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Lock Types

Every now and again I have to check what a particular lock (or enqueue) type is for and what the associated parameter values represent. This often means I have to think about the names of a couple of views and a collection of columns – then create a few column formats to make the output readable (though sometimes I can take advantage of the “print_table()” procedure that Tom Kyte published a long time ago.  It only takes a little time to get the code right but it’s a nuisance when I’m in a hurry so I’ve just scribbled out a few lines of a script that takes a lock type as an input parameter and reports all the information I want.

rem
rem     Script:         lock_types.sql
rem     Author:         Jonathan Lewis
rem     Dated:          Mar 2018
rem     Usage:          start lock_types {lock type}
rem

define m_lock_type='&1'

column  display new_value m_display

select
        case when substr(version,1,2) = '12'
        then
                'display_name'
        else
                'name'
        end                     display
from
        v$instance
;

set linesize 160
set pagesize 60
set trimspool on

column type             format a4
column name             format a32
column description      format a132
column id1_tag          format a32
column id2_tag          format a32
column is_user          format a4 heading "User"
column is_recycle       format a4 heading "Rcyc"

set feedback off
break on report skip 1

spool lock_types

select  *
from    V$lock_type
where   type = upper('&m_lock_type')
order by
        type
;

column  name                    format a42
column  parameter1              format a9
column  parameter2              format a24
column  parameter3              format a22
column  wait_class              format a14
column  display_name            format a42

select
        eve.name,
        eve.parameter1,
        eve.parameter2,
        eve.parameter3,
        eve.wait_class,
        nullif(eve.&m_display, eve.name) display_name
from
        v$event_name    eve
where
        eve.name like 'enq: ' || upper('&m_lock_type') || '%'
order by
        nullif(eve.wait_class,'Other'),
        eve.name
;

set feedback on

I’ve included a check (and hack) on the value of the major version because 12c introduced a “display_name” as well as a “name” for events, and the latter is sometimes a little more descriptive than the former, so it’s nice to have a single script that could print two different values for the versions that have them.

Here’s a sample of the output when I pass ‘IV’ as an input parameter:

TYPE NAME                             ID1_TAG                          ID2_TAG                          User Rcyc
---- -------------------------------- -------------------------------- -------------------------------- ---- ----
DESCRIPTION                                                                                                                              CON_ID
------------------------------------------------------------------------------------------------------------------------------------ ----------
IV   Library Cache Invalidation       object #                         time stamp                       NO   NO
Synchronizes library cache object invalidations across instances                                                                              0

NAME                                       PARAMETER PARAMETER2               PARAMETER3             WAIT_CLASS     DISPLAY_NAME
------------------------------------------ --------- ------------------------ ---------------------- -------------- ------------------------------------------
enq: IV -  contention                      type|mode id1                      id2                    Other

As you can see from the presence of the con_id column in v$lock_type this output came from a 12c instance. I picked the IV lock because that’s the one that prompted me to check the meanings of the id[12] and parameter[123] columns when a question about waits for the IV lock appeared recently on Oracle-L. I’ve got two reasons for carrying on with this particular example – first that it demonstrates that the descriptions can be wrong, second that it allows me to demonstrate a quick tip on translation.

The question on Oracle-L related to a 4-node RAC system and reported one instance suffering long waits on the IV enqueue on a fairly regular basis when running a particular batch task. The OP reported the following values as the p1, p2, p3 values from v$session while the wait was going on:


P1    type|mode 1230372869
P2    id1       1398361667
P3    id2                3

According to the details in v$lock_type the enqueue is about library cache invalidation across instances – and that fits the OPs complaint because the system is a RAC system. The id1 value is supposed to be an obj# (object_id), but the OP said it wasn’t; and the id2 value is supposed to be a timestamp, but 3 is an odd value for a timestamp (though it might represent – for example – the 3 second wait that is a common time-out interval for enqueues). So, clearly, the descriptions can be wrong.

Translation

Take another look at p1 and p2, and turn them into Hexadecimal:


1230372869 (dec) = 0x49560005 (hex)
1398361667 (dec) = 0x53594E43 (hex)

If you happen to be good with Hex and ASCII code you’ll know that 2-byte values in the range 41-5F are mostly the capital letters of the Roman alphabet (while 61 – 7f are mostly the lower case letters), so a second translation step gives us:


1230372869 (dec) = 0x49560005 (hex) = 'IV'  5
1398361667 (dec) = 0x53594E43 (hex) = 'SYNC'

The p1 parameter is described (correctly) as “type/mode” – this is an IV enqueue held or requested in mode 5; the p2 parameter is not an object number, it looks more like a text description of why the enqueue is being requested (the enqueue is, after all, described as being used to “synchronize library cache object invalidation”).

I still don’t know what the final parameter represents – I doubt if it’s really about a three second wait (but that could be checked by examining v$session_wait over a period of several seconds or v$session_wait_history), it might be an indication of the instance that the session is trying to synchronize with (though, again, that seems a long shot), or it might just be a “reason-code” describing why the synchronisation is necessary.

Whenever in doubt about the meaning for the various parameters, it’s always worth a quick dec -> hex -> ASCII check, just in case it offers some clues about the function of the enqueue.