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Performance tuning library cache lock & single-task message

My colleague suddenly encountered a problem today,a Database becomes very slow , and the a lot of session wait library cache lock event, Let me help him to look. DB env is 10.2.0.4 rac.

at first ,to generate a AWR manually.

exec dbms_workload_repository.create_snapshot;
@?/rdbms/admin/awrrpt

Top 5 Timed Events

Event	Waits	Time(s)	Avg Wait(ms)	% Total Call Time	Wait Class
library cache lock	 293,463	 146,216	 498	 91.5	Concurrency
CPU time	 	 10,861	 	 6.8	 
db file sequential read	 189,358	 1,042	 6	 .7	User I/O
db file scattered read	 79,436	 421	 5	 .3	User I/O
log file sync	 222,715	 396	 2	 .2	Commit

Library Cache Activity
"Pct Misses" should be very low
Namespace	Get Requests	Pct Miss	Pin Requests	Pct Miss	Reloads	Invali- dations
BODY	740	0.14	540,869	0.00	0	0
CLUSTER	5	0.00	9	0.00	0	0
INDEX	54	0.00	95	0.00	0	0
SQL AREA	32,041	2.45	24,489,123	-0.01	63	4
TABLE/PROCEDURE	855	5.15	750,579	0.01	34	0
TRIGGER	34	0.00	168,673	0.00	0	0
Back to Library Cache Statistics 
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Library Cache Activity (RAC)
Namespace	GES Lock Requests	GES Pin Requests	GES Pin Releases	GES Inval Requests	GES Invali- dations
CLUSTER	9	0	0	0	0
INDEX	95	4	0	1	0
TABLE/PROCEDURE	7,791	38	0	7	0

Riyaj Shamsudeen wrote in his blog that

Library cache locks aka parse locks are needed to maintain dependency mechanism between objects and their dependent objects like SQL etc. For example, if an object definition need to be modified or if parse locks are to be broken, then dependent objects objects must be invalidated. This dependency is maintained using library cache locks. For example, if a column is dropped from a table then all SQLs dependent upon the table must be invalidated and reparsed during next access to that object. Library cache locks are designed to implement this tracking mechanism.

The wait parameters of library cache lock & pin waits
are
p1 The address in the memory of the libraray cache handle
p2 The memory address of the lock and pin structure
p3 is encoded as 10*mode+namespace
mode = 3 shared, 5 exclusive
The namespaces are
0 cursor
1 Table, procedure & others
2 package body
3 trigger
4 index
5 cluster
6 object
7 pipe

Find blocker sessions holding the lib cache in RAC

select a.sid Holder ,a.SERIAL#,b.INST_ID,b.KGLPNUSE Sesion , b.KGLPNMOD Held, b.KGLPNREQ Req
from x$kglpn b , v$session a
where b.KGLPNHDL in (select p1raw from v$session_wait
where wait_time=0 and event like 'library cache%')
and b.KGLPNMOD <> 0
and a.saddr=b.kglpnuse ;

   HOLDER    SERIAL#    INST_ID SESION                 HELD        REQ
---------- ---------- ---------- ---------------- ---------- ----------
      6515      10005          2 C0000020F0122A20          2          0

What are the holders waiting for?

SQL> select username,program,machine,sql_id,status,wait_time,event ,p1,p2 from v$session where sid=6515;

USERNAME    PROGRAM                MACHINE       SQL_ID      STATUS   WAIT_TIME   EVENT               P1       P2
----------- ---------------     ------------- ------------- -------- ----------   ------------------ -------   ----------
REPORT      task@kybb1 (TNS V1-V3) kybb1       9u5jnnk50k3h7 KILLED     661        single-task message  0         0	  

Notice the session status was ‘KILLED‘ and event was ‘single-task message’.

what is event ‘single-task message’?

Oracle’s definition of the event:
When running single task, this event indicates that the session waits for the client side of the executable. Wait Time: Total elapsed time that this session spent in the user application.

where is the “the client side of the executable” came from? the I check the sql text.

SQL> select sql_text from v$sqlarea where sql_id='9u5jnnk50k3h7';

SQL_TEXT
--------------------------------------------------------------------------------
INSERT INTO STATQ_ZDKBYHXX_DAY 
SELECT :B1 , :B4 , C.RES_TYPE_ID BRANDID1, N.RES_TYPE_NAME, SUBSTR(E.RECORGID, 1, 8) ORGID, 
O.ORGNAME, COUNT(*) FROM TBCS.SUBS_REWARD A, TBCS.SUBSCRIBER B, IM.IM_INV_MOBTEL C, TBCS.REC_PRESENT D, 
TBCS.REC_CHANGE F, TBCS.RECEPTION E, IM.IM_RES_TYPE@IM N, TBCS.ORGANIZATION O WHERE A.REGION
 = :B1 AND B.ACTIVE = 1 AND A.REGION = B.REGION AND (EXISTS (SELECT 1 FROM TBCS.ORGANIZATION_CHILD T WHERE
 ... -- had truncated
 AND A.STATUS = 1 AND A.SERIAL

I found the SQL call a dblink ,so “the client side ” is clear.

SQL> select sysdate from dual@IM;
SYSDATE
---------
28-JAN-15

the Db link is worked fine. but blocker session status was ‘KILLED’, it is still here . then try to kill the session again, when the blocker session is gone, the Waiting for the event ‘library cache lock’ has disappeared.

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