Re: Unable to detect blocking session
From: Andy Sayer <andysayer_at_gmail.com>
Date: Wed, 21 Feb 2018 22:42:14 +0000
Message-ID: <CACj1VR4G5f8VAfRbgn34LOxUqN5NBK4iSZTbRQxZGcuahDQNdg_at_mail.gmail.com>
ANDY_at_pdb1>/
8* select * from gv$session_blockers;
9 .
ANDY_at_pdb1>
ANDY_at_pdb1>_at_blockers
Date: Wed, 21 Feb 2018 22:42:14 +0000
Message-ID: <CACj1VR4G5f8VAfRbgn34LOxUqN5NBK4iSZTbRQxZGcuahDQNdg_at_mail.gmail.com>
Procedure created.
ANDY_at_pdb1>exec do_waits
*New session:*
ANDY_at_pdb1>ed
Wrote file afiedt.buf
1 create or replace procedure do_waits
2 is
3 begin
4 dbms_lock.sleep(1000);
5* end;
ANDY_at_pdb1>/
<hangs>
My query results:
INST_ID SESS USERNAME OSUSER
SQL_ID STATE WAIT_CLASS EVENT
WAIT_TIME_MICRO BLOCKED_BY FINAL_BLOCKED_BY
---------- ---------- -------------------- ------------------------------
------------- ------------------- --------------------
------------------------------ --------------- ----------
--------------------
1 62,63959 ANDY Andrew\Andy
6d1an556t3g16 WAITING Concurrency library cache pin
215770653 79_at_1 79_at_1
(although I can't see the blocking session here because it doesn't have a
transaction, I have the identifier for it so I can easily run a follow up
query like)
1 select s.inst_id, s.sid||','||s.serial# sess,s.username, s.osuser
2 ,s.sql_id, s.state, s.wait_class, s.event, s.wait_time_micro
3
,s.blocking_Session||nvl2(s.blocking_instance,'_at_'||s.blocking_instance,null)
blocked_By,
s.final_blocking_session||nvl2(s.final_blocking_instance,'_at_'||s.final_blocking_instance,null)
final_blocked_by
4 from gv$session s
5 where s.wait_class <> 'Idle' 6 or s.taddr is not null 7* or s.sid = 79
ANDY_at_pdb1>/
INST_ID SESS USERNAME OSUSER
SQL_ID STATE WAIT_CLASS EVENT
WAIT_TIME_MICRO BLOCKED_BY FINAL_BLOCKED_BY
---------- ---------- -------------------- ------------------------------
------------- ------------------- --------------------
------------------------------ --------------- ----------
--------------------
1 62,63959 ANDY Andrew\Andy
6d1an556t3g16 WAITING Concurrency library cache pin
273724936 79_at_1 79_at_1
1 79,35243 ANDY Andrew\Andy
4bm91698ssbw8 WAITING Idle PL/SQL lock timer
284643372
When I ran your query, I got no results. I then reran the query to find that I now have results, so I did a check. It takes some time for the blocking to get registered by the gv$session_blockers view
ANDY_at_pdb1>get blockers
1 select s.inst_id, s.sid||','||s.serial# sess,s.username, s.osuser
2 ,s.sql_id, s.state, s.wait_class, s.event, s.wait_time_micro
3
,s.blocking_Session||nvl2(s.blocking_instance,'_at_'||s.blocking_instance,null)
blocked_By,
s.final_blocking_session||nvl2(s.final_blocking_instance,'_at_'||s.final_blocking_instance,null)
final_blocked_by
4 from gv$session s
5 where s.wait_class <> 'Idle' 6 or s.taddr is not null 7 or s.sid = 79;
8* select * from gv$session_blockers;
9 .
ANDY_at_pdb1>
ANDY_at_pdb1>_at_blockers
INST_ID SESS USERNAME OSUSER
SQL_ID STATE WAIT_CLASS EVENT
WAIT_TIME_MICRO BLOCKED_BY FINAL_BLOCKED_BY
---------- ---------- -------------------- ------------------------------
------------- ------------------- --------------------
------------------------------ --------------- ----------
--------------------
1 25,14247 ANDY Andrew\Andy
6d1an556t3g16 WAITING Concurrency library cache pin
1506705 79_at_1
1 79,35243 ANDY Andrew\Andy
4bm91698ssbw8 WAITING Idle PL/SQL lock timer
605400087
no rows selected
ANDY_at_pdb1>_at_blockers
INST_ID SESS USERNAME OSUSER
SQL_ID STATE WAIT_CLASS EVENT
WAIT_TIME_MICRO BLOCKED_BY FINAL_BLOCKED_BY
---------- ---------- -------------------- ------------------------------
------------- ------------------- --------------------
------------------------------ --------------- ----------
--------------------
1 25,14247 ANDY Andrew\Andy
6d1an556t3g16 WAITING Concurrency library cache pin
4486987 79_at_1 79_at_1
1 79,35243 ANDY Andrew\Andy
4bm91698ssbw8 WAITING Idle PL/SQL lock timer
608380369
INST_ID SID SESS_SERIAL# WAIT_ID WAIT_EVENT WAIT_EVENT_TEXT
BLOCKER_INSTANCE_ID
BLOCKER_SID BLOCKER_SESS_SERIAL# CON_ID
---------- ---------- ------------ ---------- ----------
----------------------------------------------------------------
------------------- ----------- -------------------- ----------
1 25 14247 47 342 library cache pin
1
79 35243 0
It's probably populated internally at the same time the final blocking
session info is calculated.
Of course, that doesn't quite explain why your issue wasn't seen by that
query a long time after the blocking was reported to have started - but it
should cast some doubt about the accuracy of the query you were using.
And just to confirm, because my wait was against library cache pin, there's nothing in gv$lock so any query relying on that is not going to find it. There are other situations that could cause similar issues, but nothing will get past gv$session (unless the blocking is outside of the database!)
Hope that helps,
Andrew
On Wed, 21 Feb 2018 at 21:11, Sandra Becker <sbecker6925_at_gmail.com> wrote:
> Andrew, > > Unfortunately, the query they ran didn't show the blocking session and > instance. What is being asked for is to show the root blocker and all > sessions it is currently blocking. Still trying to work that one out. All > good suggestions, though, that should get us going in the right direction. > > Our applications are not properly instrumented and that isn't likely to > happen given that the current direction and directives for development. > > > > On Wed, Feb 21, 2018 at 1:46 PM, Andy Sayer <andysayer_at_gmail.com> wrote: > >> Hi Sandra, >> >> You say you only found the sql*plus session because it was listed as idle >> in gv$session, that sounds like a stab in the dark. >> >> Do you still have the results and the query you did against gv$session, >> the only things interesting would be inst_id, sid, event,blocking_session, >> blocking_instance, status. From just those few columns you can see what is >> waiting, on what and who they are waiting for. >> >> Not only will this help when the waits are due to a session blocking you, >> it will show you if there is some other event being waited on which isn’t >> necessarily another session holding an enqueue. Once you do this you will >> find that you don’t need to make guesses or work through a list of typical >> situations - it tells you exactly what is going on, you might need to >> google the event name but you will be in a 100x better position to fix the >> problem. >> >> Additionally, if your application is properly instrumented via >> dbms_application_info, you can easily filter down to just the sessions you >> know are important to find out what they’re doing. >> >> I certainly wouldn’t bother with the script you shared previously, >> especially as it didn’t help you in the exact circumstance it was designed >> for. >> >> Hope that helps, >> Andrew >> >> On Wed, 21 Feb 2018 at 20:07, Sandra Becker <sbecker6925_at_gmail.com> >> wrote: >> >>> New information: we didn't actually find the blocker using v$lock (or >>> gv$lock). Someone finally looked at gv$session and saw that a sqlplus >>> session had been idle for over an hour during the timeframe the application >>> was experiencing issues. We had a script, below, that we were using to >>> locate the root blocker since we have an application issue we're also >>> trying to resolve. That script did NOT show the sqlplus session that >>> appeared to be the blocker. We think that session was the blocker because >>> as soon as the user exited the session, everything cleared almost >>> immediately. We're trying to figure out a script that will find the root >>> block, including sessions like this user's that ran an update statement, >>> but didn't actually update any rows and he didn't do a commit. If someone >>> has any suggestions, we would definitely like to hear them. >>> >>> *ROOT BLOCKER SCRIPT* >>> >>> SELECT >>> DISTINCT >>> b.username||' - '||b.blocker_sid||','||b. >>> blocker_sess_serial#||',_at_'||b.inst_id||' : '||b.sql_id blocker, >>> b.seconds_in_wait b_secs, >>> w.username||' - '||w.sid||','||w.sess_serial#||',_at_'||w.inst_id||' >>> : '||w.sql_id waiter, w.seconds_in_wait w_secs >>> FROM (SELECT >>> bb.blocker_sid, >>> bb.blocker_sess_serial#, >>> sb.inst_id, >>> sb.username, >>> sb.sql_id, >>> sb.seconds_in_wait >>> FROM gv$session_blockers bb >>> JOIN gv$session sb >>> ON bb.blocker_sid = sb.sid >>> AND bb.blocker_sess_serial# = sb.serial# >>> ) b >>> JOIN (SELECT >>> bw.blocker_sid, >>> bw.sid, >>> bw.sess_serial#, >>> sw.inst_id, >>> sw.username, >>> sw. sql_id, >>> sw.seconds_in_wait >>> FROM gv$session_blockers bw >>> JOIN gv$session sw >>> ON bw.sid = sw.sid >>> AND bw.inst_id = sw.inst_id >>> AND bw.sess_serial# = sw.serial# >>> ) w >>> ON b.blocker_sid = w.blocker_sid >>> ORDER BY w.seconds_in_wait DESC >>> / >>> >>> On Wed, Feb 21, 2018 at 12:14 PM, Powell, Mark <mark.powell2_at_dxc.com> >>> wrote: >>> >>>> Sandy, to add to what Rick's reply, the "SQL*Net message from client" >>>> indicates to me the user made an update and failed to commit it. I think >>>> Rick provided valid possibilities for EM but for why you could not directly >>>> query the information I think we would need to see what queries you used to >>>> try to find the blocker though from your final remarks you did use GV$LOCK >>>> and GV$SESSION plus sys.dbms_lock_allocated to find the blocker. Could the >>>> earlier attempt have been using the V$ version and so missed the blocker >>>> since it was on another instance? The sys.dbms_lock_allocated table would >>>> not be necessary to find the blocking session though it would identify >>>> which User Lock (UL) was involved if a UL was involved. >>>> >>>> >>>> >>>> Mark Powell >>>> Database Administration >>>> (313) 592-5148 >>>> >>>> >>>> ------------------------------ >>>> *From:* oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> >>>> on behalf of Sandra Becker <sbecker6925_at_gmail.com> >>>> *Sent:* Wednesday, February 21, 2018 10:32:14 AM >>>> *To:* oracle-l >>>> *Subject:* Unable to detect blocking session >>>> >>>> >>>> Oracle EE 12.1.0.2 on RHEL 5.11 >>>> >>>> We had a situation in our production environment yesterday where a user >>>> had a sqlplus session had an uncommitted "zero row" update on a table. >>>> This kept the actual application from processing orders using that same >>>> table. The sqlplus session was initiated from SQL*Plus Release >>>> 11.1.0.6.0. The wait event on the application session was holding a user >>>> lock, which was apparently blocked, with a wait event of "SQL*Net message >>>> from client". Once the user's sqlplus session was exited, all application >>>> sessions resumed normal functions without any intervention. The user who >>>> issues the update is a tier1 support person, so we can't lock out their >>>> access for such activites to prevent future occurrences. >>>> >>>> What we are trying to understand is why we were unable to see that the >>>> user's sqlplus session was blocking either through EM or through queries >>>> looking at gv$session and gv$session_blockers. We found the application >>>> locks by joining gv$lock, gv$session and sys.dbms_lock_allocated. Any >>>> ideas why we could see the blocking or suggestions on what we can look at >>>> so we don't miss it again? >>>> >>>> >>>> -- >>>> Sandy B. >>>> >>>> >>> >>> >>> -- >>> Sandy B. >>> >>> > > > -- > Sandy B. > >
-- http://www.freelists.org/webpage/oracle-lReceived on Wed Feb 21 2018 - 23:42:14 CET