelanyway?


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Non-Parallel Architecture?Introduction


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Parallel Architecture Introduction


Configuration

Dictionary Views


Conclusion


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Non-Parallel

EM P

Server

Process

User Process

select * from emp;

Parallel

Deg 2

Slave

0

Rea ding 1st

Half

Slave 1

Re a ding 2nd

Half

EMP

QC

User Process

select /*+

parallel(emp,2) */

* from emp;


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The Degree of Parallelism (DOP) refers to thenumber of discrete threads of work

The default DOP for an Instance is calculatedas cpu_count * parallel_threads_per_cpu Used if I dont specify a DOP in a hint or table

definition

The maximum number of PX slaves is :- DOP * 2 Plus the Query Coordinator But this is per Data Flow Operation

And the slaves will be re-used



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Inter-process communication is through message buffers (alsoknown as table queues)

These can be stored in the shared pool or the large pool

Slave

0

Sorting A- P

Slave

1

Sorting Q- Z

Slave

2

Reading 1st

Half

Slave 3

Reading 2nd

Half

EMP

QC

QC

(RANGER)

User Process

select *from emp

order by

name;


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This slide intentionally leftblank


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Methods of invoking Parallel Execution

Table / Index LevelALTERTABLE emp PARALLEL(DEGREE 2);

Optimizer HintsSELECT /*+ PARALLEL(emp) */ *

FROM emp;

Note Using Parallel Execution implies that you willbe using the Cost-based Optimiser

As usual, appropriate statistics are vital Statement Level

ALTER INDEX emp_idx_1 REBUILD

PARALLEL 8;



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Configuration Introduction


Configuration

Dictionary Views


Conclusion


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parallel_automatic_tuning First introduced in Oracle 8i

This is the first parameter you should set - to TRUE

An alternative point of view dont use it! Deprecated in 10G and default is FALSE but much of

the same functionality is implemented

Ensures that message queues are stored in theLarge Pool rather than the Shared Pool

It modifies the values of other parameters As well as the 10g default values, the followingsections show the values whenparallel_automatic_tuning is set to TRUE on previousversions

Configuration


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parallel_adaptive_multi_user First introduced in Oracle 8

Default Value FALSE (TRUE in 10g)

Automatic Tuning Default TRUE Designed when using PX for online usage

As workload increases, new statements will havetheir degree of parallelism down-graded.

Configuration

Effective Oracle by Design Tom Kyte

This provides the best of both worlds and what usersexpect from a system. They know that when it is busy,it will run slower.


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parallel_max_servers Default - cpu_count * parallel_threads_per_cpu * 2

(if using automatic PGA management) * 5 e.g. 1 CPU * 2 * 2 * 5 = 20 on my laptop

The maximum number of parallel execution slavesavailable for all sessions in this instance.

Watch out for the processes trap!

parallel_min_servers Default - 0 May choose to increase this if PX usage is constant

to reduce overhead of starting and stopping slaveprocesses.

Configuration

More on this subject in tomorrows presentation


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parallel_execution_message_size

Default Value 2148 bytes

Automatic Tuning Default 4Kb

Maximum size of a message buffer May be worth increasing to 8Kb, depending on wait

event analysis.

However, small increases in message size could leadto large increases in large pool memoryrequirements

Remember that DOP2 relationship and multiplesessions

Configuration


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Metalink Note 201799.1 contains full detailsand guidance for setting all parameters

Ensure that standard parameters are also set

appropriately large_pool_size Modified by parallel_automatic_tuning

Calculation in Data Warehousing Guide

Can be monitored using v$sgastat

processes Modified by parallel_automatic_tuning

sort_area_size

For best results use automatic PGA management

Be aware of _smm_px_max_size

Configuration


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Dictionary Views Introduction


Configuration Dictionary Views


Conclusion


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Parallel-specific Dictionary Views

SELECT table_nameFROM dict

WHERE table_name LIKE 'V%PQ%' OR table_name like 'V%PX%;

TABLE_NAME------------------------------

V$PQ_SESSTATV$PQ_SYSSTATV$PQ_SLAVEV$PQ_TQSTATV$PX_BUFFER_ADVICEV$PX_SESSION

V$PX_SESSTATV$PX_PROCESSV$PX_PROCESS_SYSSTAT

Also GV$PQ_SESSTAT and GV$PQ_TQSTAT with INST_ID

Dictionary Views


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v$pq_sesstat Provides statistics relating to the current session

Useful for verifying that a specific query is using

parallel execution as expectedSELECT * FROM v$pq_sesstat;

STATISTIC LAST_QUERY SESSION_TOTAL

------------------------------ ---------- -------------

Queries Parallelized 1 1

DML Parallelized 0 0

DDL Parallelized 0 0DFOTrees 1 1

Server Threads 3 0

Allocation Height 3 0

Allocation Width 1 0

Local Msgs Sent 217 217

Distr Msgs Sent 0 0

Local Msgs Recv'd 217 217

Distr Msgs Recv'd 0 0

Dictionary Views


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v$pq_sysstat

The instance-level overview

Various values, including information to help set

parallel_min_servers and parallel_max_servers v$px_process_sysstat contains similar information

SELECT * FROM v$pq_sysstat WHERE statistic like Servers%;

STATISTIC VALUE

------------------------------ ----------

Servers Busy 0

Servers Idle 0

Servers Highwater 3

Server Sessions 3

Servers Started 3

Servers Shutdown 3

Servers CleanedUp 0

Dictionary Views


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v$pq_slave Gives information on the activity of individual PX slaves

v$px_process contains similar information

SELECT slave_name, status, sessions, msgs_sent_total, msgs_rcvd_total

FROM v$pq_slave;

SLAV STAT SESSIONS MSGS_SENT_TOTALMSGS_RCVD_TOTAL

---- ---- ---------- --------------- ---------------

P000 BUSY 3 465 508

P001 BUSY 3 356 290P002 BUSY 3 153 78

P003 BUSY 3 108 63

P004 IDLE 2 249 97

P005 IDLE 2 246 97

P006 IDLE 2 239 95

P007 IDLE 2 249 96

Dictionary Views


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v$pq_tqstat Shows communication relationship between slaves Must be executed from a session thats been using parallel

operations refers to this session

Example 1 Attendance Table (25,481 rows)break on dfo_number on tq_id

SELECT /*+ PARALLEL(attendance, 4) */ *FROM attendance;

SELECT dfo_number, tq_id, server_type, process, num_rows, bytesFROM v$pq_tqstatORDERBY dfo_number DESC, tq_id, server_type DESC, process;

DFO_NUMBER TQ_ID SERVER_TYP PROCESS NUM_ROWS BYTES---------- ---------- ---------- ---------- ---------- ----------

1 0 Producer P000 6605 114616Producer P001 6102 105653Producer P002 6251 110311Producer P003 6523 113032Consumer QC 25481 443612

Dictionary Views


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Example 2 - with a sort operation

SELECT /*+ PARALLEL(attendance, 4) */ *FROM attendanceORDERBY amount_paid;

DFO_NUMBER TQ_ID SERVER_TYP PROCESS NUM_ROWS BYTES---------- ---------- ---------- ---------- ---------- ----------

1 0 Ranger QC 372 13322Producer P004 5744 100069Producer P005 6304 110167Producer P006 6303 109696Producer P007 7130 124060Consumer P000 15351 261380

Consumer P001 10129 182281Consumer P002 0 103Consumer P003 1 120

1 Producer P000 15351 261317Producer P001 10129 182238Producer P002 0 20Producer P003 1 37Consumer QC 25481 443612

Dictionary Views


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So why the unbalanced slaves? Check the list of distinct values in amount_paid

SELECT amount_paid, COUNT(*)

FROM attendance

GROUP BY amount_paid

ORDERBY amount_paid

/

AMOUNT_PAID COUNT(*)

----------- ----------

200 1

850 1900 1

1000 7

1150 1

1200 15340

1995 10129

4000 1

Dictionary Views


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v$px_session and v$px_sesstat Query to show slaves and physical reads

break on qcsid on server_set

SELECT stat.qcsid, stat.server_set, stat.server#, nam.name, stat.valueFROM v$px_sesstat stat, v$statname nam

WHERE stat.statistic# = nam.statistic#AND nam.name = physical readsORDERBY 1,2,3

QCSID SERVER_SET SERVER# NAME VALUE---------- ---------- ---------- -------------------- ----------

145 1 1 physical reads 0

2 physical reads 03 physical reads 0

2 1 physical reads 632 physical reads 563 physical reads 61 physical reads 4792

Dictionary Views


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v$px_process

Shows parallel execution slave processes, status andsession information

SELECT * FROM v$px_process;

SERV STATUS PID SPID SID SERIAL#

---- --------- ---------- ------------ ---------- ----------

P001 IN USE 18 7680 144 17

P004 IN USE 20 7972 146 11

P005 IN USE 21 8040 148 25P000 IN USE 16 7628 150 16

P006 IN USE 24 8100 151 66

P003 IN USE 19 7896 152 30

P007 AVAILABLE 25 5804

P002 AVAILABLE 12 6772

Dictionary Views


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Monitoring the SQL being executed by slaves

set pages 0

column sql_text format a60

select p.server_name,sql.sql_text

from v$px_process p, v$sql sql, v$session s

WHERE p.sid = s.sidAND p.serial# = s.serial#

AND s.sql_address = sql.address AND s.sql_hash_value = sql.hash_value

/

9i Results

P001 SELECTA1.C0 C0,A1.C1 C1,A1.C2 C2,A1.C3 C3,A1.C4 C4,A1.C5 C5,

A1.C6 C6,A1.C7 C7 FROM :Q3000 A1 ORDERBY A1.C0

10g Results

P001 SELECT /*+ PARALLEL(attendance, 2) */ * FROM attendance

ORDERBY amount_paid

Dictionary Views


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Additional information in standard DictionaryViews

e.g. v$sysstat

SELECT name, value FROM v$sysstat WHERE name LIKE 'PX%';

NAME VALUE

---------------------------------------------- ----------

PX local messages sent 4895

PX local messages recv'd 4892

PX remote messages sent 0PX remote messages recv'd 0

Dictionary Views


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Monitoring the adaptive multi-user algorithm We need to be able to check whether operations are

being downgraded and by how much

Downgraded to serial could be a particular problem!SELECT name, value FROM v$sysstat WHERE name LIKE 'Parallel%'

NAME VALUE

---------------------------------------------------------------- ----------

Parallel operations not downgraded 546353

Parallel operations downgraded to serial 432

Parallel operations downgraded 75 to 99 pct 790Parallel operations downgraded 50 to 75 pct 1454

Parallel operations downgraded 25 to 50 pct 7654


Monitoring the adaptive multi-user algorithm We need to be able to check whether operations are

being downgraded and by how much

Downgraded to serial could be a particular problem!SELECT name, value FROM v$sysstat WHERE name LIKE 'Parallel%'

NAME VALUE

------------------ ---------------------------------------------- ----------

Parallel operations not downgraded 546353

P*ssed-off users 432

Parallel operations downgraded 75 to 99 pct 790Parallel operations downgraded 50 to 75 pct 1454



Dictionary Views


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Statspack Example Report (Excerpt)

During overnight batch operation

Mainly Bitmap Index creation Slightly difficult to read

Parallel operations downgraded 1 0




Parallel operations downgraded to 1Parallel operations not downgrade 22

With one stream downgraded to serial, the rest of theschedule may depend on this one job.

Dictionary Views


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Tracing and Wait Events Introduction




Conclusion


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Tracing Parallel Execution operations is morecomplicated than standard tracing One trace file per slave (as well as the query

coordinator)

Potentially 5 trace files even with a DOP of 2

May be in background_dump_dest oruser_dump_dest (usually background_dump_dest)


Optimizing Oracle Performance Millsap and HoltThe remaining task is to identify and analyze all of the

relevant trace files. This task is usually simple


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Much simpler in 10g Use trcsess to generate a consolidated trace file for

QC and all slaves

exec dbms_session.set_identifier(PX_TEST');

REM tracefile_identifier is optional, but might make things easier for you

alter session set tracefile_identifier=PX_TEST';

exec dbms_monitor.client_id_trace_enable(PX_TEST');

REM DO WORK

exec dbms_monitor.client_id_trace_disable(PX_TEST);

GENERATE THE CONSOLIDATED TRACE FILE AND THEN RUN IT THROUGH TKPROF

trcsess output=/ora/admin/TEST1020/udump/PX_TEST.trc clientid=PX_TEST/ora/admin/TEST1020/udump/*px_test*.trc /ora/admin/TEST1020/bdump/*.trc

tkprof /ora/admin/TEST1020/udump/DOUG.trc/ora/admin/TEST1020/udump/DOUG.out



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This is what one of the slaves looks like

C:\oracle\product\10.2.0\admin\ORCL\udump>cd ../bdump

C:\oracle\product\10.2.0\admin\ORCL\bdump>more orcl_p000_2748.trc

*** SERVICE NAME:(SYS$USERS) 2006-03-0710:57:29.812

*** CLIENT ID:(PX_TEST) 2006-03-07 10:57:29.812

*** SESSION ID:(151.24) 2006-03-0710:57:29.812

WAIT #0: nam='PXDeq: Msg Fragment' ela= 13547sleeptime/senderid=268566527 passes=1 p3=0 obj#=-1 tim=3408202924

=====================

PARSING IN CURSOR #1 len=60 dep=1 uid=70 oct=3 lid=70 tim=3408244715hv=1220056081 ad='6cc64000'

select /*+ parallel(test_tab3, 2) */ count(*)

from test_tab3

ENDOF STMT



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Many more wait events and more time spentwaiting

The various processes need to communicate with

each other Metalink Note 191103.1 lists the wait events related

to Parallel Execution

But be careful of what Idle means



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Events indicating consumers or QC are waitingfor data from producers PX Deq: Execute Reply

PX Deq: Table Q Normal

Although considered idle events, if these waits areexcessive, it could indicate a problem in theperformance of the slaves

Investigate the slave trace files



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Events indicating producers are quicker thanconsumers (or QC)

PX qref latch

Try increasingparallel_execution_message_size as

this might reduce the communications overhead

Although it could make things worse if the consumer isjust taking time to process the incoming data.



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Messaging Events PX Deq Credit: need buffer

PX Deq Credit: send blkd

Although there may be many waits, the time spentshould not be a problem.

If it is, perhaps you have an extremely busy server that

is struggling to cope Reduce DOP?

Increase parallel_execution_message_size?

Dont use PX?



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Query Coordinator waiting for the slaves toparse their SQL statements

PX Deq: Parse Reply

If there are any significant waits for this event, this mayindicate you have shared pool resource issues.

Or youve encountered a bug!



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Partial Message Event

PX Deq: Msg Fragment

May be eliminated or improved by increasing

parallel_execution_message_size

Not an issue on recent tests



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Example Excerpt from an overnight Statspack Report

Event Waits Timeouts Time (s) (ms) /txn

direct Path read 2,249,666 0 115,813 51 25.5

PXDeq: Execute Reply 553,797 22,006 75,910 137 6.3PX qref latch 77,461 39,676 42,257 546 0.9

library cache pin 27,877 10,404 31,422 1127 0.3

db file scattered read 1,048,135 0 25,144 24 11.9

Direct Path Reads Sort I/O

Read-ahead

PX Slave I/O

The average wait time SAN!



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Event Waits Timeouts Time (s) (ms) /txn

direct Path read 2,249,666 0 115,813 51 25.5

PXDeq: Execute Reply 553,797 22,006 75,910 137 6.3

PX qref latch 77,461 39,676 42,257 546 0.9

library cache pin 27,877 10,404 31,422 1127 0.3

db file scattered read 1,048,135 0 25,144 24 11.9

PX Deq: Execute Reply

Idle event QC waiting for a response from slaves

Some waiting is inevitable

PX qref latch

Largely down to the extreme use of Parallel Execution Practically unavoidable but perhaps we could increase

parallel_execution_message_size?

Library cache pin? Need to look at the trace files



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Conclusion

Introduction




Conclusion


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Plan / Test / Implement Asking for trouble if you dont!

Hardware

Its designed to suck the server dry Trying to squeeze a quart into a pint pot will make

things slow down due to contention

Tune the SQL first All the old rules apply

The biggest improvements come from doing lessunnecessary work in the first place

Even if PX does make things go quickly enough, itsgoing to use a lot more resources doing so

Conclusion


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Dont use it for small, fast tasks They wont go much quicker They might go slower They will use more resources

Dont use it for online Not unless its a handful of users With a predictable maximum number of concurrent

activities Who understand the implications and wont go crazy

when something takes four times as long as normal! It gives a false initial perception of high performance and

isnt scalable Okay, Tom, set parallel_adaptive_multi_user to TRUE

Conclusion


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The slower your I/O sub-system, the morebenefit you are likely to see from PX

But shouldnt you fix the underlying problem?

More on this in the next presentation

Consider whether PX is the correct parallelsolution for overnight batch operations

A single stream of parallel jobs?

Parallel streams of single-threaded jobs?

Unfortunately youll probably have to do some workto prove your ideas!

Conclusion


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Tuning & Tracing Parallel Execution(An Introduction)

Doug Burns

([email protected])(oracledoug.blogspot.com)(doug.burns.tripod.com)

introduction to parallel execution 4119

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