Tuning based on Wall Clock Time instead of Elapsed Time
Wall Clock Time Tuning in Databases
Gerwin Hendriksen
Principal Consultant
Version 2.0
13 June 2017
© 2016 Gerwin Hendriksen
Agenda:
© 2016 Gerwin Hendriksen
• Tuning with AWR:
• What about parallel?
• What about WCT?
• How to get WCT data?
• What can we do more?
• Q/A
What about Parallel…
© 2016 Gerwin Hendriksen
Tuning with AWR…
© 2016 Gerwin Hendriksen
The normal way of database performance tuning is the
usage of AWR data.
Problems occur with this way of tuning when introducing
parallel.
So lets first checkout an AWR report…
AWR Reports…
© 2016 Gerwin Hendriksen
• AWR Snapshot: 166-167
• AWR Snapshot: 167-168
• AWR Snapshot: 168-169
• AWR Snapshot: 169-170
• AWR Snapshot: 170-171
So lets see the details:
© 2016 Gerwin Hendriksen
…and what is different for the different statements…
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163264128
Yes! We did some tuning…
© 2016 Gerwin Hendriksen
Based on AWR we did some tuning, we saw that the
system had still some resources available, so increasing
parallel processes looked to be very smart…
BUT…. IS IT REALLY SMART???
So what about the End-User?
© 2016 Gerwin Hendriksen
• AWR doesn’t give us
the information!!
• Oracle made SQL-
Monitoring…
Lets checkout
Oracle SQL-Monitoring…
Wall Clock Time YEAH !!!
© 2016 Gerwin Hendriksen
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• P2-5tm2zdkuyb3vk_10041458
• P4-8qzbnban8mdsm_10041535
• P8-dv0jap9p5vfr5_10041506
• P16-gmfh78v3n2f55_10041509
• P32-b23ua8kyvnq5d_10041515
• P64-8dn43389f4qvk_10041523
• P128-cc05cd0hz4brx_10041528
Questions Rising…
© 2016 Gerwin Hendriksen
• How can we checkout a change in performance from an
end-user perspective when changing the number of
parallel threads used for a query?
• The same thing applies for checking out overall database
performance from an end-user perspective. What did the
change do for the end-user performance?
The data for SQL-Monitoring comes from the columns:
• tm_delta_time
• tm_delta_cpu_time
• tm_delta_db_time
• delta_time
Data for SQL-Monitoring:
© 2016 Gerwin Hendriksen
Out of views:
• ASH: v$active_session_history
• AWR: dba_hist_active_sess_history
Blog Dominic Brooks !
Active Session History…
© 2016 Gerwin Hendriksen
AWR and ASH:
© 2016 Gerwin Hendriksen
WCT vs Elapsed Time…
© 2016 Gerwin Hendriksen
Session
Session
Parallel 1
Parallel 2
Parallel 3
Parallel 4
WCT for Non Parallel = Elapsed Time
WCT for Parallel WCT Improvement
Elapsed Time in Parallel = Sum of all used system time of involved threads
THREADS
How to get Wall Clock Time…
© 2016 Gerwin Hendriksen
• ASH samples every second. If we find an entry for a
session in ASH, we can say that it was running for one
second.
• AWR takes one out of ten ASH samples. If we find an
entry for session in AWR, we can say that it was running
for 10 seconds.
• In both above claims is some error, but with lots of
samples and longer running queries it works fine!!
Next to get Wall Clock Time…
© 2016 Gerwin Hendriksen
Parallel Server processes run under different sessions, but
have the same coordinator process (qc_session_id) and so
can be counted as elapse time for one and the same query.
Lets Dive into ASH and AWR…
DEMO TIME!!!
Calculate Wall Clock Time…1
© 2016 Gerwin Hendriksen
Specific statement in ASH (v$active_session_history):
WCT in seconds =
<ASH Samples containing one of related sessions>
x 1 second
ELA (DB-Time) in seconds =
<ASH Sampl. sum of occurrence of each related session>
x 1 second
Calculate Wall Clock Time…2
© 2016 Gerwin Hendriksen
Specific stat. in AWR (dba_hist_active_sess_history):
Cumalative WCT in seconds =
<AWR Samples contain at least one of related sessions>
x 10 seconds
Cumalative ELA (DB-Time) in seconds =
<AWR Sampl. sum of occurrence of each related session>
x 10 seconds
Calculate Wall Clock Time…3
© 2016 Gerwin Hendriksen
For one execution in AWR (dba_hist_sqlstat):
Estimation Parallel =
Cum. ELA / Cum. WCT
Estimation WCT per exection =
ELA from AWR Report / Est. Parallel / Number of Exec
© 2016 Gerwin Hendriksen
Data gathered…
© 2016 Gerwin Hendriksen
See the Excel:
WCT Calculated:
© 2016 Gerwin Hendriksen
WCT SQL Monitoring:
© 2016 Gerwin Hendriksen
Conclusion…
© 2016 Gerwin Hendriksen
• ASH and AWR can be used to calculate Wall Clock Time
for statements running more than 30 seconds quite
reliable.
• So extra data should be gathered from AWR to be able to
make good tuning decissions when parallel is used. In
EXADATA systems this is really important.
