Re: Result caching
- From: "Job Miller" <dmarc-noreply@xxxxxxxxxxxxx> (Redacted sender "jobmiller@xxxxxxxxx" for DMARC)
- To: "dmarc-noreply@xxxxxxxxxxxxx" <dmarc-noreply@xxxxxxxxxxxxx>, "rajendra.pande@xxxxxxx" <rajendra.pande@xxxxxxx>, "oracle-l@xxxxxxxxxxxxx" <oracle-l@xxxxxxxxxxxxx>
- Date: Fri, 23 Jan 2015 15:37:49 +0000 (UTC)
> thus added code to disable cardinality feedback at the session level on the
>top of the script and my results became consistent.
Is this query meaningful, or just a sample query used to test out your test
harness?
If cardinality feedback is indeed working as advertised and fixing a problem
with the limitations of stats collection or a data specific issue, do you want
to lock your users into bad performance for consistency of testing sake or do
you want to fix the problem to get good production results the first time?
Job
From: stephen van linge <dmarc-noreply@xxxxxxxxxxxxx>
To: "dmarc-noreply@xxxxxxxxxxxxx" <dmarc-noreply@xxxxxxxxxxxxx>;
"rajendra.pande@xxxxxxx" <rajendra.pande@xxxxxxx>; "oracle-l@xxxxxxxxxxxxx"
<oracle-l@xxxxxxxxxxxxx>
Sent: Thursday, January 22, 2015 2:17 PM
Subject: Re: Result caching
I worked offline with Iggy and was able to deduce the cause of the issue, the
following is a summary of our efforts and the result (in bold for those of you
that don't want to read the details). I ran a trace and tkprof on both the
first and second runs of the script, and we noticed that there was significant
physical I/O occurring on the first run and not on the second run. Upon closer
examination, each run had a different plan hash, thus Iggy directed me to view
v$sql_shared_cursor
(http://docs.oracle.com/cd/E11882_01/server.112/e40402/dynviews_3059.htm#REFRN30254).
We noticed that the "USE_FEEDBACK_STATS" flag was the only difference between
the plans, and so upon further examination Iggy found that the cause of the
divergent plans could be cardinality feedback in 11g r2
(https://blogs.oracle.com/optimizer/entry/cardinality_feedback). Since this
query is likely to be run more "here and there" instead of in rapid succession,
I judged that the more production-accurate result for us would be to benchmark
the query without cardinality feedback enabled, and thus added code to disable
cardinality feedback at the session level on the top of the script and my
results became consistent. Stephen
From: stephen van linge <dmarc-noreply@xxxxxxxxxxxxx>
To: "rajendra.pande@xxxxxxx" <rajendra.pande@xxxxxxx>;
"dmarc-noreply@xxxxxxxxxxxxx" <dmarc-noreply@xxxxxxxxxxxxx>;
"oracle-l@xxxxxxxxxxxxx" <oracle-l@xxxxxxxxxxxxx>
Sent: Tuesday, January 20, 2015 9:17 AM
Subject: Re: Result caching
@IggyOk I'll look at the trace files, this'll be a good excuse to get to know
trace files and tkprof better. @RajendraThe first run of the benchmarking
script (runs the query itself 5 times) is just the first time I hit "execute"
on it. The re-run is the second time the benchmarking script is run with the
same query (another 5 times). It certainly could be the instrumentation, but
as the query in question is a SELECT and is being run exactly as-is over and
over again, I believe the possibility of an error is minute. Stephen
From: "rajendra.pande@xxxxxxx" <rajendra.pande@xxxxxxx>
To: dmarc-noreply@xxxxxxxxxxxxx; oracle-l@xxxxxxxxxxxxx
Sent: Tuesday, January 20, 2015 9:10 AM
Subject: RE: Result caching
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{}#yiv3790300871 How do you tell the difference between a re-run (average .6
seconds) and a second run (average 70 seconds)My thought is there is some issue
with the instrumentation. Another question is how do you validate the results.
Is it possible that there is some error in the re-run that is not caught
Regards
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On
Behalf Of stephen van linge
Sent: Tuesday, January 20, 2015 11:56 AM
To: oracle-l@xxxxxxxxxxxxx
Subject: Result caching Hi, I'm trying to put together a benchmarking wrapper
that we can place scripts inside. The script is simple in design, it does the
following: 1) Loop over the query that's being benchmarked x number of times
(configurable).2) Throw out the first 2 runs and average the rest of the
runs.3) Return the average duration in milliseconds of the runs from (2). So
far this has worked great, however I'm having some weird results. A query I'm
benchmarking as an unoptimized case takes ~70 seconds to run on average as
reported from the benchmarking wrapper (with 5 runs) which is all fine and
dandy, but if I try to run the wrapper again (with 5 more runs), it completes
in 0.6 seconds on average. I had two thoughts: 1) Maybe the execution plan is
being cached. But this doesn't explain why it consistently ran so slow for 5
runs.2) Maybe the results are being cached. I reproduced the issue in our DR
server and verified that the results cache didn't change in size, so this is
not the issue.3) Maybe it has something to do with the fact that the query
being benchmarked is always being benchmarked without bind variables (hardcoded
bind values of a slow case to make the wrapper more simple). We are on
single-instance Oracle 11gR2 and I'm running this all through PL/SQL developer.
I can include the benchmarking wrapper script if necessary. This is more
academic at this point, when we run the unoptimized query through the
application, it consistently runs at around 70 seconds a run regardless of the
number of times run. Thank you for your time, Stephen Van Linge
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