I’ve been having issues for a long time now with an Exadata RAC database that has user reports experiencing library cache lock waits. The challenge is to figure out what is holding the library cache locks that the queries are waiting on.
My starting point on library cache locks has always been this Oracle support document:
How to Find which Session is Holding a Particular Library Cache Lock (Doc ID 122793.1)
But it doesn’t tell you how to find the session across nodes of a RAC database.
I also found this helpful blog post that briefly addresses finding the session across RAC nodes: Library cache lock and library cache pin waits
I’ve spent many hours over more than a year now dealing with these waits without a lot of success so I finally tried to build a script that I could run regularly to try to capture information about the sessions holding the library cache locks.
First, I knew from Oracle’s document that the x$kgllk table could be used to find the blocking session on a single node so I included queries against this table in my script and set it up so I would ssh into every node of the cluster and run a queries like this against each node:
-- sessions on this instance that are waiting on
-- library cache lock waits
-- unioned with
-- sessions on this instance that are holding locks that other
-- sessions on this instance are waiting on.
insert into myuser1.library_cache_lock_waits
(
SOURCETABLE,
BLOCKER,
SAMPLE_TIME,
INST_ID,
SID,
USERNAME,
STATUS,
OSUSER,
MACHINE,
PROGRAM,
LOGON_TIME,
LAST_CALL_ET,
KGLLKHDL,
KGLLKREQ,
USER_NAME,
KGLNAOBJ,
sql_id,
SQL_FULLTEXT
)
(select
'X\$KGLLK',
'N',
sysdate,
(select INSTANCE_NUMBER from v\$instance),
s.SID,
s.USERNAME,
s.STATUS,
s.OSUSER,
s.MACHINE,
s.PROGRAM,
s.LOGON_TIME,
s.LAST_CALL_ET,
x.KGLLKHDL,
x.KGLLKREQ,
x.USER_NAME,
x.KGLNAOBJ,
s.sql_id,
q.SQL_FULLTEXT
from
v\$session s, x\$kgllk x, v\$sql q
where
x.kgllkses=s.saddr and
s.sql_id=q.sql_id(+) and
s.event='library cache lock' and
x.KGLLKREQ > 0 and
q.child_number(+)=0)
union all
(select
'X\$KGLLK',
'Y',
sysdate,
(select INSTANCE_NUMBER from v\$instance),
s.SID,
s.USERNAME,
s.STATUS,
s.OSUSER,
s.MACHINE,
s.PROGRAM,
s.LOGON_TIME,
s.LAST_CALL_ET,
x.KGLLKHDL,
x.KGLLKREQ,
x.USER_NAME,
x.KGLNAOBJ,
s.sql_id,
q.SQL_FULLTEXT
from
v\$session s, x\$kgllk x, v\$sql q,
x\$kgllk x2
where
x.kgllkses=s.saddr and
s.sql_id=q.sql_id(+) and
x.KGLLKREQ = 0 and
x2.KGLLKREQ > 0 and
x2.KGLLKHDL = x.KGLLKHDL and
q.child_number(+)=0);
commit;
The dollar signs are escaped with a backslash because these queries are part of a Unix shell script. I picked a few columns that I thought would be helpful from v$session and joined to v$sql to get the text of the blocking and blocked SQL. Note that I ran these queries as SYSDBA. Here is an example of my test case where the blocker and blocked sessions are both on one node:
SOURCETABLE B Sample Time INST_ID SID USERNAME STATUS OSUSER MACHINE PROGRAM Logon Time LAST_CALL_ET KGLLKHDL KGLLKREQ USER_NAME KGLNAOBJ SQL_ID RESOURCE_NAME1 RESOURCE_NAME2 SQL_FULLTEXT
------------------------------ - ------------------- ---------- ---------- ------------------------------ -------- ------------------------------ ---------------------------------------------------------------- ------------------------------------------------ ------------------- ------------ ---------------- ---------- ------------------------------ ------------------------------------------------------------ ------------- ------------------------------ ------------------------------ --------------------------------------
X$KGLLK N 2014-02-17 17:19:01 1 1183 MYUSER1 ACTIVE myuser2 MYMACHINE sqlplus.exe 2014-02-17 17:18:57 5 00000005F9E7D148 2 MYUSER1 TEST g4b4j3a8mms0z select sum(b) from test
X$KGLLK Y 2014-02-17 17:19:03 1 995 MYUSER1 ACTIVE myuser2 MYMACHINE sqlplus.exe 2014-02-17 17:18:52 10 00000005F9E7D148 0 MYUSER1 TEST gv7dyp7zvspqg alter table test modify (a char(100))
Next, I noticed that on gv$session when a session was waiting on library cache lock waits sometimes FINAL_BLOCKING_INSTANCE and FINAL_BLOCKING_SESSION were populated and that might lead me to the session holding the lock. Also, this query and the ones following can run in a less privileged account – you don’t need SYSDBA.
drop table lcl_blockers;
create table lcl_blockers as
select distinct
s.INST_ID,
s.SID,
s.USERNAME,
s.STATUS,
s.OSUSER,
s.MACHINE,
s.PROGRAM,
s.LOGON_TIME,
s.LAST_CALL_ET,
s.sql_id
from
gv\$session s,
gv\$session s2
where
s2.FINAL_BLOCKING_INSTANCE=s.INST_ID and
s2.FINAL_BLOCKING_SESSION=s.SID and
s2.event='library cache lock';
insert into myuser1.library_cache_lock_waits
(
SOURCETABLE,
BLOCKER,
SAMPLE_TIME,
INST_ID,
SID,
USERNAME,
STATUS,
OSUSER,
MACHINE,
PROGRAM,
LOGON_TIME,
LAST_CALL_ET,
sql_id,
SQL_FULLTEXT
)
select
'GV\$SESSION',
'Y',
sysdate,
s.INST_ID,
s.SID,
s.USERNAME,
s.STATUS,
s.OSUSER,
s.MACHINE,
s.PROGRAM,
s.LOGON_TIME,
s.LAST_CALL_ET,
s.sql_id,
q.SQL_FULLTEXT
from
lcl_blockers s, gv\$sql q
where
s.sql_id=q.sql_id(+) and
s.INST_ID=q.INST_ID(+) and
q.child_number(+)=0
order by s.INST_ID,s.sid;
commit;
When this works – sporadically in my tests – it shows the same sort of information the previous queries do for same node locking. Here is an example of these gv$session queries catching the blocker:
SOURCETABLE B Sample Time INST_ID SID USERNAME STATUS OSUSER MACHINE PROGRAM Logon Time LAST_CALL_ET KGLLKHDL KGLLKREQ USER_NAME KGLNAOBJ SQL_ID RESOURCE_NAME1 RESOURCE_NAME2 SQL_FULLTEXT
------------------------------ - ------------------- ---------- ---------- ------------------------------ -------- ------------------------------ ---------------------------------------------------------------- ------------------------------------------------ ------------------- ------------ ---------------- ---------- ------------------------------ ------------------------------------------------------------ ------------- ------------------------------ ------------------------------ --------------------------------------
GV$SESSION Y 2014-02-17 17:19:05 1 995 MYUSER1 ACTIVE myuser2 MYMACHINE sqlplus.exe 2014-02-17 17:18:52 12 gv7dyp7zvspqg alter table test modify (a char(100))
Lastly, I got a cross node query working that uses the view gv$ges_blocking_enqueue. The key to making this query was that the pid column in gv$ges_blocking_enqueue is the same as the spid column in gv$process.
-- join gv$ges_blocking_enqueue, gv$session, gv$process to show
-- cross node library cache lock blockers. Blocked session will
-- have event=library cache lock.
drop table ges_blocked_blocker;
create table ges_blocked_blocker as
(select distinct
'N' blocker,
s.INST_ID,
s.SID,
s.USERNAME,
s.STATUS,
s.OSUSER,
s.MACHINE,
s.PROGRAM,
s.LOGON_TIME,
s.LAST_CALL_ET,
s.sql_id,
s.process,
p.spid,
e.RESOURCE_NAME1,
e.RESOURCE_NAME2
from
gv\$session s,
gv\$process p,
gv\$ges_blocking_enqueue e
where
s.event='library cache lock' and
s.inst_id=p.inst_id and
s.paddr=p.addr and
p.inst_id=e.inst_id and
p.spid=e.pid and
e.blocked > 0)
union
(select distinct
'Y',
s.INST_ID,
s.SID,
s.USERNAME,
s.STATUS,
s.OSUSER,
s.MACHINE,
s.PROGRAM,
s.LOGON_TIME,
s.LAST_CALL_ET,
s.sql_id,
s.process,
p.spid,
e.RESOURCE_NAME1,
e.RESOURCE_NAME2
from
gv\$session s,
gv\$process p,
ges_blocked b,
gv\$ges_blocking_enqueue e
where
s.inst_id=p.inst_id and
s.paddr=p.addr and
p.inst_id=e.inst_id and
p.spid=e.pid and
b.RESOURCE_NAME1=e.RESOURCE_NAME1 and
b.RESOURCE_NAME2=e.RESOURCE_NAME2 and
e.blocker > 0);
insert into myuser1.library_cache_lock_waits
(
SOURCETABLE,
BLOCKER,
SAMPLE_TIME,
INST_ID,
SID,
USERNAME,
STATUS,
OSUSER,
MACHINE,
PROGRAM,
LOGON_TIME,
LAST_CALL_ET,
sql_id,
SQL_FULLTEXT,
RESOURCE_NAME1,
RESOURCE_NAME2
)
select
'GV\$GES_BLOCKING_ENQUEUE',
s.blocker,
sysdate,
s.INST_ID,
s.SID,
s.USERNAME,
s.STATUS,
s.OSUSER,
s.MACHINE,
s.PROGRAM,
s.LOGON_TIME,
s.LAST_CALL_ET,
s.sql_id,
q.SQL_FULLTEXT,
s.RESOURCE_NAME1,
s.RESOURCE_NAME2
from
ges_blocked_blocker s, gv\$sql q
where
s.sql_id=q.sql_id(+) and
s.INST_ID=q.INST_ID(+) and
q.child_number(+)=0
order by s.INST_ID,s.sid;
commit;
Here is some example output from my gv$ges_blocking_enqueue script. I edited my username, machine name, etc. to obscure these.
SOURCETABLE B Sample Time INST_ID SID USERNAME STATUS OSUSER MACHINE PROGRAM Logon Time LAST_CALL_ET KGLLKHDL KGLLKREQ USER_NAME KGLNAOBJ SQL_ID RESOURCE_NAME1 RESOURCE_NAME2 SQL_FULLTEXT
------------------------------ - ------------------- ---------- ---------- ------------------------------ -------- ------------------------------ ---------------------------------------------------------------- ------------------------------------------------ ------------------- ------------ ---------------- ---------- ------------------------------ ------------------------------------------------------------ ------------- ------------------------------ ------------------------------ --------------------------------------
GV$GES_BLOCKING_ENQUEUE N 2014-02-17 17:19:55 2 301 MYUSER1 ACTIVE myuser2 MYMACHINE sqlplus.exe 2014-02-17 17:19:46 7 g4b4j3a8mms0z [0x426d0373][0x224f1299],[LB][ 1114440563,575607449,LB select sum(b) from test
GV$GES_BLOCKING_ENQUEUE Y 2014-02-17 17:19:55 1 497 MYUSER1 ACTIVE myuser2 MYMACHINE sqlplus.exe 2014-02-17 17:19:41 13 gv7dyp7zvspqg [0x426d0373][0x224f1299],[LB][ 1114440563,575607449,LB alter table test modify (a char(100))
The alter table command on node 1 is holding the lock while the select statement on node 2 is waiting on the library cache lock.
So, I’ve got this going on a script that runs every 15 minutes in production. It worked great in my test case but time will tell if it yields any useful information for our real problems.
– Bobby
p.s. I’ve uploaded a zip of my scripts: zip
Here is a description of the included files:
Testcase to create a library cache lock:
create.sql – creates a table with one character first column CHAR(1)
alter.sql – alters table expanding CHAR column
query.sql – queries table – waits on library cache lock wait if run while alter.sql is running
all.sh – top level script – you will need to edit to have the host names for your RAC cluster and to have your own userid and password
lcl.sh – x$ table script that is run on each node. Only key thing is that our profile required a 1 to be entered to choose the first database from a list. You may not need that line.
resultstable.sql – create table to save results
dumpresults.sql – dump out all results
dumpresultsnosql.sql – dump out all results except sql text so easier to read.
Here is the definition of the results table:
create table myuser1.library_cache_lock_waits
(
SOURCETABLE VARCHAR2(30),
BLOCKER VARCHAR2(1),
SAMPLE_TIME DATE,
INST_ID NUMBER,
SID NUMBER,
USERNAME VARCHAR2(30),
STATUS VARCHAR2(8),
OSUSER VARCHAR2(30),
MACHINE VARCHAR2(64),
PROGRAM VARCHAR2(48),
LOGON_TIME DATE,
LAST_CALL_ET NUMBER,
KGLLKHDL RAW(8),
KGLLKREQ NUMBER,
USER_NAME VARCHAR2(30),
KGLNAOBJ VARCHAR2(60),
SQL_ID VARCHAR2(13),
RESOURCE_NAME1 VARCHAR2(30),
RESOURCE_NAME2 VARCHAR2(30),
SQL_FULLTEXT CLOB
);
P.P.S. This was all tested only on Exadata running 11.2.0.2.
Oracle documentation on Library Cache:
The library cache is a shared pool memory structure that stores executable SQL and PL/SQL code. This cache contains the shared SQL and PL/SQL areas and control structures such as locks and library cache handles. In a shared server architecture, the library cache also contains private SQL areas.
Oracle 12c Concepts manual diagram with library cache
ppps. Someone emailed me to point out that I left out the definition of the ges_blocked table. I need to go back and review this because I am not sure that I didn’t make a mistake in my all.sh script. I found a script from this time that included the following create table statement for the ges_blocked table:
create table ges_blocked as
select
s.INST_ID,
s.SID,
s.USERNAME,
s.STATUS,
s.OSUSER,
s.MACHINE,
s.PROGRAM,
s.LOGON_TIME,
s.LAST_CALL_ET,
s.sql_id,
s.process,
p.spid,
e.RESOURCE_NAME1,
e.RESOURCE_NAME2
from
gv\$session s,
gv\$process p,
gv\$ges_blocking_enqueue e
where
s.event='library cache lock' and
s.inst_id=p.inst_id and
s.paddr=p.addr and
p.inst_id=e.inst_id and
p.spid=e.pid and
e.blocked > 0;
4/11/22
Standalone script on GitHub:
librarycachelocks.sql
