1. 1
Oracle Background Process
Communication, Exotic Wait Events
and Some Tracing too
https://blog.tanelpoder.com
@tanelpoder

2. 2
Advanced Oracle Troubleshooting Training
By Tanel Poder | https://blog.tanelpoder.com/seminar/
• 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
• 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
• How many wait events?
• DISPLAY_NAME in 12c
Wait Events

5. 5
Wait Events - DISPLAY_NAME column

6. 6
• Demo
• Will be available at https://youtube.com/tanelpoder
Transient Wait Events

7. 7
Transient Wait Events - Process State Dump
Where did the
“read request”
wait events
go?!

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
• http://download.oracle.com/owsf_2003/40248_cai.ppt
KST Tracing

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
• 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
• 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
• Examples
Nested (multi-level) Wait Events - Process State Dump

13. 13
• Examples
Nested (multi-level) Wait Events - Process State Dump

14. 14
Inter-Process Communication & Background Processes

15. 15
Oracle 12c Process Models (Unix/Linux)

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 11.2.0.3+ can avoid foreground wakeup syscall overhead
• Foregrounds poll for sync completion instead of waiting for semaphore post
• _use_adaptive_log_file_sync
• https://fritshoogland.wordpress.com/2015/09/29/how-the-log-writer-and-foreground-processes-
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
• 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
• @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
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
• The video will be uploaded to:
• https://youtube.com/tanelpoder
• Gluent & Hive new LLAP architecture webinar (7th Feb 2018)
• https://gluent.com/event/gluent-hive-llap/
• Advanced Oracle Troubleshooting Training:
• https://blog.tanelpoder.com/seminar
• Follow @tanelpoder:
• https://twitter.com/tanelpoder
Thanks! Hopefully this was fun!