Re: using ASH to track down library cache lock contention
- From: Adric Norris <landstander668@xxxxxxxxx>
- To: oracle-l <oracle-l@xxxxxxxxxxxxx>
- Date: Thu, 18 Sep 2014 16:03:36 -0500
I appreciate the suggestions. Will definitely give them a try, and see what
turns up.
On Thu, Sep 18, 2014 at 3:51 PM, Tim Gorman <tim@xxxxxxxxx> wrote:
> A mystery! Love it!
>
> I recall that in astronomy, one finds "black holes" by the behavior of the
> things near to it.
>
> In this case, the waits on "library cache lock" are the behavior of the
> things near to the black hole. So, the black hole itself is hopefully
> something not mundane, something exclusively modifying the library cache,
> which I understand could be just about anything, such as an everyday SQL
> statement, but perhaps there was something out of the ordinary being
> executed?
>
> Since you've got it nailed down already to a 10 minute period, perhaps you
> could run something like...
>
> SQL> select instance_number, sql_opname, session_state, event, count(*)
> 2 from dba_hist_active_sess_history
> 3 where sample_time >= to_timestamp( '2014-09-15 14:19', 'YYYY-MM-DD
> HH24:MI' )
> 4 and sample_time < to_timestamp( '2014-09-15 14:28', 'YYYY-MM-DD
> HH24:MI' )
> 5 and sql_opname not in ('SELECT','UPDATE','INSERT','DELETE')
> 6 group by instance_number, sql_opname, session_state, event;
>
>
> This should filter out the majority of "usual" traffic and perhaps reveal
> an unexpected ALTER SYSTEM FLUSH SHARED_POOL running from a cron script
> somewhere? Unlikely I agree, but perhaps something suspect may pop out?
>
> For follow-up investigation, you could look at whatever of interest popped
> up and see if it started prior to the arrival of the flood of "library
> cache lock" events?
>
> Another approach might be to search sessions by the parameter values of
> the event...
>
> SQL> select parameter1, parameter2, parameter3 from v$event_name where
> name = 'library cache lock';
>
> PARAMETER1 PARAMETER2 PARAMETER3
> -------------------- -------------------- --------------------
> handle address lock address 100*mode+namespace
>
>
> Granted the parameters for the "library cache lock" event are addresses
> within the SGA, and not really useful in and of themselves, but perhaps
> these values can be used as IDs to identify the commonly-sought object,
> without caring what the value means. For example...
>
> SQL> select instance_number, p2, sql_opname, session_state, event,
> count(*)
> 2 from dba_hist_active_sess_history
> 3 where sample_time >= to_timestamp( '2014-09-15 14:19', 'YYYY-MM-DD
> HH24:MI' )
> 4 and sample_time < to_timestamp( '2014-09-15 14:28', 'YYYY-MM-DD
> HH24:MI' )
> 5 and p2 in (select distinct p2 from dba_hist_active_sess_history
> 6 where event = 'library cache lock'
> 7 and sample_time >= to_timestamp( '2014-09-15 14:19',
> 'YYYY-MM-DD HH24:MI' )
> 8 and sample_time < to_timestamp( '2014-09-15 14:28',
> 'YYYY-MM-DD HH24:MI' )
> 9 )
> 10 and event <> 'library cache lock'
> 11 group by instance_number, p2, sql_opname, session_state, event;
>
>
> ...or something like that, for P1, P2 (shown), and P3?
>
> Hope this helps...
>
> Thanks!
>
> -Tim
>
>
>
>
>
> On 9/18/14, 13:27, Adric Norris wrote:
>
> I'm trying to track down a recent issue on an 11.2.0.3 Solaris SPARC64
> system, where the majority of app-related database sessions unexpectedly
> hung for about 10 minutes. The condition cleared just as we were getting
> onto the system for troubleshooting, unfortunately, so we weren't able to
> capture much detail while the problem was ongoing... I'm therefore trying
> to identify the cause primarily via ASH data.
>
> At this point, we're fairly confident that it was due to contention for a
> library cache lock.
>
> SQL> select instance_number, session_state, event, count(*)
> 2 from dba_hist_active_sess_history
> 3 where sample_time >= to_timestamp( '2014-09-15 14:19', 'YYYY-MM-DD
> HH24:MI' )
> 4 and sample_time < to_timestamp( '2014-09-15 14:28', 'YYYY-MM-DD
> HH24:MI' )
> 5 group by instance_number, session_state, event
> 6 having count(*) >= 40
> 7 order by 4 desc, 2, 3, 1;
>
> INSTANCE_NUMBER SESSION EVENT COUNT(*)
> --------------- ------- ------------------------------ ----------
> 1 WAITING library cache lock 6264
> 3 WAITING library cache lock 6216
> 2 WAITING library cache lock 6031
> 1 WAITING db file sequential read 50
> 2 WAITING db file scattered read 41
>
> Unfortunately, the blocking-session info isn't captured for this type of
> wait. ASH does grab the P1/P2/P3 details, but everything I'm finding so far
> indicates that you have to match this information against various X$ tables
> while the issue is actually occurring. So the question, of course, is
> whether or not it's possible to definitively identify the offending session
> using just the ASH data.
>
> Any suggestions on how to accomplish this?
>
> Thanx!
>
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>
>
>
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