Re: kkslce [KKSCHLPIN2] mutex contention on hard parse of PQ with binds

From: Mohamed Houri <mohamed.houri_at_gmail.com>
Date: Tue, 15 Sep 2015 11:33:41 +0200
Message-ID: <CAJu8R6hGVptyWh=b7=i2W=RcGS6zc4jWG-iJsrmxAMOW8+BCbg_at_mail.gmail.com>

*"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_at_jlcomp.demon.co.uk>:

>
>
> 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
> _at_jloracle
> ------------------------------
> *From:* oracle-l-bounce_at_freelists.org [oracle-l-bounce_at_freelists.org] on
> behalf of Stefan Knecht [knecht.stefan_at_gmail.com]
> *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
>
>
>

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Received on Tue Sep 15 2015 - 11:33:41 CEST