Introduction to Parallel Execution

Tuning & Tracing Parallel Execution (An Introduction) Doug Burns (dougburns@yahoo.com)

Introduction Introduction Parallel Architecture Configuration Dictionary Views Tracing and Wait Events Conclusion

Parallel Query Option introduced in 7.1 Now called Parallel Execution Parallel Execution splits a single large task into multiple smaller tasks which are handled by separate processes running concurrently. Full Table Scans Partition Scans Sorts Index Creation And others … Introduction

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! Isn’t Oracle’s Instance architecture parallel anyway?

Non-Parallel Architecture? Introduction

Parallel Architecture Introduction Parallel Architecture Configuration Dictionary Views Tracing and Wait Events Conclusion

Parallel Architecture Non-Parallel Parallel Deg 2

The Degree of Parallelism (DOP) refers to the number of discrete threads of work The default DOP for an Instance is calculated as cpu_count * parallel_threads_per_cpu Used if I don’t 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 Parallel Architecture

Parallel Architecture Inter-process communication is through message buffers (also known as table queues) These can be stored in the shared pool or the large pool

Parallel Architecture This slide intentionally left blank

Methods of invoking Parallel Execution Table / Index Level ALTER TABLE emp PARALLEL(DEGREE 2); Optimizer Hints SELECT /*+ PARALLEL(emp) */ * FROM emp; Note Using Parallel Execution implies that you will be using the Cost-based Optimiser As usual, appropriate statistics are vital Statement Level ALTER INDEX emp_idx_1 REBUILD PARALLEL 8; Parallel Architecture

Configuration Introduction Parallel Architecture Configuration Dictionary Views Tracing and Wait Events Conclusion

parallel_automatic_tuning First introduced in Oracle 8i This is the first parameter you should set - to TRUE An alternative point of view – don’t use it! Deprecated in 10G and default is FALSE but much of the same functionality is implemented Ensures that message queues are stored in the Large Pool rather than the Shared Pool It modifies the values of other parameters As well as the 10g default values, the following sections show the values when parallel_automatic_tuning is set to TRUE on previous versions Configuration

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 have their degree of parallelism down-graded. Configuration Effective Oracle by Design Tom Kyte ‘ This provides the best of both worlds and what users expect from a system. They know that when it is busy, it will run slower.’

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 slaves available 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 slave processes. Configuration More on this subject in tomorrow’s presentation

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 lead to large increases in large pool memory requirements Remember that DOP 2 relationship and multiple sessions Configuration

Metalink Note 201799.1 contains full details and 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 Metalink Note 201799.1 contains full details and guidance for all relevant parameters Configuration

Dictionary Views Introduction Parallel Architecture Configuration Dictionary Views Tracing and Wait Events Conclusion

Parallel-specific Dictionary Views SELECT table_name FROM dict WHERE table_name LIKE 'V%PQ%' OR table_name like 'V%PX%‘; TABLE_NAME ------------------------------ V$PQ_SESSTAT V$PQ_SYSSTAT V$PQ_SLAVE V$PQ_TQSTAT V$PX_BUFFER_ADVICE V$PX_SESSION V$PX_SESSTAT V$PX_PROCESS V$PX_PROCESS_SYSSTAT Also GV$PQ_SESSTAT and GV$PQ_TQSTAT with INST_ID Dictionary Views

v$pq_sesstat Provides statistics relating to the current session Useful for verifying that a specific query is using parallel execution as expected SELECT * FROM v$pq_sesstat; STATISTIC LAST_QUERY SESSION_TOTAL ------------------------------ ---------- ------------- Queries Parallelized 1 1 DML Parallelized 0 0 DDL Parallelized 0 0 DFO Trees 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

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 Cleaned Up 0 Dictionary Views

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_TOTAL MSGS_RCVD_TOTAL ---- ---- ---------- --------------- --------------- P000 BUSY 3 465 508 P001 BUSY 3 356 290 P002 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

v$pq_tqstat Shows communication relationship between slaves Must be executed from a session that’s been using parallel operations – refers to this session Example 1 – Attendance Table (25,481 rows) break on dfo_number on tq_id SELECT /*+ PARALLEL (attendance, 4) */ * FROM attendance; SELECT dfo_number, tq_id, server_type, process, num_rows, bytes FROM v$pq_tqstat ORDER BY dfo_number DESC, tq_id, server_type DESC, process; DFO_NUMBER TQ_ID SERVER_TYP PROCESS NUM_ROWS BYTES ---------- ---------- ---------- ---------- ---------- ---------- 1 0 Producer P000 6605 114616 Producer P001 6102 105653 Producer P002 6251 110311 Producer P003 6523 113032 Consumer QC 25481 443612 Dictionary Views

Example 2 - with a sort operation SELECT /*+ PARALLEL (attendance, 4) */ * FROM attendance ORDER BY amount_paid; DFO_NUMBER TQ_ID SERVER_TYP PROCESS NUM_ROWS BYTES ---------- ---------- ---------- ---------- ---------- ---------- 1 0 Ranger QC 372 13322 Producer P004 5744 100069 Producer P005 6304 110167 Producer P006 6303 109696 Producer P007 7130 124060 Consumer P000 15351 261380 Consumer P001 10129 182281 Consumer P002 0 103 Consumer P003 1 120 1 Producer P000 15351 261317 Producer P001 10129 182238 Producer P002 0 20 Producer P003 1 37 Consumer QC 25481 443612 Dictionary Views

So why the unbalanced slaves? Check the list of distinct values in amount_paid SELECT amount_paid, COUNT(*) FROM attendance GROUP BY amount_paid ORDER BY amount_paid / AMOUNT_PAID COUNT(*) ----------- ---------- 200 1 850 1 900 1 1000 7 1150 1 1200 15340 1995 10129 4000 1 Dictionary Views

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.value FROM v$px_sesstat stat, v$statname nam WHERE stat.statistic# = nam.statistic# AND nam.name = ‘physical reads’ ORDER BY 1,2,3 QCSID SERVER_SET SERVER# NAME VALUE ---------- ---------- ---------- -------------------- ---------- 145 1 1 physical reads 0 2 physical reads 0 3 physical reads 0 2 1 physical reads 63 2 physical reads 56 3 physical reads 61 physical reads 4792 Dictionary Views

v$px_process Shows parallel execution slave processes, status and session information SELECT * FROM v$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 25 P000 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

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.sid AND p.serial# = s.serial# AND s.sql_address = sql.address AND s.sql_hash_value = sql.hash_value / 9i Results P001 SELECT A1.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 ORDER BY A1.C0 10g Results P001 SELECT /*+ PARALLEL (attendance, 2) */ * FROM attendance ORDER BY amount_paid Dictionary Views

Additional information in standard Dictionary Views 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 0 PX remote messages recv'd 0 Dictionary Views

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$sysstat WHERE name LIKE 'Parallel%' NAME VALUE ---------------------------------------------------------------- ---------- Parallel operations not downgraded 546353 Parallel operations downgraded to serial 432 Parallel operations downgraded 75 to 99 pct 790 Parallel operations downgraded 50 to 75 pct 1454 Parallel operations downgraded 25 to 50 pct 7654 Parallel operations downgraded 1 to 25 pct 11873 Dictionary Views Monitoring the adaptive multi-user algorithm We need to be able to check whether operations are being downgraded and by how much Downgraded to serial could be a particular problem! SELECT name, value FROM v$sysstat WHERE name LIKE 'Parallel%' NAME VALUE ------------------ ---------------------------------------------- ---------- Parallel operations not downgraded 546353 P*ssed-off users 432 Parallel operations downgraded 75 to 99 pct 790 Parallel operations downgraded 50 to 75 pct 1454 Parallel operations downgraded 25 to 50 pct 7654 Parallel operations downgraded 1 to 25 pct 11873

Statspack Example Report (Excerpt) During overnight batch operation Mainly Bitmap Index creation Slightly difficult to read Parallel operations downgraded 1 0 Parallel operations downgraded 25 0 Parallel operations downgraded 50 7 Parallel operations downgraded 75 38 Parallel operations downgraded to 1 Parallel operations not downgrade 22 With one stream downgraded to serial, the rest of the schedule may depend on this one job. Dictionary Views

Tracing and Wait Events Introduction Parallel Architecture Configuration Dictionary Views Tracing and Wait Events Conclusion

Tracing Parallel Execution operations is more complicated 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 or user_dump_dest (usually background_dump_dest) Tracing and Wait Events Optimizing Oracle Performance Millsap and Holt ‘ The remaining task is to identify and analyze all of the relevant trace files. This task is usually simple …’

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 Tracing and Wait Events

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 <SNIPPED> *** SERVICE NAME:(SYS$USERS) 2006-03-07 10:57:29.812 *** CLIENT ID:(PX_TEST) 2006-03-07 10:57:29.812 *** SESSION ID:(151.24) 2006-03-07 10:57:29.812 WAIT #0: nam= 'PX Deq: Msg Fragment' ela= 13547 sleeptime/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=3408244715 hv=1220056081 ad='6cc64000' select /*+ parallel(test_tab3, 2) */ count(*) from test_tab3 END OF STMT Tracing and Wait Events

Many more wait events and more time spent waiting 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 Tracing and Wait Events

Events indicating consumers or QC are waiting for data from producers PX Deq: Execute Reply PX Deq: Table Q Normal Although considered idle events, if these waits are excessive, it could indicate a problem in the performance of the slaves Investigate the slave trace files Tracing and Wait Events

Events indicating producers are quicker than consumers (or QC) PX qref latch Try increasing parallel_execution_message_size as this might reduce the communications overhead Although it could make things worse if the consumer is just taking time to process the incoming data. Tracing and Wait Events

Messaging Events PX Deq Credit: need buffer PX Deq Credit: send blkd Although there may be many waits, the time spent should 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? Don’t use PX? Tracing and Wait Events

Query Coordinator waiting for the slaves to parse their SQL statements PX Deq: Parse Reply If there are any significant waits for this event, this may indicate you have shared pool resource issues. Or you’ve encountered a bug! Tracing and Wait Events

Partial Message Event PX Deq: Msg Fragment May be eliminated or improved by increasing parallel_execution_message_size Not an issue on recent tests Tracing and Wait Events

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 PX Deq: 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 Direct Path Reads Sort I/O Read-ahead PX Slave I/O The average wait time – SAN! Tracing and Wait Events

Event Waits Timeouts Time (s) (ms) /txn direct Path read 2,249,666 0 115,813 51 25.5 PX Deq: 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 Tracing and Wait Events

Conclusion Introduction Parallel Architecture Configuration Dictionary Views Tracing and Wait Events Conclusion

Plan / Test / Implement Asking for trouble if you don’t! Hardware It’s 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 less unnecessary work in the first place Even if PX does make things go quickly enough, it’s going to use a lot more resources doing so Conclusion

Don’t use it for small, fast tasks They won’t go much quicker They might go slower They will use more resources Don’t use it for online Not unless it’s a handful of users With a predictable maximum number of concurrent activities Who understand the implications and won’t go crazy when something takes four times as long as normal! It gives a false initial perception of high performance and isn’t scalable Okay, Tom, set parallel_adaptive_multi_user to TRUE Conclusion

The slower your I/O sub-system, the more benefit you are likely to see from PX But shouldn’t you fix the underlying problem? More on this in the next presentation Consider whether PX is the correct parallel solution for overnight batch operations A single stream of parallel jobs? Parallel streams of single-threaded jobs? Unfortunately you’ll probably have to do some work to prove your ideas! Conclusion

Tuning & Tracing Parallel Execution (An Introduction) Doug Burns (dougburns@yahoo.com) (oracledoug.blogspot.com) (doug.burns.tripod.com)

Introduction to Parallel Execution

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