*"cursor: pin S wait on X"* is a symptom of a session wanting a cursor on
*S*hared mode but another session is holding the same cursor in e*X*lusive
mode.
Typically the cursor holding the eXclusive lock is hard parsing. It might
also be due to Extended Cursor Sharing kicking in and optimizing an
execution plan for each execution.This is why, as far as you are using bind
variables, I would have started by checking the reason for which your
cursor is not shared (using *nonshared.sql* script of Tanel Poder for
example)
Best regards
Mohamed
2015-09-15 10:37 GMT+02:00 Jonathan Lewis <jonathan@xxxxxxxxxxxxxxxxxx>:
Are the 4 out of 8 isolated to one of the two slave sets, or are they
scattered randomly across the two slave sets. (I would suspect the former).
It's probably expected behaviour (at least in some circumstances) but an
extreme example of slow parse times. I have seen a couple of cases in 11g
where PX slaves actually managed to get different execution plans for the
same query at the same time because the slaves re-optimized the query - if
this is possible, then the slaves could be in a position where they are
attempting to re-optimize the query, but are still waiting for the QC to
finish its parse call.
Gathering related information:
From v$sql - with one call to execute a new version of the statement, how
many parse_calls does the parallel statement report.
If you enable 10053 for a serial and a parallel version, does the parallel
trace show much more work than the serial ? Does the 12c serial show much
more work than the 11g serial?
If you enable 10046 trace can you see the 4 PX slaves doing work that
looks like re-optimization after they get the child mutex
Speculative question
Do the queries involve partitioned tables ?
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
@jloracle
------------------------------
*From:* oracle-l-bounce@xxxxxxxxxxxxx [oracle-l-bounce@xxxxxxxxxxxxx] on
behalf of Stefan Knecht [knecht.stefan@xxxxxxxxx]
*Sent:* 15 September 2015 09:12
*To:* oracle-l-freelists
*Subject:* kkslce [KKSCHLPIN2] mutex contention on hard parse of PQ with
binds
Hi all
Fairly odd event. After an upgrade to 12.1.0.2 we've been seeing very high
"cursor: pin S wait on X" waits. But here's the catch - they only happen
under very special (and seemingly odd) circumstances:
- Query runs in parallel
- Query uses bind variables
- Query is hard parsed
If we run the same query in serial, or with literal values, or we re-use a
cursor, no waits are seen.
The waits are huge (up to 0.3 seconds) and are seen on the PX slaves only.
Say we're using 8 slaves, 4 of them will show these waits. 4 won't. And
only if we're using binds. No binds, no waits.
Does anyone have an explanation for what could be happening?
I've already tried disabling some of the new fancy adaptive features, and
most bind related features (peeking, px bind peeking, bind value captures,
etc). Heck, it even happens with opt features set to 11.2.
On a very similar database, that's still on 11.2.0.3, we're also seeing
the same behavior. The key difference being that the waits are very short -
less than 0.01 seconds. But they're still there, and I don't see why they
would be in the first place. What causes Oracle to pin that mutex, if a
bind variable is used in the query?
A bug has been filed. So we'll see what comes out of that. It would seem
odd that we'd be the first ones to hit this, it's not exactly exotic to run
a parallel query with binds.
Stefan
