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Oracle X$TRACE, Exotic Wait Event Types and Background Process Communication


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In this session we will look into some internals of Oracle background process communication and also some special types of wait events that most people aren’t aware of. We will use some exotic tracing for internals research and fun and some of this stuff is actually useful in real life too! I’m not going to reveal everything upfront, as this is a secret internals hacking session after all ;-)

We will use various techniques to research what the “reliable message” wait event is about and how reliable background process communication is orchestrated in Oracle.

This is a hacking session, not formal structured training, so I’ll just do free form demos and talk (probably no slides, just hacking stuff on the command line).

The video will be available at
You might be interested in my full week Advanced Oracle Troubleshooting training at

Published in: Technology

Oracle X$TRACE, Exotic Wait Event Types and Background Process Communication

  1. 1. 1 Oracle Background Process Communication, Exotic Wait Events and Some Tracing too @tanelpoder
  2. 2. 2 Advanced Oracle Troubleshooting Training By Tanel Poder | • This seminar is focused entirely on Oracle troubleshooting – understanding what exactly the Oracle database is doing right now or what was it doing when the problem occurred. You will gain the skill to systematically work out the reasons for crashes, hangs, bad performance or other misbehavior. • The seminar will take you well beyond the typical high-level abstractions like the “database is slow” or “instance is hung”. After all, an Oracle instance is just a bunch of processes that access shared caches, perform I/O and coordinate work with each other. They can be measured in very high detail, both inside Oracle and at OS level. Understanding that is the core foundation of this class and helps you to drill down to the deepest levels of Oracle’s doings – using the right tool for the right problem. • You’ll also get fully downloadable videos for personal use!
  3. 3. 3 • This is a hacking session, not formal training with slides and structure • Mostly hands on in sqlplus and shell • Demos will break • Wait Events • DISPLAY_NAME • Transient wait events • KST Tracing / X$TRACE • Multi-level (nested) wait events • Background Process communication • KSR Channels, Actions & Messages • X$TRACING the reliable message wait event Topics
  4. 4. 4 • How many wait events? • DISPLAY_NAME in 12c Wait Events
  5. 5. 5 Wait Events - DISPLAY_NAME column
  6. 6. 6 • Demo • Will be available at Transient Wait Events
  7. 7. 7 Transient Wait Events - Process State Dump Where did the “read request” wait events go?!
  8. 8. 8 • KST = Kernel Server Trace • @oddc kst • Always enabled in-memory ring buffer tracing • trace_enabled = true (enable in memory tracing) • _trace_buffers = ALL:256 (trace buffer sizes per process) • X$TRACE & X$TRACE_EVENTS • @xt.sql @xtall.sql • ALTER TRACING ENABLE “event#:level:OPID” • ALTER TRACING ENABLE “10706:1:ALL” -- Global Enqueue KST tracing • Multiple IPC and RAC CIC events are always enabled by default • @grp event x$trace • KST Tracing
  9. 9. 9 KST Tracing • KST Trace buckets are dumped on errorstack dump (ORA-600/ORA-7445) • DIAG process dumps KST buckets globally upon RAC instance failure • Store the cross instance communication history that preceded the crash
  10. 10. 10 • Usually they show up when complex communication is needed between Oracle DB and ASM/Grid/CSS processes • ALTER DATABASE DATAFILE x RESIZE....; • Demo (if have time) Nested (multi-level) Wait Events
  11. 11. 11 • ALTER TABLESPACE x ONLINE • Tablespace on ASM - software mirrored by ASM • Control file read ends up wanting to read from ASM mirror disk instead • KSL SNAP END “suspends” time accounting for the 1st wait event and resumes later Nested (multi-level) Wait Events - Example
  12. 12. 12 • Examples Nested (multi-level) Wait Events - Process State Dump
  13. 13. 13 • Examples Nested (multi-level) Wait Events - Process State Dump
  14. 14. 14 Inter-Process Communication & Background Processes
  15. 15. 15 Oracle 12c Process Models (Unix/Linux)
  16. 16. 16 • Not all background processes communicate the same way • Unix semaphores are just used for process sleep/wakeup - not for messaging “payload” • Similar with thread-level post/wait with futexes • LGWR in can avoid foreground wakeup syscall overhead • Foregrounds poll for sync completion instead of waiting for semaphore post • _use_adaptive_log_file_sync • work-together-on-commit/ • ORADEBUG works by sending a SIGUSR2 signal to the inspected process • The signal handler in the target process will do the dumping • RAC cross-instance calls are also different • Higher level messaging over network sockets Background process communication - sleep/wakeup (post/wait)
  17. 17. 17 • Used for storing & exchanging message payloads • @channels.sql 1=1 • V$CHANNEL_WAITS • X$MESSAGES • X$KSBTABACT (background process “action“ list) KSR Communication Channels
  18. 18. 18 • @segcached soe.% • @grp status,dirty v$bh • alter session set “_serial_direct_read”=always (and reparse) • Run a query that forces a segment level checkpoint before scan • SQL trace • @xt • @xtall Tracing the reliable message wait event
  19. 19. 19 PARSING IN CURSOR #140325242489736 len=58 dep=0 uid=0 oct=3 lid=0 tim=2512232719414 hv= SELECT /*+ FULL(o) NO_PARALLEL */ COUNT(*) FROM soe.orders END OF STMT PARSE #140325242489736:c=0,e=126,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,plh=630573765,tim=2 EXEC #140325242489736:c=0,e=37,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,plh=630573765,tim=251 WAIT #140325242489736: nam='Disk file operations I/O' ela= 27 FileOperation=8 fileno=0 WAIT #140325242489736: nam='SQL*Net message to client' ela= 2 driver id=1413697536 #byt WAIT #140325242489736: nam='reliable message' ela= 179 channel context=23031522976 chan WAIT #140325242489736: nam='enq: KO - fast object checkpoint' ela= 264748 name|mode=126 WAIT #140325242489736: nam='direct path read' ela= 48 file number=13 first dba=1469858 WAIT #140325242489736: nam='direct path read' ela= 990 file number=13 first dba=513059 WAIT #140325242489736: nam='direct path read' ela= 156 file number=13 first dba=513152 WAIT #140325242489736: nam='direct path read' ela= 1092 file number=13 first dba=513280 Direct Path Read • alter session set serial_direct_read=always (and reparse)
  20. 20. 20 • The video will be uploaded to: • • Gluent & Hive new LLAP architecture webinar (7th Feb 2018) • • Advanced Oracle Troubleshooting Training: • • Follow @tanelpoder: • Thanks! Hopefully this was fun!