Re: enq: TM contention
- From: Tim Gorman <tim@xxxxxxxxx>
- To: oracle-l@xxxxxxxxxxxxx
- Date: Fri, 16 Dec 2011 09:15:45 -0700
Sandra,
Get to the session level, either looking at the "live" GV$SESSION view
or at the "historical" GV$ACTIVE_SESSION_HISTORY view or at the
"archive" DBA_HIST_ACTIVE_SESS_HISTORY view. Look at sessions waiting
on the event, and look at the SQL statement (via the SQL_ID and
DBMS_XPLAN) being executed. That alone should provide a huge amount of
insight into what is going on.
My guess, just to throw out a hypothesis: there is a long-running
CREATE INDEX command running that is blocking INSERT/UPDATE/DELETE
commands against the table?
Essentially, a exclusive "TM" enqueue is taken by a session to while an
object's basic structure is being modified, while a shared "TM" enqueue
(along with a "TX" enqueue) is taken by a DML command to prevent DDL
from being performed during the transaction.
So, if you try to CREATE INDEX on a table which is being modified by a
transaction, you immediately get ORA-00054 ("resource is busy") because
the CREATE INDEX could not obtain the exclusive "TM" enqueue because the
active transactions were holding shared "TM" enqueues on the table in
question.
Conversely, if a long-running CREATE INDEX is running and people attempt
to perform DDL, the requested shared "TM" enqueue will queue up waiting
for the exclusive "TM" enqueue to finish, hence your "enq: TM -
contention" waits by sessions.
Examining the session-level views mentioned above may prove me
completely 180-degrees wrong, but that's probably the general situation.
Hope this helps...
-Tim
On 12/16/2011 7:32 AM, Sandra Becker wrote:
> Oracle EE 10.2.0.5
>
> We have been experiencing periods where we see a lot of enq: TM
> contention waits. Customers notice performance degradation.
> Generally it clears on its own within 10 minutes or so. The only
> cause I've been able to locate in my searches (both google& MOS) is a
> non-indexed foreign key. This is definitely not the cause in our
> case. I haven't had an unindexed foreign key in over 2 years and
> verified it hadn't popped up unexpectedly.
>
> Does anyone know of another possibility for causing this wait?
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