Re: session wait fun

From: Niall Litchfield <niall.litchfield_at_gmail.com>
Date: Thu, 23 Jan 2014 20:38:27 +0000
Message-ID: <CABe10sYLByX8sU6aLqK3k=FKKxjdmeHs6g0NG7n3b_yTYObVTg_at_mail.gmail.com>

I'm struggling to see how slow storage could produce *this* issue. I'd *expect* (aka guess) that current system time is maintained by vktm purely in memory. I'd probably next take a snapper snap of vktm and (possibly but carefully) a snap of *all* sessions with a say 5s interval on the basis that the problem appears to be time related (hence vktm) and system wide and across waits. Of course the problem may make such snaps useless :( On Jan 20, 2014 5:57 PM, "John D Parker" <orclwzrd_at_yahoo.com> wrote:

> Here's what I have been able to determine. The epoch wait time is indeed
> bogus. I did run strace -T -ttt on both lgwr and vktm. Those traces look
> "normal" to me. I don't see any elapsed times that are screwy. What I
> believe is happening based on reading the comments on this thread is the
> following. the "SGA Time" is not getting updated properly in my 11.2.0.3
> RAC environment due to some kind of slowdown in the backend storage that
> plugs ups the work and delays the updating of that time. The queries
> against v$session are yielding only the KSLWTSTIME value instead of the
> calculated delta because the "SGA Time" is incorrect or invalid. This in
> return yields the Epoch wait duration. Assuming that all my "supposes" and
> "assumptions" are correct this would be a bug. The upside is that I have a
> team mate seeing similar but not exactly the same behavior on an Exadata
> system. Their bad wait times can be traced back to slowdowns in processing
> parallel query jobs. Me thinks that the diff slowdowns are plugging up the
> same internal oracle pipe and thus preventing the "SGA Time" from getting
> updated properly. Patching is not likely to happen anytime soon so I'm
> trying to think up a work around. They are also not likely to fix the cause
> of the backend slowdown I perceive because they don't see it in their tiny
> lens.
>
> whew! I'm open to suggestions.
>
> John
>
> ------------------------------
> *From:* Jared Still <jkstill_at_gmail.com>
> *To:* John D Parker <orclwzrd_at_yahoo.com>
> *Cc:* oracle-l <oracle-l_at_freelists.org>
> *Sent:* Saturday, January 18, 2014 6:51 PM
> *Subject:* Re: session wait fun
>
> Just wondering if you had an opportunity to trace a session.
>
> It would be interesting to find know what the problem turned out to be.
>
> Jared Still
> Certifiable Oracle DBA and Part Time Perl Evangelist
> Sr Oracle DBA at Pythian
> Pythian Blog http://www.pythian.com/blog/author/still/
> Oracle Blog: http://jkstill.blogspot.com
> Home Page: http://jaredstill.com
>
>

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Received on Thu Jan 23 2014 - 21:38:27 CET